Combi v HW Tank

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www.GymRats.uk

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Dec 2, 2021, 9:41:31 AM12/2/21
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Trying to decide as and when our boilers finally die (both are at the
"care-home" stage of their lives) whether to try and find space for a
pressurised HW tank rather than stick with the inherent fragility of
combi's.

Obviously the downside of stored HW is reduced flue heat recovery from
condensing (that can't make much of a difference though can it?) but I
feel the advantages outweigh the disadvantages.

Adding the potential of a solar-thermal HW in the summer and looking at
my mothers "Ideal" system boiler that is only the second boiler ever
fitted to the house in 55 years has me thinking.

And... depending on whatever other "green agenda" nonsense is forced
upon us it may well be more suitable for utilising solar p.v. if my
backwards-running electricity meter ever has to be replaced.

Any pros or cons I've missed?

Cheers - Pete

Dave Plowman (News)

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Dec 2, 2021, 10:02:51 AM12/2/21
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In article <soalum$j1l$1...@dont-email.me>,
I stuck with a cold water header tank and stored hot water when I changed
boilers not that long ago. Means things can go on pretty well as normal
for a day or so if the water is cut off. Much longer if I was careful. And
like my high flow shower, and the ability to fill a bath quickly.

But your choice if you could put the room these things take up to better
use.

--
*Even a blind pig stumbles across an acorn now and again *

Dave Plowman da...@davenoise.co.uk London SW
To e-mail, change noise into sound.

Fredxx

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Dec 2, 2021, 10:05:48 AM12/2/21
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On 02/12/2021 14:41, www.GymRats.uk wrote:
> Trying to decide as and when our boilers finally die (both are at the
> "care-home" stage of their lives) whether to try and find space for a
> pressurised HW tank rather than stick with the inherent fragility of
> combi's.
>
> Obviously the downside of stored HW is reduced flue heat recovery from
> condensing (that can't make much of a difference though can it?) but I
> feel the advantages outweigh the disadvantages.

I would say time to getting hot water should also be factored in with a
combi. They can waste an awful lot of water if used constantly.

A HW tank loses an awful lot of heat in the day. So if HW is drawn only
occasionally a combi wins hands down. Also if space is at a premium.

> Adding the potential of a solar-thermal HW in the summer and looking at
> my mothers "Ideal" system boiler that is only the second boiler ever
> fitted to the house in 55 years has me thinking.
>
> And... depending on whatever other "green agenda" nonsense is forced
> upon us it may well be more suitable for utilising solar p.v. if my
> backwards-running electricity meter ever has to be replaced.
>
> Any pros or cons I've missed?

It's not uncommon that any excess electrical power from solar is used to
(pre)heat HW.

John Rumm

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Dec 2, 2021, 10:17:38 AM12/2/21
to
On 02/12/2021 14:41, www.GymRats.uk wrote:

> Trying to decide as and when our boilers finally die (both are at the
> "care-home" stage of their lives) whether to try and find space for a
> pressurised HW tank rather than stick with the inherent fragility of
> combi's.

System boilers these days share much of the design and parts list of a
combi - just lack a diversion valve and secondary plate heat exchanger.

> Obviously the downside of stored HW is reduced flue heat recovery from
> condensing (that can't make much of a difference though can it?) but I
> feel the advantages outweigh the disadvantages.

With a modern fast recovery cylinder (which all unvented ones will be)
they can swallow the full output of a modest sized boiler. (my system
boiler is 24kW, and the cylinder coil will take up to 22kW). So the
boiler will be in condensing mode for much of the reheat.

> Adding the potential of a solar-thermal HW in the summer and looking at
> my mothers "Ideal" system boiler that is only the second boiler ever
> fitted to the house in 55 years has me thinking.

Yup additional coils can be specced for a cylinder for solar thermal
input. (and immersion heaters can be used to mop up solar PV).

About the only heat source you can't easily integrate with an unvented
cylinder is solid fuel.

> And... depending on whatever other "green agenda" nonsense is forced
> upon us it may well be more suitable for utilising solar p.v. if my
> backwards-running electricity meter ever has to be replaced.
>
> Any pros or cons I've missed?

Chapter and verse here:

http://wiki.diyfaq.org.uk/index.php/Unvented_DHW

Specifically when compared to a combi (of the "normal" type rather than
ones with a large internal store of water):

You get high pressure water to all taps from both.
You get higher flow rate from the cylinder (mains supply being good
enough), and that can also be at a higher temp to allow mixing down to
final use temp.
Neither need header tanks

Both need reliable cold mains supply.

Combi will be simpler and cheaper and take less space. Heating
performance ought to be similar. Hot water performance will be
significantly poorer, but can still be adequate for most users if the
boiler is sized large enough.

System boilers at the posh end of the market can do more advanced tricks
somewhat more easily like use weather compensation or multiple heating
zones.

Unvented hot water cylinders need servicing, but that is an easily
DIYable job (check strainer is clear, check emergency valves operate and
reseal, check pressure in expansion vessel).


--
Cheers,

John.

/=================================================================\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\=================================================================/

Fredxx

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Dec 2, 2021, 11:14:45 AM12/2/21
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On 02/12/2021 15:17, John Rumm wrote:
> On 02/12/2021 14:41, www.GymRats.uk wrote:
>
>> Trying to decide as and when our boilers finally die (both are at the
>> "care-home" stage of their lives) whether to try and find space for a
>> pressurised HW tank rather than stick with the inherent fragility of
>> combi's.
>
> System boilers these days share much of the design and parts list of a
> combi - just lack a diversion valve and secondary plate heat exchanger.
>
>> Obviously the downside of stored HW is reduced flue heat recovery from
>> condensing (that can't make much of a difference though can it?) but I
>> feel the advantages outweigh the disadvantages.
>
> With a modern fast recovery cylinder (which all unvented ones will be)
> they can swallow the full output of a modest sized boiler. (my system
> boiler is 24kW, and the cylinder coil will take up to 22kW). So the
> boiler will be in condensing mode for much of the reheat.

