Why loft vents for boiler and immersion cylinders?
Baker@nowhere.com Dave Baker
affair. The lower outlet to the coil inside the immersion cylinder tees off
to a vent into the small tank in the loft which therefore vents the boiler
side of things.
The heated water in the cylinder fills from the bottom and the outlet at the
top to the taps also tees off to a vent into the large loft tank. All fairly
normal and so far so good.
A few years ago I was up in the loft for the first time in ages and it was
dripping wet in there. Everything stored had gone musty and water was
running off the roof felt. Turned out the boiler vent was pumping over into
the small tank which was consequently full of hot water and producing clouds
of condensation. I tried the pump on its lowest setting and it still
happened. I examined the pipe runs for the whole system and concluded that
there was no point in a vent being there. The boiler fills up under gravity
feed from the small tank and any excess pressure can just go straight back
up there if it wants. The boiler certainly isn't going to explode and anyway
there's a blow-off valve on the back of it.
So I dug out an old radiator valve, slapped it on the end of the vent in the
loft and turned it off. It's been that way for years and hasn't made a scrap
of difference to anything other than the loft is now dry. So what purpose,
if any, was the vent really serving?
The vent to the main part of the cylinder doesn't overflow of course because
it isn't pumped but again I see no point to it other than maybe to let any
air in the cylinder get out. However wouldn't the air be immediately
displaced out of a hot tap somewhere the first time the system was filled if
there was no vent?
--
Dave Baker
www.pumaracing.co.uk
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Doctor Drivel
"Dave Baker" <Dave Ba...@nowhere.com> wrote in message
news:4409b3bb$0$7112$6d36...@titian.nntpserver.com...
You have converted the system to a combined heat and vent:
1. The pipe should be 22mm
2. The boiler should suitable for sealed systems (high limit stat)
Firstly, find out if the boiler is suitable. If yes, then have a 22mm pipe
from F&E tank to system an deemove the vent pipe with the valve on. . If
not, converts it back to a two pipe system and fit an air ejector.
Screwfix No.90641
Also fit a Magnaclean filter on the return after dosing with desludger and
flushing out. This will elimiante sludge which may accumulate in the ejector
and cause problems.........and protect the whole system
Aidan
Dave Baker wrote:
> So I dug out an old radiator valve, slapped it on the end of the vent in the
> loft and turned it off. It's been that way for years and hasn't made a scrap
> of difference to anything other than the loft is now dry.
With all due respect, you're daft.
Take it off.
> So what purpose,
> if any, was the vent really serving?
In normal operation it releases any air that gets into the system.
Typically dissoled oxygen in the boiler return wil be released from
solution when the water is haeted in the boiler and is discharged from
the open vent.
In the event of a control failure, it is a safety device; it releases
steam, prevents the system becoming over-pressurized and bursting. In
your system, such a failure would probably now cause the contents of
the heating system to be discharged into the loft and the boiler would
dry-fire to destruction.
It was pumping over probably because it was piped incorrectly. Get
someone competent to fix that problem. Conversion to a sealed system
would be an improvement, if the boiler is suitable; if you'd like that
done, get someone competent to do that.
> The boiler certainly isn't going to explode and anyway
> there's a blow-off valve on the back of it.
I find your faith in a cheap safety valve to be touching.
They don't have to do anything in normal operation. If called upon to
operate in anger, a disturbing number fail to work. Many have been
dripping inconspicuously for years, the water evaporates from the hot
safety valve and the accumulation of limescale deposits concretes the
valve immovably shut. Have you tested the valve?
>
> The vent to the main part of the cylinder doesn't overflow of course because
> it isn't pumped but again I see no point to it other than maybe to let any
> air in the cylinder get out. However wouldn't the air be immediately
> displaced out of a hot tap somewhere the first time the system was filled if
> there was no vent?
Kin 'ell! Is this a wind up? Leave it alone.
Immersion heater contacts frequently weld themselves together, so the
heater fails on. The open vent would then discharge steam. If you block
it, the steam will blow the contents of the cylinder into the storage
tank. If that discharge route is blocked, this happens;
http://www.waterheaterblast.com
That is a small water heater (12 US gallons). Drivel loves posting that
link to promote his thermal stores, but someone proposes plugging the
vent on a domestic hot water storage system and he makes no comment; he
is a dangerous idiot.
Unvented water heaters are good, but they have at least 3 sequential
safety devices for each heating system. They are costly. A vented
cylinder cannot be converted.
Andy Hall
>
>Kin 'ell! Is this a wind up? Leave it alone.
>
>Immersion heater contacts frequently weld themselves together, so the
>heater fails on. The open vent would then discharge steam. If you block
>it, the steam will blow the contents of the cylinder into the storage
>tank. If that discharge route is blocked, this happens;
>
>http://www.waterheaterblast.com
>
>That is a small water heater (12 US gallons). Drivel loves posting that
>link to promote his thermal stores, but someone proposes plugging the
>vent on a domestic hot water storage system and he makes no comment; he
>is a dangerous idiot.
>
>Unvented water heaters are good, but they have at least 3 sequential
>safety devices for each heating system. They are costly. A vented
>cylinder cannot be converted.
An interesting story is the sinking of the Fleur de Lys, a fishing
vessel.
This one went down, fortunately not killing anybody, when its 120
litre HW cylinder, a class 4 type (weakest rating normally used for
open vented systems) was being used in a pressurised system.
A firm of plumbers who didn't know what they were doing had replaced
the cylinder and fitted the wrong type.
http://www.maib.gov.uk/cms_resources/dft_masafety_504222.pdf
The cause of the explosion was listed as follows:
"The hot water storage cylinder ruptured because the water in it
overheated and steam generated, causing overpressure. The rupture of
the cylinder probably occurred at about 3.4bar, four times working
pressure of the cylinder. The pressure relief valve must have been set
at this pressure or above, or it was seized in the closed position.
The cylinder probably overheated when the immersion heater thermostat
failed to shut off the electrical supply to the heater. There were no
safety devices fitted to the system to prevent overheating and
overpressure as a result of thermostat failure."
Returning to the original point of blocking the vent - it is quite
common to have an isolating gate valve etc. on the cold feed to the
cylinder. If this is closed when the heat source is still on and
the vent is blocked, a potential bomb will have been created.
We can see from the fishing boat example that there does not need to
be a great deal of pressure at all to achieve an unfortunate
situation.
--
.andy
Doctor Drivel
"Aidan" <amc...@hotmail.com> wrote in message
news:1141510790....@v46g2000cwv.googlegroups.com...
I disagree. Heat banks give better performance as they can take higher
pressures becaus eof the plate heat exchangers, open vented and a minute to
zero chance of an explosion. Unvented cylinders require an annual service
charge. That alone is enough to discount them totally...especially when a
safe, service free, better performing unit is available....the heat bank.
Baker@nowhere.com Dave Baker
>
>
>
> > So I dug out an old radiator valve, slapped it on the end of the vent in
the
> > loft and turned it off. It's been that way for years and hasn't made a
scrap
> > of difference to anything other than the loft is now dry.
>
> With all due respect, you're daft.
I'm actually a rather competent engineer who was hoping for reasoned opinion
on the merits of this vent.
> Take it off.
No thanks. As I say everything works very well as is.
>
> > So what purpose,
> > if any, was the vent really serving?
>
> In normal operation it releases any air that gets into the system.
> Typically dissoled oxygen in the boiler return wil be released from
> solution when the water is haeted in the boiler and is discharged from
> the open vent.
Firstly, the water in the boiler system always has an open vent path via the
fill point at the bottom of the tank in the loft. If water can get down that
I see no reason why dissolved oxygen can't get back up it.
Secondly, The vast bulk of the water in the system is in the radiators and
that's where the vast bulk of any dissolved oxygen will end up. There is no
way for bottom fed radiators to have this oxygen removed than by bleeding
from the valves at the top. A bleed valve at the top of the boiler system
would surely do the same rather than having to have a permanently open vent.
>
> In the event of a control failure, it is a safety device; it releases
> steam, prevents the system becoming over-pressurized and bursting. In
> your system, such a failure would probably now cause the contents of
> the heating system to be discharged into the loft and the boiler would
> dry-fire to destruction.
Nonsense. The system isn't pressurised other than by the head of water from
the loft tank. The contents of the system can't be discharged into the loft
other than by the same pipe from the bottom of the loft tank which is
continuously filling it up anyway.
>
> It was pumping over probably because it was piped incorrectly.
Possibly but it was certainly ok for many years and the only change made to
the system in 19 years was a new pump which was fitted by a Corgi plumber.
Now it's possible this was fitted to pump in the opposite direction from the
original pump but I can't instantly see how this would make a deal of
difference anyway. I'm open to suggestions as how it might. At present the
vent is on the return side of the pump so if anything I would think this
would give the least chance of pumping over.
Get
> someone competent to fix that problem. Conversion to a sealed system
> would be an improvement, if the boiler is suitable; if you'd like that
> done, get someone competent to do that.
>
> > The boiler certainly isn't going to explode and anyway
> > there's a blow-off valve on the back of it.
>
> I find your faith in a cheap safety valve to be touching.
>
> They don't have to do anything in normal operation. If called upon to
> operate in anger, a disturbing number fail to work. Many have been
> dripping inconspicuously for years, the water evaporates from the hot
> safety valve and the accumulation of limescale deposits concretes the
> valve immovably shut. Have you tested the valve?
It's been checked, cleaned and the washer replaced a couple of times and is
fine. In any case the system is always vented to the bottom of the loft tank
as I have said.
> >
> > The vent to the main part of the cylinder doesn't overflow of course
because
> > it isn't pumped but again I see no point to it other than maybe to let
any
> > air in the cylinder get out. However wouldn't the air be immediately
> > displaced out of a hot tap somewhere the first time the system was
filled if
> > there was no vent?
>
> Kin 'ell! Is this a wind up? Leave it alone.
I have every intention of leaving it alone because it cause no problems as I
have made clear. Please try to confine yourself to the physics and
hydraulics of the issue rather than pontificating.
>
> Immersion heater contacts frequently weld themselves together, so the
> heater fails on. The open vent would then discharge steam. If you block
> it, the steam will blow the contents of the cylinder into the storage
> tank. If that discharge route is blocked, this happens;
There is no electric heater in the tank.
>
> http://www.waterheaterblast.com
>
> That is a small water heater (12 US gallons). Drivel loves posting that
> link to promote his thermal stores, but someone proposes plugging the
> vent on a domestic hot water storage system and he makes no comment; he
> is a dangerous idiot.
>
> Unvented water heaters are good, but they have at least 3 sequential
> safety devices for each heating system. They are costly. A vented
> cylinder cannot be converted.
--
Doctor Drivel
> Typically dissoled oxygen in the boiler return wil be released from
> solution when the water is haeted in the boiler and is discharged from
> the open vent.
>
> In the event of a control failure, it is a safety device; it releases
> steam, prevents the system becoming over-pressurized and bursting. In
> your system, such a failure would probably now cause the contents of
> the heating system to be discharged into the loft and the boiler would
> dry-fire to destruction.
If the boiler has a blow-off valve the boiler is most probably suitable for
sealed systems ..... and a combined feed and expansion pipe. If so, then he
has a one pipe F&E system, except the pipe is probably 15mm not 22mm. Then
it will not dry fire itself to destruction as a high limit is on the boiler.
Baker@nowhere.com Dave Baker
I've just been up into the loft and airing cupboard and mapped out all the
pipe runs. If you look at this basic diagram.
http://www.diydata.com/planning/central_heating/boiler.htm
Then my pump would be on the blue line immediately adjacent to the boiler,
pumping into it and then back out into the line that feeds the vent and then
the rads and cylinder. The vent is therefore in the wrong place, before the
rads rather than after them and hence seeing all the pump pressure. Turning
the pump round wouldn't help because then the feed from the bottom of the
loft tank would see this pressure instead thus similarly reducing flow
through the rads.
So at present I only get normal flow because the vent is blocked and the
expansion/feed pipe does any venting required. When I get round to fixing
all the niggles in the system I'll reorganise everything to correct this.
As I say though, whether vented, for the first 15 years here, or unvented
for the last 4 I've always had piping hot rads and hot water and I guess
just the pressure head from the loft tank has sufficed to keep the system
running ok. Black mark to the guy who fitted all this before I moved in I
suppose.
Doctor Drivel
> Sorted :)
>
> I've just been up into the loft and airing cupboard and mapped out all the
> pipe runs. If you look at this basic diagram.
>
> http://www.diydata.com/planning/central_heating/boiler.htm
>
> Then my pump would be on the blue line immediately adjacent to the boiler,
> pumping into it and then back out into the line that feeds the vent and
> then
> the rads and cylinder. The vent is therefore in the wrong place, before
> the
> rads rather than after them and hence seeing all the pump pressure.
> Turning
> the pump round wouldn't help because then the feed from the bottom of the
> loft tank would see this pressure instead thus similarly reducing flow
> through the rads.
>
> So at present I only get normal flow because the vent is blocked and the
> expansion/feed pipe does any venting required. When I get round to fixing
> all the niggles in the system I'll reorganise everything to correct this.
>
> As I say though, whether vented, for the first 15 years here, or unvented
> for the last 4 I've always had piping hot rads and hot water and I guess
> just the pressure head from the loft tank has sufficed to keep the system
> running ok. Black mark to the guy who fitted all this before I moved in I
> suppose.
The older it is, the near the time to when it boils over. Get it sorted
ASAP.
Andy Hall
Ba...@nowhere.com> wrote:
>Sorted :)
>
>I've just been up into the loft and airing cupboard and mapped out all the
>pipe runs. If you look at this basic diagram.
>
>http://www.diydata.com/planning/central_heating/boiler.htm
>
>Then my pump would be on the blue line immediately adjacent to the boiler,
>pumping into it and then back out into the line that feeds the vent and then
>the rads and cylinder. The vent is therefore in the wrong place, before the
>rads rather than after them and hence seeing all the pump pressure. Turning
>the pump round wouldn't help because then the feed from the bottom of the
>loft tank would see this pressure instead thus similarly reducing flow
>through the rads.
>
>So at present I only get normal flow because the vent is blocked and the
>expansion/feed pipe does any venting required. When I get round to fixing
>all the niggles in the system I'll reorganise everything to correct this.
>
>As I say though, whether vented, for the first 15 years here, or unvented
>for the last 4 I've always had piping hot rads and hot water and I guess
>just the pressure head from the loft tank has sufficed to keep the system
>running ok. Black mark to the guy who fitted all this before I moved in I
>suppose.
If your pump is on the blue line adjacent to the boiler, it's the pump
that's in the wrong place relative to the other pipes.
It's not so much an issue of whether the pump is before the heating
circuit or after it, but rather that there is a pressure differential
between the feed/expansion pipe and the vent.
If your set up is as shown on the web site except that the pump is
adjacent to the boiler, you have a classic pumping over situation
because the two pipes to the tank are on opposite sides of the pump.
Actually, the setup in the diagram is not a lot better because the F/E
pipe and the vent have the boiler between them. Since there is a
level of flow resistance through the boiler there will be a pressure
differential so in the case illustrated there is a possibility of air
being sucked down the vent if the boiler flow resistance is high -
e.g. with pump on high speed. If you imagine the flow through the
boiler stopped (for the sake of illustration) you can see that the
effect would be that the water level in the tank would rise and air
would enter through the vent. In normal operation it would depend on
the pump setting, the height of the tank and the flow resistance of
the boiler and heating/HW circuits.
A better design would have the flow from the boiler going to an air
separator - e.g. www.bes.ltd.uk part 11334 - and the F/E and vent
pipes separately joining it. The pump goes after that followed by
motorised valve(s) and the heating and HW circuits. The returns of
those would be commoned and go directly to the boiler.
This achieves two key objectives:
- The F/E and vent pipes are close together. This means that there is
very little pressure differential and no chance of sucking down or
pumping over. Where a separator isn't used, but the pipes join the
circuit, it is recommended that there is not more than a 150mm space
between the two connection points.
- There is a clear path from the boiler flow to the vent. This helps
with venting but also the safety purpose of an easy path for the
escape of steam in the event of control or other failure.
An additional benefit of the air separator is that the system becomes
very easy to vent and little or no air should collect in the radiators
after the initial filling and expulsion of dissolved air.
--
.andy
fred
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Aidan
Dave Baker wrote:
> > With all due respect, you're daft.
>
> I'm actually a rather competent engineer who was hoping for reasoned opinion
> on the merits of this vent.
Stick to racing engines. I'm an HVAC Engineer. I suggest you take it
off.
> > Take it off.
>
> No thanks. As I say everything works very well as is.
So far.
> Firstly, the water in the boiler system always has an open vent path via the
> fill point at the bottom of the tank in the loft. If water can get down that
> I see no reason why dissolved oxygen can't get back up it.
It's often on the return and it's commonly lower. The oxygen won't get
to it, it will corrode the radiators,
> Secondly, The vast bulk of the water in the system is in the radiators and
> that's where the vast bulk of any dissolved oxygen will end up. There is no
> way for bottom fed radiators to have this oxygen removed than by bleeding
> from the valves at the top. A bleed valve at the top of the boiler system
> would surely do the same rather than having to have a permanently open vent.
>
> >
> > In the event of a control failure, it is a safety device; it releases
> > steam, prevents the system becoming over-pressurized and bursting. In
> > your system, such a failure would probably now cause the contents of
> > the heating system to be discharged into the loft and the boiler would
> > dry-fire to destruction.
>
> Nonsense. The system isn't pressurised other than by the head of water from
> the loft tank. The contents of the system can't be discharged into the loft
> other than by the same pipe from the bottom of the loft tank which is
> continuously filling it up anyway.
It would be discharged into the F&E tank through the cold feed pipe.
> >
> > It was pumping over probably because it was piped incorrectly.
>
> Possibly but it was certainly ok for many years
You may think so. I do not. You didn't notice anything amiss, this
doesn't mean it was OK.
> the system in 19 years was a new pump which was fitted by a Corgi plumber.
Ye're doomed, doomed I tell ye.
> Now it's possible this was fitted to pump in the opposite direction from the
> original pump but I can't instantly see how this would make a deal of
> difference anyway. I'm open to suggestions as how it might. At present the
> vent is on the return side of the pump so if anything I would think this
> would give the least chance of pumping over.
>
> > I find your faith in a cheap safety valve to be touching.
> >
> > They don't have to do anything in normal operation. If called upon to
> > operate in anger, a disturbing number fail to work. Many have been
> > dripping inconspicuously for years, the water evaporates from the hot
> > safety valve and the accumulation of limescale deposits concretes the
> > valve immovably shut. Have you tested the valve?
>
>
> > Kin 'ell! Is this a wind up? Leave it alone.
>
> I have every intention of leaving it alone because it cause no problems as I
> have made clear. Please try to confine yourself to the physics and
> hydraulics of the issue rather than pontificating.
I have done. You're convinced you know everything worth knowing about
it and can't believe that someone else may know more.
Take it off.
> Dave Baker
> www.pumaracing.co.uk
Tony Williams
Dave Baker <Dave Ba...@nowhere.com> wrote:
> http://www.diydata.com/planning/central_heating/boiler.htm
> Then my pump would be on the blue line immediately adjacent to
> the boiler, pumping into it..............
That's where our pump is.... in the return line,
and immediately next to the boiler. It is the
original 1970 plumbing and works ok.
> .............and then back out into the line that
> feeds the vent and then the rads and cylinder. The vent is
> therefore in the wrong place, before the rads rather than after
> them and hence seeing all the pump pressure.
ISTM that if the radiator loops are free-flowing, then
the pump will pull a low pressure on the pipe feeding
the loops, lower than the head required by the vent pipe.
Radiator circulation is tickety-boo.
But if the radiator loops get blocked, (by sludge, or
later fitted valves?) then the pump will preferentially
do a local circulation, in just the loop between the
header tank and the vent pipe. Any chance that that
is what you are seeing?
BTW: My old plumbing book has a gruesome tale about
a plumber who blocked a vent pipe with a cork, to try
and reduce the amount of water lost during a drain
down. The resultant vaccuum caused parts of the
plumbing to collapse inwards. Blocking a vent pipe
may not be good news.
--
Tony Williams.
David Hansen
<Dave Ba...@nowhere.com> wrote this:-
>I'm actually a rather competent engineer
So you claim. However, in my view a competent engineer would have
found out what was going on and then sorted it out at source, rather
then dealing with the symptoms without knowing what they were doing.
Finding out what was going on is fairly simple, as it is an
elementary part of building services engineering. For example in a
library one can find, "Faber and Kell's Heating and Air-conditioning
of Buildings", the book on the subject. In there one should find
some examples of piping layouts.
Finding out would also involve tracing out the pipes, something you
say you have only just done.
>who was hoping for reasoned opinion on the merits of this vent.
From my 1984 edition of the above book, page 190, "In no
circumstances should the feed and expansion pipe be combined with
the system vent pipe as case D. This practice was shown to be
dangerous more than 50 years ago."
The reason why it is dangerous should be obvious. It is to do with
what happens if this one pipe is restricted or blocked, especially
if it is only 15mm. You are living on borrowed time and should sort
it out immediately.
BTW, your one safety valve may have been cleaned and the washer
replaced a few times. What pressure is it set at? How often is the
operation of the valve tested? Can it pass the necessary volume of
hot water/steam?
--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54
Dave Plowman (News)
Dave Baker <Dave Ba...@nowhere.com> wrote:
> Possibly but it was certainly ok for many years and the only change made
> to the system in 19 years was a new pump which was fitted by a Corgi
> plumber. Now it's possible this was fitted to pump in the opposite
> direction from the original pump but I can't instantly see how this
> would make a deal of difference anyway. I'm open to suggestions as how
> it might. At present the vent is on the return side of the pump so if
> anything I would think this would give the least chance of pumping over.
Is there a bypass fitted? Commonly just a link pipe between feed and
expansion with perhaps a gate valve in it?
--
*The fact that no one understands you doesn't mean you're an artist
Dave Plowman da...@davenoise.co.uk London SW
To e-mail, change noise into sound.
Andy Hall
<to...@ledelec.demon.co.uk> wrote:
>In article <440a3ab2$0$27800$6d36...@titian.nntpserver.com>,
> Dave Baker <Dave Ba...@nowhere.com> wrote:
>
>> http://www.diydata.com/planning/central_heating/boiler.htm
>
>> Then my pump would be on the blue line immediately adjacent to
>> the boiler, pumping into it..............
>
> That's where our pump is.... in the return line,
> and immediately next to the boiler. It is the
> original 1970 plumbing and works ok.
>
>> .............and then back out into the line that
>> feeds the vent and then the rads and cylinder. The vent is
>> therefore in the wrong place, before the rads rather than after
>> them and hence seeing all the pump pressure.
>
> ISTM that if the radiator loops are free-flowing, then
> the pump will pull a low pressure on the pipe feeding
> the loops, lower than the head required by the vent pipe.
> Radiator circulation is tickety-boo.
>
> But if the radiator loops get blocked, (by sludge, or
> later fitted valves?) then the pump will preferentially
> do a local circulation, in just the loop between the
> header tank and the vent pipe. Any chance that that
> is what you are seeing?
>
All of this can be very easily avoided and the risk of pumping over
eliminated by positioning the FE and vent pipes on the same side of
the boiler.
There should be a clear path from the boiler flow side to the vent
with nothing in the way in terms of pumps or valves.
The FE pipe can be connected to a point on the circuit within 150mm of
where the vent pipe connects. The pump is located after that,
followed by the motorised valve(s).
An even better solution is to use an Aerjec air separator which will
do the correct plumbing connections in one place as well as helping to
purge the system of air as it is circulated following refilling and in
general.
The flow through the radiators then becomes irrelevant and if
inhibitor is used, no significant corrosion or sludgng either.
Another issue to consider is that if TRVs are used then the flow
resistance through the CH circuit will increase anyway as they start
to close, so it having an arrangement where avoiding pumping
over/sucking down is on the basis of clear path through the CH circuit
is never going to be clear of potential problems.
--
.andy
Doctor Drivel
"David Hansen" <SENDdavi...@spidacom.co.uk> wrote in message
news:3ebl02dd48a9kkm8v...@4ax.com...
> On Sun, 5 Mar 2006 00:17:08 -0000 someone who may be "Dave Baker"
> <Dave Ba...@nowhere.com> wrote this:-
>
>>I'm actually a rather competent engineer
>
> So you claim. However, in my view a competent engineer would have
> found out what was going on and then sorted it out at source, rather
> then dealing with the symptoms without knowing what they were doing.
>
> Finding out what was going on is fairly simple, as it is an
> elementary part of building services engineering. For example in a
> library one can find, "Faber and Kell's Heating and Air-conditioning
> of Buildings", the book on the subject. In there one should find
> some examples of piping layouts.
>
> http://www.foyles.co.uk/foyles/display.asp?K=181613010522833&M=310&WHERE=(keyword+INCLUDES+'heating')&SS=(
>
> Finding out would also involve tracing out the pipes, something you
> say you have only just done.
>
>>who was hoping for reasoned opinion on the merits of this vent.
>
> From my 1984 edition of the above book, page 190, "In no
> circumstances should the feed and expansion pipe be combined with
> the system vent pipe as case D. This practice was shown to be
> dangerous more than 50 years ago."
I don't know what context this passage was written. Combined feed and
expansion pipes are common and inceasingly copmmon as 99% of boioer has a
high limit stat If a boiler has a high limit stat (suitabkle for sealed
system) it can have a combined feed & expansion pipe that must be 22mm
minimum.
> The reason why it is dangerous should be obvious. It is to do with
> what happens if this one pipe is restricted or blocked,
A a normal sealed system in millions of homes:- If the blow-off valve is
seized and the main boiler stat fails as well, the only protection is the
high limit device. It is exactly the same protection level with a combined
feed and expansion pipe, except that to block a 22mm pipe will a hell of a
lot more difficult on the flow at high level, where they are supposed to be,
and are specifically for this reason (no crud at high level), than a
blow-off valve failing. Which means a combined feed and expansion pipe setup
is safer. The reason why 22mm pipe is used as a minum (some require that
size because of system size), is so that the probability of being blocked is
minimum. A blow-off pipe on a sealed system in most domestic homes is only
15mm.
> especially
> if it is only 15mm. You are living on
> borrowed time and should sort
> it out immediately.
He says he has a blow-off valve. If so, the explosion danger is minimum.
If the boiler is suitable for a sealed systemn, which it appears to be with
a blow off valve on it (I am assuming the valve is integral with the boiler,
not an add on), then it will have a high limit stat which improve boiler
protection. If not suitable for sealed systems then he has no boiler
protection.
Tony Williams
> ISTM that if the radiator loops are free-flowing, then
> the pump will pull a low pressure on the pipe feeding
> the loops, lower than the head required by the vent pipe.
> Radiator circulation is tickety-boo.
Followup to myself.
We don't have the vent pipe taking off from the
feed to the radiators loop. Ours is a 4-pipe boiler,
one pair feeding the radiators (pumped in the return),
and the other pair doing the gravity feed to the h/w
storage tank. That second pair does an upslope to the
top of the h/w tank, with a dropdown into the h/w tank,
and the upslope continuing up to become the vent pipe.
--
Tony Williams.
Aidan
Dave Baker wrote:
> > In the event of a control failure, it is a safety device; it releases
> > steam, prevents the system becoming over-pressurized and bursting. In
> > your system, such a failure would probably now cause the contents of
> > the heating system to be discharged into the loft and the boiler would
> > dry-fire to destruction.
>
> Nonsense. The system isn't pressurised other than by the head of water from
> the loft tank. The contents of the system can't be discharged into the loft
> other than by the same pipe from the bottom of the loft tank which is
> continuously filling it up anyway.
You and Drivel have misunderstood what I have written. It is not
nonsense, it is common sense.
If the boiler thermostat fails in the ON position, the temperature of
the water will quite likely exceed 100 degC. The water in the boiler
will boil, producing steam. This happens. Your heating system will THEN
be pressurized by the steam and the steam pressure will become greater
than the static pressure provided by the head of water in the F&E tank.
The water in the heating system will be pushed up the cold feed pipe
into the F&E tank. It will be pushed out rapidly, probably faster than
the F&E overflow pipe can remove it. There will be no water in the
boiler and it will continue firing.
This assumes that the cold feed pipe is clear and that there is not a
stopcock (acting as a non return valve) fitted to it. The cold feed on
my heating system was totally blocked with limescale when I bought the
house. This is a common fault. I had to cut out the tee and replace it.
There had been a longstanding leak from the heating system through the
coil in the indirect cylinder.
With a non-fubarred system, such a failure would cause steam to be
blown out of the open vent, but cold make-up water would enter the
system through the cold feed. This could continue indefinitely.
You can't get 2-way flow (steam up & water down) in one pipe, which is
probably what Faber & Kell were referring to. Their book (20 years
since I had a copy) deals with commercial installations on which such a
combined CF &OV would be inadvisable.
Aidan
Tony Williams wrote:
> BTW: My old plumbing book has a gruesome tale about
> a plumber who blocked a vent pipe with a cork, to try
> and reduce the amount of water lost during a drain
> down. The resultant vaccuum caused parts of the
> plumbing to collapse inwards. Blocking a vent pipe
> may not be good news.
Good point sir! Probably the copper hot water storage cylinder, which
will be crushed by atmospheric pressure if you were to drain it without
relieving the vacuum (hose out bathroom window, no hot taps open).
On unvented hot water storage cylinders the mandatory T&P relief valve
also acts as a vacuum breaker valve (I think, goes off to get book to
check).
Andy Hall
>
>Tony Williams wrote:
>
>> BTW: My old plumbing book has a gruesome tale about
>> a plumber who blocked a vent pipe with a cork, to try
>> and reduce the amount of water lost during a drain
>> down. The resultant vaccuum caused parts of the
>> plumbing to collapse inwards. Blocking a vent pipe
>> may not be good news.
>
>Good point sir! Probably the copper hot water storage cylinder, which
>will be crushed by atmospheric pressure if you were to drain it without
>relieving the vacuum (hose out bathroom window, no hot taps open).
>
True... I was able very easily to collapse a vented system cylinder
to a very small size using a domestic vacuum cleaner - it was quite
surprising.
As a heating engineer who visited to service my parents' system on one
occasion remarked ".. as thin as bloody fag paper.."
--
.andy
Doctor Drivel
>
> Tony Williams wrote:
>
>> BTW: My old plumbing book has a gruesome tale about
>> a plumber who blocked a vent pipe with a cork, to try
>> and reduce the amount of water lost during a drain
>> down. The resultant vaccuum caused parts of the
>> plumbing to collapse inwards. Blocking a vent pipe
>> may not be good news.
>
> Good point sir! Probably the copper hot water storage cylinder, which
> will be crushed by atmospheric pressure if you were to drain it without
> relieving the vacuum (hose out bathroom window, no hot taps open).
I have seen that happen a few times, evn being open vented and not blocked
Dave Fawthrop
wrote:
Done that myself :-( Should have known better. :-(
--
Dave Fawthrop <dave hyphenologist co uk>
Freedom of Speech, Expression, Religion, and Democracy are
the keys to Civilization, together with legal acceptance of
Fundamental Human rights.
Doctor Drivel
>
> Dave Baker wrote:
>
>> > In the event of a control failure, it is a safety device; it releases
>> > steam, prevents the system becoming over-pressurized and bursting. In
>> > your system, such a failure would probably now cause the contents of
>> > the heating system to be discharged into the loft and the boiler would
>> > dry-fire to destruction.
>>
>> Nonsense. The system isn't pressurised other than by the head of water
>> from
>> the loft tank. The contents of the system can't be discharged into the
>> loft
>> other than by the same pipe from the bottom of the loft tank which is
>> continuously filling it up anyway.
>
> You and Drivel have misunderstood what I have written. It is not
> nonsense, it is common sense.
I never mosunderstodd at all and I agree with you. I was looking into what
he had. If the boiler is a sealed compatible boiler (which it "appears" to
be) then there is no immediate chance of explosion, and as I said get it
sorted ASAP.
> If the boiler thermostat fails in the ON position, the temperature of
> the water will quite likely exceed 100 degC. The water in the boiler
> will boil, producing steam. This happens. Your heating system will THEN
> be pressurized by the steam and the steam pressure will become greater
> than the static pressure provided by the head of water in the F&E tank.
If a sealed compatible boiler then the high limit cuts in. Another
protection level.
> The water in the heating system will be pushed up the cold feed pipe
> into the F&E tank. It will be pushed out rapidly, probably faster than
> the F&E overflow pipe can remove it. There will be no water in the
> boiler and it will continue firing.
>
> This assumes that the cold feed pipe is clear and that there is not a
> stopcock (acting as a non return valve) fitted to it. The cold feed on
> my heating system was totally blocked with limescale when I bought the
> house. This is a common fault. I had to cut out the tee and replace it.
> There had been a longstanding leak from the heating system through the
> coil in the indirect cylinder.
That is why on combined feed & expansion pipes it has to be 22mm minimum.
The one pipe has system water move up and down the pipe, so water with
inhibitor will be passing the tee reducing the liklihood of only fresh water
being there leaving behind scale. In very hard water areas it is best to
fit a 28mm tee and a small length of 28mm pipe. An aerator can be fitted
instead of a tee. Have compression joints for disconnection purposes and
the unit can have the odd clear out in case. The aerators have a greater
volume inside, so les lilely to scaleup, and are recommended by the likes of
Baxi and Potterton on combined feed and expansion pipes.
> With a non-fubarred system, such a failure would cause steam to be
> blown out of the open vent, but cold make-up water would enter the
> system through the cold feed. This could continue indefinitely.
>
> You can't get 2-way flow (steam up & water down) in one pipe, which is
> probably what Faber & Kell were referring to. Their book (20 years
> since I had a copy) deals with commercial installations on which such a
> combined CF &OV would be inadvisable.
Yep. You can also get water circulation in one pipe. I have come across
that before.
Aidan
Doctor Drivel wrote:
> I never mosunderstodd at all and I agree with you. I was looking into what
> he had. If the boiler is a sealed compatible boiler (which it "appears" to
> be) then there is no immediate chance of explosion, and as I said get it
> sorted ASAP.
You can't have read it then. Re the pressure relief valve, he said;
> It's been checked, cleaned and the washer replaced a couple of times and is
> fine. In any case the system is always vented to the bottom of the loft tank
> as I have said.
He has dismantled the safety valve.
The ones I've seen in the past 10 or 15 years were factory pre-set, no
user adjustable parts. It's probably a spring-loaded antique and the
setting has been lost in dismantling. The boiler is probably a similar
vintage. Almost certainly not suitable for sealed system operation and
no manual-reset secondary thermostat.
Now it hasn't got an open vent or a working safety valve either.
Doctor Drivel
>
> Doctor Drivel wrote:
>
>> I never misunderstodd at all and I agree with you. I was looking into
>> he had. If the boiler is a sealed compatible boiler (which it "appears"
>> to
>> be) then there is no immediate chance of explosion, and as I said get it
>> sorted ASAP.
>
> You can't have read it then. Re the pressure relief valve, he said;
>
>> It's been checked, cleaned and the
>> washer replaced a couple of times and is
>> fine. In any case the system is always
>> vented to the bottom of the loft tank
>> as I have said.
>
> He has dismantled the safety valve.
..and cleaned it.
> The ones I've seen in the past 10 or 15 years were factory pre-set, no
> user adjustable parts. It's probably a spring-loaded antique and the
> setting has been lost in dismantling.
Some of them you could split and clean, he has done that. That does not
mean it is now unoperatational.
> The boiler is probably a similar
> vintage. Almost certainly not suitable for sealed system operation and
> no manual-reset secondary thermostat.
"every" boiler I have seen with an integral pressure relief valve has been
suitable for a sealed system (why is it there?), that is going back to
boilers made 30 years ago (a few still around). In ye olden dayes, a
presure relief valve was fitted to the flow pipe of open vented boilers.
This would blow off in the kitchen. It was made illegal and required a pipe
to outside. Then they never required one at all on open vented systems.
> Now it hasn't got an open vent or a working safety valve either.
I told him to check the suitability of the boiler for sealed systems. I
would be confident the valve would work.
David Hansen
<Min...@nospam.com> wrote this:-
>I don't know what context this passage was written.
"A traditional boiler, loft tank, vented cylinder type
affair."
>He says he has a blow-off valve.
Correct. However, that does not answer the three questions I asked
about it.
David Hansen
<Min...@nospam.com> wrote this:-
>That is why on combined feed & expansion pipes it has to be 22mm minimum.
Here you appear to be mixing up two pipes. The first pipe is the
feed and expansion pipe, the second pipe is the vent pipe. This is
strange as you got it right earlier in the thread.
>The one pipe has system water move up and down the pipe,
The name feed and expansion pipe is something of a clue. In such a
pipe water will/should indeed move up and down, as the system is
warmed up or cools down. As it happens water will also move up and
down the vent pipe, because the level of water in the vent pipe
will/should be the same as the level of water in the F & E tank.
Doctor Drivel
> On Sun, 5 Mar 2006 13:58:44 -0000 someone who may be "Doctor Drivel"
> <Min...@nospam.com> wrote this:-
>
>>That is why on combined feed &
>>expansion pipes it has to be 22mm minimum.
>
> Here you appear to be mixing up two pipes.
Nope. A combined feed and expansion pipe is one pipe. Legal on systems
which sealed system compatible boilers may be fitted. And no need to have a
blow-off valve either.
>>The one pipe has system water move up and down the pipe,
>
> The name feed and expansion pipe is something of a clue.
Yes it is. I have the clue and also explained. You should understand what
it is.
David Hansen
<Min...@nospam.com> wrote this:-
>> Here you appear to be mixing up two pipes.
>
>Nope.
Yet you then go on to make precisely the same mistake again.
>A combined feed and expansion pipe is one pipe.
A feed and expansion pipe is indeed one pipe. However, as I said and
you snipped, you appear to be mixing up this one pipe with the vent
pipe, which is another pipe. As I said, this is strange as you got
it right earlier.
Bluff and bluster on this again, if you wish.
Doctor Drivel
> On Sun, 5 Mar 2006 16:01:37 -0000 someone who may be "Doctor Drivel"
> <Min...@nospam.com> wrote this:-
>
>>> Here you appear to be mixing up two pipes.
>>
>>Nope.
>
> Yet you then go on to make precisely the same mistake again.
>
>>A combined feed and expansion pipe is one pipe.
>
> A feed and expansion pipe is indeed one pipe. However, as I said and
> you snipped, you appear to be mixing up this one pipe with the vent
> pipe, which is another pipe. As I said, this is strange as you got
> it right earlier.
>
> Bluff and bluster on this again, if you wish.
It is the vent pipe as air works its way up this pipe. Air doesn't care. It
will go up any pipe.
Christian McArdle
> you snipped, you appear to be mixing up this one pipe with the vent
> pipe, which is another pipe. As I said, this is strange as you got
> it right earlier.
At the risk of agreeing with Drivel, a combined feed and expansion pipe also
handles the venting. There is only one 22mm pipe from the system to the
loft. This pipe handles feeding of the system with water, expansion from the
system into the header tank and venting of air or steam to the loft space.
It splits into 2 at the bottom of the tank and a vent only pipe then
continues above the tank.
Christian.
Doctor Drivel
"Christian McArdle" <cmcar...@nospam.yahooxxxx.co.uk> wrote in message
news:44105f44$0$270$4d4e...@read.news.uk.uu.net...
>> A feed and expansion pipe is indeed one pipe. However, as I said and
>> you snipped, you appear to be mixing up this one pipe with the vent
>> pipe, which is another pipe. As I said, this is strange as you got
>> it right earlier.
>
> At the risk of agreeing with Drivel,
As I know more about it than anyone else just read and take heed.
> a combined feed and expansion pipe also
> handles the venting. There is only one 22mm pipe from the system to the
> loft. This pipe handles feeding of the system with water, expansion from
> the
> system into the header tank and venting of air or steam to the loft space.
> It splits into 2 at the bottom of the tank and a vent only pipe then
> continues above the tank.
It doesn't split at all. It is one 22mm pipe to the bottom of the F&E tank.
Been standard for years. As 95% of all boilers are sealed system
compatible, doing it this way saves on pipe, time and no pump over or
drawing of air in.
Potterton/Baxi specify a air ejector when the combined Feed and expansion
pipe meets the system. Works very well. With this method extremely low
heads can be attained (200mm from top of boiler).
legin
having googled and followed recent threads I was thinking that the best
probable solution would indeed be to fit a heatbank in my self build. I
went with the specific question in mind. How do you keep the
condednsing boiler condensing if a temperature of circa 75 degrees is
required for the heatbank. I even suggested mixing the return from the
heatbank with the return from the underfloor heating to get the return
temp down enough to get lull the boiler into condensing mode. Apart
from some exhibitors being unsure of how a system could be piped, the
overall conclusion was to fit an unvented cylinder. This would take
better advantage of a condensing boiler.
Anyone any better advice to offer.
Regards
Legin
Andy Hall
wrote:
One thing to bear in mind is that a boiler does not reach Nirvana or
orgasm when it "goes into condensing mode".
What actually happens for boilers designed for condensing is that
there is an efficient heat exchanger able to work with a temperature
differential between return and flow of 20 degrees and possible more.
The lower the return temperature, the more efficiently the boiler is
running. This happens regardless of whether it is condensing or not.
What happens at the dew point - around 54 degrees - below which
condensing takes place, is that the *rate of change* of efficiency
with return temperature increases below this point.
This is as a result of the contribution of the latent heat that is
released from the phase change of water from gas (i.e. steam from
combustion) to liquid. Note that I am talking about steam in the
true and invisible sense here and not "steam" that is visible and is
in fact, water vapour.
There isn't a step change of efficiency at the dew point - just a knee
in the curve.
In terms of optimising overall efficiency, anything that can be done
to reduce the temperature of the return water to the boiler will help.
UFH is possibly one aspect for this. Another, if you are going to
use radiators, is to make sure that these give the heat required when
you run them at 70 degree flow and 50 return. Conventionally,
radiators are sized based on 82 and 70. To achieve 70/50 requires
them to be approximately a third higher nominal output when choosing
from the manufacturer datasheet. The effect of doing this is to run
the boiler at a lower temperature overall for more of the heating
season.
Where a heatbank is used for water heating, normally the objective is
to maintain it at as high a temperature as is reasonably practicable -
i.e. 82 degrees. THe implication is that the return would be at
around 60 degrees and you are out of condensing range.
This is not really a problem for several reasons:
- The cylinder will cool as HW is run with cool water replacing hot at
the bottom. The plate heat exchangers used are able to trasnfer heat
very efficently to the cold water (100-200kW equivalent is typical),
so quite a drop in temperature in water returned to the cylinder.
- The point at which the boiler fires can be arranged to be when the
water is quite a bit cooler such that in any case there is a low
return temperature to the boiler. It is only going to reach the 62
degree point relatively late in the reheat cycle if adjusted
correctly.
There is not really a lot of point, therefore, in striving to achieve
55 return by reducing the flow to 75 max. because also you will lose
heat storage capacity in the heatbank
--
.andy
Doctor Drivel
"legin" <ni...@anon987654.fslife.co.uk> wrote in message
news:1141928466....@i39g2000cwa.googlegroups.com...
>I went to the recent Homebuilding show at the Nec. The reason was that
> having googled and followed recent threads I was thinking that the best
> probable solution would indeed be to fit a heatbank in my self build. I
> went with the specific question in mind. How do you keep the
> condednsing boiler condensing if a temperature of circa 75 degrees is
> required for the heatbank. I even suggested mixing the return from the
> heatbank with the return from the underfloor heating to get the return
> temp down enough to get lull the boiler into condensing mode.
A plate heat exchanger is "highly" efficient. It can take in 80C primary
water from the store and have 25C coming out of the plate, transferring all
the heat to the incoming cold mains. This 25C water is pumped directly into
the bottom of the store, where the return pipe to the condensing boiler is.
So, very cool water at the bottom. Water baffles prevent de-stratification.
Look at:
http://www.heatweb.com/products/cylinders/gxv/operation/gxoperation.html
Ignore the solar and gravity circulation. Dedicated UFH heat banks have a
cooler lower section for UFH. Look at the return from the plate heat ex, it
goes into the bottom of the store. Also the UFH blend cool water the bottom
with warmer water a bout 1/4 to 1/3 of the way up. The boiler return is
right at the bottom where all the cool water returns promoting condensing.
A store may be 75C at the top, but stratification will ensure that the lower
section may be around 25 to 30C. All looking good. With two cylinder
stats to eliminate inefficient boiler cycling, the boiler is only called in
to reheat the whole store with one long efficient burn. The return water is
a very low temperature and promotes condensing efficiency. The ignorant
salesmen who spoke to you think, oh run temp of 75C, dew-point 54C, so no
condensing. Total tripe. You need to understand the operation. Look at
http://www.sedbuk.com Look at the ACV HeatMaster, a combined boiler/thermal
store. It is one of the most efficient boilers there.
"Direct" boiler heated heat banks (using a plate) are very efficient.
Thermal stores use an inefficient (to the plate) coil for DHW take off.
These do not promote a lower return temp as much.
> Apart from some exhibitors being unsure
> of how a system could be piped, the
> overall conclusion was to fit an unvented cylinder.
That is tripe.
http://www.waterheaterblast.com
And an annual service charge for one too. Say it lasts 20 years that is
£1600 minimum just to service it at today's rates; that is to have a
cylinder of hot water standing there. In reality it will be much, much,
more. At that price you could afford to replace a thermal store every 10
years and still ahead of the game. Many stores are guaranteed 20 years for
the cylinder anyway.
> This would take
> better advantage of a condensing boiler.
It would not. Most unvented cylinders do not have a quick recovery coil.
Better than Part L, not quick recovery.
Doctor Drivel
"Andy Hall" <an...@hall.nospam> wrote in message
news:ruu0125deuect2fa2...@4ax.com...
Wrong. Modern banks are 75C
> THe implication is that the return would be at
> around 60 degrees and you are out of condensing
> range.
Wrong. See my explanation.
> This is not really a problem for several reasons:
>
> - The cylinder will cool as HW is run with cool water replacing hot at
> the bottom. The plate heat exchangers used are able to trasnfer heat
> very efficently to the cold water (100-200kW equivalent is typical),
> so quite a drop in temperature in water returned to the cylinder.
>
> - The point at which the boiler fires can be arranged to be when the
> water is quite a bit cooler such that in any case there is a low
> return temperature to the boiler. It is only going to reach the 62
> degree point relatively late in the reheat cycle if adjusted
> correctly.
>
> There is not really a lot of point, therefore, in striving to achieve
> 55 return by reducing the flow to 75 max. because also you will lose
> heat storage capacity in the heatbank
You size to suit.
David Hansen
McArdle" <cmcar...@nospam.yahooxxxx.co.uk> wrote this:-
>At the risk of agreeing with Drivel, a combined feed and expansion pipe also
>handles the venting. There is only one 22mm pipe from the system to the
>loft. This pipe handles feeding of the system with water, expansion from the
>system into the header tank and venting of air or steam to the loft space.
>It splits into 2 at the bottom of the tank and a vent only pipe then
>continues above the tank.
You appear to be describing a particular system. If so then it is
far more dangerous than one with separate feed & expansion and vent
pipes. Unless, of course, other precautions are taken that are
roughly equivalent.
Doctor Drivel
> On Thu, 9 Mar 2006 17:00:52 -0000 someone who may be "Christian
> McArdle" <cmcar...@nospam.yahooxxxx.co.uk> wrote this:-
>
>>At the risk of agreeing with Drivel, a combined feed and expansion pipe
>>also
>>handles the venting. There is only one 22mm pipe from the system to the
>>loft. This pipe handles feeding of the system with water, expansion from
>>the
>>system into the header tank and venting of air or steam to the loft space.
>>It splits into 2 at the bottom of the tank and a vent only pipe then
>>continues above the tank.
>
> You appear to be describing a particular system. If so then it is
> far more dangerous than one with separate feed & expansion and vent
> pipes. Unless, of course, other precautions are taken that are
> roughly equivalent.
It is not more dangerous at all, as long as the boiler is sealed system
compatible. The problem is having on pipe over the F&E tank, teeing off
this and entering the F&E tank at the bottom is that air can be drawn on the
vent section. The vent section is totally surplus to requirements, as air
will vent via the pipe to the bottom of the F&E tank.
A problem with a 22mm combined feed and expansion is that the water in the
pipe/F&E tank can move up and down. If the tank is full of fresh water then
scaling may occur at the tee causing a blockage which can be dangerous. One
of the reasons the tee and pipe should be 22mm.
Because of this some makers stipulate that the 22mm combined feed and
expansion should be minimum length to minimise fresh water scaling at the
tee. Others stipulate a 3 port air ejector, which is a largish cylinder
about 1.5 inches in dia, so reduces the risk. Many engineers don't like air
ejectors as they are a point that could collect sludge, again causing a
blockage. The fitting of a Magnaclean filter solves that problem.
Many boiler are now being fitted in lofts. The problem with this is getting
the pressure relief valve to the outside (this could be ugly), so open
vented boilers are used. This creates a situation of low head. Combined feed
and expansion pipes solves this. Baxi say 200mm from top of boiler to
underside of F&E tank, using an air ejector and one 22mm pipe. It works
brilliantly. One problem with a combined feed and expansion is that they
are at times a pig to fill up, with air locks about the place. In truth it
is usually because no auto air vents are used at key points. The ejector
makes sure easy filling and no air problems.
Christian McArdle
> temperature of circa 75 degrees is required for the
> heatbank.
It is the return temperature that determines condensing efficiency, not the
flow temperature. The temperature at the bottom of the heatbank, provided
the thermostatic control is good, should actually be quite low.
Christian.
legin
>flow temperature. The temperature at the bottom of the heatbank, provided
>the thermostatic control is good, should actually be quite low.
>Christian.
If that is the case, what will be the flow temp. Assuming that the
boiler will lift the temp say 20 degrees. If the return temp is say 45
then the flow temp will be 65. Going through a heat bank previously
heated to say 75, then it will have a cooling effect on the heatbank.
Thanks to Andy for the detailed response. I understand what you are
saying about the knee in the curve. However at this stage it would be
nice to get it right and have the most efficient system. That to me
means making sure that the condensing boiler actually condenses. The
heat bank technology is completely new to me. Is there not a boiler out
there that could supply heating to the heatbank at say 75 degrees until
it is totally satisfied then reduce its temp down to say 65, to ensure
that the return is well within the condensing range. I know that this
would mean using a S plan to supply the heatbank and not actually use
it to feed the rad circuits and underfloor circuits. Or could it be
plumbed as a heatbank supplying the rad/ ufh circuits, with a low
return as Christian suggests. Would the boiler then be capable of
lifting the temp say 25 degrees. Also dare i say thanks to drivel. I
know his ramblings have been the source of a lot of banter, but I have
got to admit that it does amuse me. More importantly he has raised my
awareness of heatbanks to the point where I am now seriously
considering using one.
Legin
Andy Hall
wrote:
Yes.
I have the MAN (now MHG) Micromat EC and it will do precisely that.
http://www.mhg.de/en/products/gas_units/micromat_ec.html
In the UK, this unit is sold by Eco Hometec and targetted (among other
places) for the homebuild market. MHS boilers sell it as the Strata
1 more for commercial applications where several can be clustered for
more output.
http://www.mhsboilers.com/boilers/premix/strata1.htm
http://www.eco-hometec.co.uk/EC30.htm
I mention the MHS site because it has much better brochures and
technical manuals and the product is the same.
This boiler has a wide modulating range - down to 3kW on some models -
and a very good build quality. Servicability is also good in that all
major components push fit and clip onto the back plate and can be
swapped very quickly.
The internal controller has
- Weather compensation. A sensor located outside provides the
temperature and the boiler will adjust output as that changes. For
UFH this can be helpful because heat output from the floor can't be
quickly adjusted. Because of the insulating effect of the house an
inside sensor knows about outside temperature change much later than
if it is measured directly
- Analogue sensing of HW cylinder/store temperature. There is a
sensor which fits into a pocket in the cylinder which places it in the
middle rather than the surface. The boiler knows the store
temperature and a reheat cycle can be triggered at a programmable
temperature.
- Optionally, analogue sensing of room temperature. There is a
programmer made by Siemens for this boiler which gives it the actual
room temperature to use rather than just a controller telling the
boiler to switch on and off. This provides better results when
combined with the weather compensator.
- Analogue control of the pump. Water flow is optimised to match the
boiler output and thus the temperature difference across the heat
exchanger is optimised.
- Control of motorised valves. There is an option on the boiler to
have an internal diverter valve, but a better solution is to use
either an external diverter valve or zone valves (S plan). There
are installer settings on the controller which allow the boiler to
control either scenario directly - i.e. you don't need an external
controller.
The controller has about 40 different combinations of operational
settings for different max flow temperatures, valve operations,
external controls. You can set the weather compensation curve,
which basically means the weather compensated flow temperatures at 20
and -1 degrees outside.
Max. flow temperatures in CH mode can be set for systems with radiator
outputs designed for conventional boilers (85), condensing (70) and
UFH (55). My system had a conventional boiler originally and I
changed radiators where needed to give enough output at 70.
If you want to run radiators and UFH, then the normal way is to run
either a separate zone or one derived from the main CH with a separate
pump and mixing valve which will blend some of the UFH return water
with flow at 70 degrees to provide a low flow temperature for the UFH.
In CH operation, I find that the boiler will modulate down to a flow
temperature of about 40 degrees or so when the weather is relatively
warm and will sit all day doing that. In colder weather, it's rare
that it goes over 60 degrees. There can be as much as a 25 degree
difference across the heat exchanger. The pump speed drops to as low
as 35% of max output.
HW reheat operation depends on one of two things. The cylinder
temperature has dropped below the low set point (in my case 55 degrees
and a storage temperature of 60 degrees) - it would probably be 85 and
80 or possibly 85 and 75 for a store depending on size. If the boiler
detects the cylinder temperature dropping rapidly - i.e you just
started running a bath and a shower, then it will start much earlier.
This prevents regular reheats when small amounts of water are being
used, but begins reheat earlier when there are large amounts.
The boiler controller switches over the zone valves and winds the
boiler and pump up to full power. The return temperature is low
until the last couple of minutes of the cycle, whereupon the power
level and pump speeds are gradually reduced. This prevents the
cylinder temperature from overshooting and maintains the boiler return
temperature as low as possible.
In practice all of this works very well. However the boiler does
cost around 60-100% more than other good quality boilers.
--
.andy
Doctor Drivel
Please use paragraphs :( The great thing about a heat bank is that a
simpler, cheaper, less complicated boiler can be fitted. The way the
cylinder is arranged ensures stratification has low temperatures at the
bottom of the cylinder. The 75C is only at the top section of the cylinder,
for DHW purposes. The lower section will always be much lower temperature
than this. There are baffles in the cylinder to prevent water mixing. The
plate heat exchanger can ensure that water fed back into the bottom of the
heat bank is around 25C. You could insist on a double pass plate heat
exchanger be fitted which would ensure this sort of return temperature and
high flowrates in DHW on full load.
Another point about a heat banks is that a smaller boiler can be used.
Boilers operating directly on heating systems are sized for "peak" use. On
heat banks/thermal stores only "average" use is required. The thermal
storage fills the peaks. But also when fitting a boiler "directly" to a
heat bank, not via a coil, a "very large" boiler can be fitted too. This
will give pronto DHW recovery. So you have the best of both to choose from.
Most boilers modulate their output these days, and the most cost effective
boilers are in the 25 - 30kW range. So getting the largest you can at a
good price is worth it as the heat bank will not complain and still no
cycling.
I personally would consider a Glow HXi30, 30kW, boiler mated to a heat bank.
It is a Vaillant underneath, stainless heat exchanger, cheap enough, quiet
and very, very good. They do have different sizes and the boiler keeps a
constant output modulating along the way. Going over in kW size is not a
problem
http://www.uselessenergy.org.uk/boilers_prices.asp
Do not use S plans and the likes. Take everything off the heat bank using
pumps which don't restrict flow. It is a wonderful central neutral spot.
Insist on "two" cylinder anti-cycle stats on the heat bank. This prevents
anti-cycling. When heating, the store is heated to 75C, then it cools to
60C before the boiler cuts in. So no on-off, on-off of the boiler on the
one cylinder stat. The stats are placed so that the bottom of the store
still remains very, very cool. Look at the DPS diagrams
http://www.heatweb.com
So-called professional people who say a condensing boiler will not condense
when mated to a thermal store/heat bank are just plainly thick.
Christian McArdle
> boiler will lift the temp say 20 degrees. If the return temp is say 45
> then the flow temp will be 65. Going through a heat bank previously
> heated to say 75, then it will have a cooling effect on the heatbank.
Nope. If you take water out at 25C and return it at 45C, then it will heat
the heatbank, even if most of it was previously at 75C and the hot water is
deposited at the top.
> Is there not a boiler out there that could supply heating to the
> heatbank at say 75 degrees until it is totally satisfied then
> reduce its temp down to say 65, to ensure that the return is
> well within the condensing range.
Worcester Bosch Greenstar System with the optional diverter valve allows
separate setting of water and CH temperatures. You can further subzone the
heating side with S-Plan if you have multiple zones. I have the boiler
without the diverter and was only aware of it from reading the instructions.
I seriously wish I had bought the option, as it would have enabled me to run
the heating at a much lower temp.
Christian.
Doctor Drivel
The W-Bosch setup. Do they extra controls for DHW and CH and it diverts to
suit? I can't remember now.
IIRC, the W-B maintains a constant flow temp and modulates to maintain that.
You can install your own diverter valve and fit a weather compensator. It
will switch in and out the boiler to maintain the compensator setpoint,
which will be low most of the time. Compensators have anti-cycle control on
them.
Christian McArdle
to
> suit? I can't remember now.
It's a diverter, rather than a 3 way. DHW priority and separate flow temp
controls. The DHW side is limited to 75C. The CH side to 85C. I would have
preferred it the other way round as I have indirect heat bank heating and
would have liked 82-85C for the hot water and 70C for the heating, but it
should be possible to swap the sides (possibly with a relay to invert the
priority).
> IIRC, the W-B maintains a constant flow temp and modulates to maintain
that.
That is my understanding and why I think it would be possible to run the hot
water from the CH side.
> You can install your own diverter valve and fit a weather compensator. It
> will switch in and out the boiler to maintain the compensator setpoint,
> which will be low most of the time. Compensators have anti-cycle control
on
> them.
How would you get it to go hotter for the water with an external diverter?
Christian.
Doctor Drivel
>> The W-Bosch setup. Do they do
>> extra controls for DHW and CH and it diverts
>> to suit? I can't remember now.
>
> It's a diverter, rather than a 3 way.
> DHW priority and separate flow temp
> controls.
Is this two separate knobs on teh panel: DHW and CH temps?
> The DHW side is limited to 75C.
> The CH side to 85C. I would have
> preferred it the other way round as
> I have indirect heat bank heating and
> would have liked 82-85C for the hot
> water and 70C for the heating,
Sounds Greek to me. Oh being PR here is that racist?
> but it should be possible to
> swap the sides (possibly with a
> relay to invert the priority).
See the tech dept, they will proabbly tell you how to reverse
>> IIRC, the W-B maintains a constant
>> flow temp and modulates to maintain
>> that.
>
> That is my understanding and why I
> think it would be possible to run the hot
> water from the CH side.
>
>> You can install your own diverter valve
>> and fit a weather compensator. It
>> will switch in and out the boiler to maintain
>> the compensator setpoint,
>> which will be low most of the time.
>> Compensators have anti-cycle control
>> on them.
>
> How would you get it to go hotter for
> the water with an external diverter?
Set the boiler to maximum temp. When DHW calls, the compensator is switched
out via a relay. When DHW is satisfied back onto compensator control. The
compensator switches the burner (could be in the room stat circuit), the
relay just takes this in and out to what flow temp it wants to maintain.
The boiler should have anti-cycle control, so no constant click-click, of
the burner, and the compensator should have anti-cycle control too. But it
may not modulate the burner as the setpoint is maximum on the boiler, and
may only modulate to what that is set to.
Some boilers have all this built-in. The MAN/Eco-Hometec, Viessmann ranges
do. But these are serious money, but with quality to suit. It would be
cheaper, and simpler, to have two cheaper quality boilers, such as the Glow
Worms (Vaillants), which are small in physical size too. One for DHW only
set to max temperature, and one for CH only on a weather compensator. No
3-way valves. Having a few isolation valves could bring one in to do both if
one is down, so backup if needed.
Christian McArdle
Yes. The combi and system both have the same panel with DHW and CH temp
controls. In normal use, the system boiler DHW temp knob has no function
unless the diverter is specified. The new range may be different, I have no
idea. The system boiler still needs the DHW temp control even without the
diverter, as it is used by the software to control the installer menus.
In actual fact, both DHW and CH are normally limited to 75C. To get more on
the CH side, you have to remove a stop on the knob that physically prevents
you turning it up further. Unfortunately, no such stop exists to remove on
the DHW side. Presumably they didn't conceive of heat banks and TMVs in the
design.
Christian.
Doctor Drivel
>> Is this two separate knobs on the
>> panel: DHW and CH temps?
>
> Yes. The combi and system both
> have the same panel with DHW
> and CH temp controls. In normal
> use, the system boiler DHW temp
> knob has no function unless the
> diverter is specified. The new range
> may be different, I have no idea.
> The system boiler still needs the
> DHW temp control even without the
> diverter, as it is used by the software
> to control the installer menus.
On the heating boiler there is no DHW know at all, so I assume this cannot
have this three way valve.
> In actual fact, both DHW and CH
> are normally limited to 75C. To get
> more on the CH side, you have to
> remove a stop on the knob that
> physically prevents you turning it
> up further. Unfortunately, no such
> stop exists to remove on
> the DHW side. Presumably they
> didn't conceive of heat banks and
> TMVs in the design.
Does the DHW mode modulate? The boiler doesn't really care if water is sent
to CH or DHW. Changing over the CH and DHW pots so that the DHW markings
turn the CH and reversing the ports on the 3-way valve should get around the
low temp settings on the DHW.
Is this 3-ways valve a kit to add on? Or factory fitted?
legin
>the heatbank, even if most of it was previously at 75C and the hot water is
>deposited at the top.
Now I am totally confused! Assuming that we are starting from cold, UFH
is on and as you have stated the return temp is at 25C. Boiler lifts
the temp by 20C so we now have a flow at 45C. How can that possibly
heat the bank to any higher than 45C.
Lets assume no UFH and we are calling for hot water. The heat bank is
heated to say 75C. UFH now calling; If water is subsequently passed
through this at 45C how can it still heat the bank. My initial
assumption is that it would draw heat from the bank. If not then how
does a thermal store actually heat the hot water as it passes through
its coil?
Thanks
Legin
Doctor Drivel
> >Nope. If you take water out at 25C and return it at 45C, then it will
> >heat
>>the heatbank, even if most of it was previously at 75C and the hot water
>>is
>>deposited at the top.
>
> Now I am totally confused! Assuming that we are starting from cold, UFH
> is on and as you have stated the return temp is at 25C. Boiler lifts
> the temp by 20C so we now have a flow at 45C. How can that possibly
> heat the bank to any higher than 45C.
The water requires another pass through the boiler to gain take it to 65C
You could put two boilers in series so the first raise to 45C and then input
to the second boiler which then raise it again. Realistically, the boiler
will raise about 35C, so nearly two passes of heat bank water through the
boiler.
> Lets assume no UFH and we are calling for
> hot water.
So 75C water passing thought the plate heat exchanger and cooling the bottom
of the cylinder.
> The heat bank is heated to
> say 75C. UFH now calling;
So water out the top at 75C for DHW and water out the bottom UFH section at
about 50C - 60C.
> If water is subsequently passed
> through this at 45C
> how can it still heat the bank.
I don't know what you mean. I think you mean that the return water (water
in the bottom of the heat bank) is about 25C. The boiler only comes in when
the store of water cools significantly to reheat in one long efficient burn.
The store temp would be around 60C at the top before the boiler comes in.
So, water enters the boiler at say 25C the boiler raises it 35C, that is 60C
entering the top of the store, then as the store water is passed through the
boiler it re-heats. A tall thin store may mean the top temp never gets
below 65C, and most of the store water is much less than this. But if you
want 75C entering the top of the store you fit a blending valve on the
boilers flow and return pipe. This ensures the store water passes only once
through the boiler, and only water of 75C enters the top of the store.
See:
<http://www.heatweb.com/products/cylinders/gxv/operation/gxoperation.html>
On the boiler return there is a blending valve.
> My initial assumption is that it would
> draw heat from the bank. If not then how
> does a thermal store actually heat the
> hot water as it passes through its coil?
See above.
Doctor Drivel
>> If that is the case, what will be the flow temp. Assuming that the
>> boiler will lift the temp say 20 degrees. If the return temp is say 45
>> then the flow temp will be 65. Going through a heat bank previously
>> heated to say 75, then it will have a cooling effect on the heatbank.
>
> Nope. If you take water out at 25C and return it at 45C, then it will heat
> the heatbank, even if most of it was previously at 75C and the hot water
> is
> deposited at the top.
>
>> Is there not a boiler out there that could supply heating to the
>> heatbank at say 75 degrees until it is totally satisfied then
>> reduce its temp down to say 65, to ensure that the return is
>> well within the condensing range.
>
> Worcester Bosch Greenstar System with the optional diverter valve allows
> separate setting of water and CH temperatures.
I checked this out. It cannot operate at different temperatures on CH and
DHW. The tech man on the phone was a bit slow with poor product knowledge,
but he said the boiler can only give out one temperature on CH and DHW with
the diverting valves fitted, which can only be fitted on system boilers.
It can be done externally, but the modulation would not work that well, as
you are just switching in and out the boiler to weather compensator or pipe
stat. You can brake into the control circuitry, if you know what you are
doing, and ad another temp pot, but that is very much another thing and the
guarantee would be void.