I've gone the other way, using a thermal store. That really swallows the
boiler output!

It has the advantage of being at low pressure and doesn't require servicing.

Robin

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Dec 2, 2021, 11:23:11 AM12/2/21
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On 02/12/2021 15:17, John Rumm wrote:
Is another possible factor now the ease of conversion to heat pump? (A
cylinder seems a plus for that but I've not looked at the likelihood of
that making much difference in the overall scale of the buggeration
involved.)

--
Robin
reply-to address is (intended to be) valid

John Rumm

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Dec 2, 2021, 11:50:32 AM12/2/21
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OOI how are you extracting the DHW from it, with a coil or an external
plate heat exchanger and a pump?

> It has the advantage of being at low pressure and doesn't require
> servicing.

Yup thermal store is another good way to go. Easy to get heat into it,
and can take solid fuel as well as heat from a heat pump, and can be
direct rather than indirect so full boiler output and condensing
efficiency as well. Slightly more complicated getting the heat out, but
has the advantage you could run rads of UFH from it as well if its big
enough.

I did look at doing one for here, but in the end it worked out a bit
more expensive than going unvented if buying a COTS solution, and more
hassle if going homebuilt.

NY

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Dec 2, 2021, 11:52:41 AM12/2/21
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"John Rumm" <see.my.s...@nowhere.null> wrote in message
news:WtidnWW9s6oQfjX8...@brightview.co.uk...
Having had various types of CH and HW systems in different houses I would
rate them (least preferable first, most preferable last)

- cylinder/tank-fed
- cylinder/mains-fed
- combi/mains-fed

I'm not sure whether combi/tank-fed is an option, but I'd put it
second-to-least-preferable if it did.

Combi has the advantage over cylinder that the supply of HW is endless: the
HW doesn't run cold so you have to wait a long time for the cylinder to heat
up before it can run warm again.

Mains-fed gives a *much* higher flow rate and is easier to balance hot and
cold in a shower, because any other tap running in the house will affect the
flow of both HW and CW equally.

Our present house *used* to have cylinder/tank-fed and the flow of hot water
was puny (so it took ages for the water to run warm given the long pipe runs
in our house) because some so-called qualified CH engineer had used 15 mm
rather than 22 mm for the low-pressure HW pipes. Normally with tank fed, the
HW pipes are larger diameter to allow a higher flow rate even though the
pressure is a lot lower.

When that cylinder started to leak and needed replacing, the CH engineer
looked at our boiler and said that based on its age it was due to be
replaced, and when he removed it he confirmed this: the heat exchanger was
badly corroded.

We had the boiler replaced with a combi, the header-tank decommissioned
(it's too big to get out through the loft hatch) and the cylinder removed
and the pipework replumbed for combi heat-on-demand.

Taps run warm a lot quicker. The length of pipe runs to the kitchen could
have been shortened a bit (reducing the run-warm time even more) by not
feeding them via where the cylinder had been, but we decided not to pay this
extra expense. The shower no longer goes scalding hot every time the washing
machine needs more water etc.


The only downside was a trivial one: one of our showers had an ancient
copper head like you get on camp sites, in school dormitories etc. And when
the engineer tested that shower after finishing the work, the extra
pressure/flow blew the perforated disc off! No great problem. It looked naff
and we were planning to replace it anyway, so I chose a head that was rated
for our flow rate.

Tim+

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Dec 2, 2021, 1:06:44 PM12/2/21
to
NY <m...@privacy.invalid> wrote:

>
> Having had various types of CH and HW systems in different houses I would
> rate them (least preferable first, most preferable last)
>
> - cylinder/tank-fed
> - cylinder/mains-fed
> - combi/mains-fed
>

Whilst acknowledging that combis have some advantages, there are few things
more miserable than a house with no heating *and* no hot water.

For that reason alone, I won’t have the “eggs all in one basket “ approach
of combis in my house.

Tim

--
Please don't feed the trolls

John Rumm

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Dec 2, 2021, 1:41:31 PM12/2/21
to
On 02/12/2021 18:06, Tim+ wrote:
> NY <m...@privacy.invalid> wrote:
>
>>
>> Having had various types of CH and HW systems in different houses I would
>> rate them (least preferable first, most preferable last)
>>
>> - cylinder/tank-fed
>> - cylinder/mains-fed
>> - combi/mains-fed

My own experience is that the unvented cylinder trounces the combi in
every way... Massively faster delivery of water, at higher temperatures,
and people can have long showers without turning off the house heating.

One option less commonly considered is using a combi *with* a
conventional cylinder - main fed hot water to showers, and perhaps
kitchen taps, and then cylinder for rapid bath fills.

> Whilst acknowledging that combis have some advantages, there are few things
> more miserable than a house with no heating *and* no hot water.
>
> For that reason alone, I won’t have the “eggs all in one basket “ approach
> of combis in my house.

You could always go for the Dr Drivel solution, two combis!

(other backup solutions exist like an inline electric hot water heater)

NY

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Dec 2, 2021, 1:44:37 PM12/2/21
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"Tim+" <tim.d...@gmail.com> wrote in message
news:185198214.660161022.242...@news.individual.net...
Fair enough. I can see your point there.

But if I had a cylinder, I'd make sure it was mains-fed unless the house had
several storeys and hence a large head of water. One storey (tank in loft,
cylinder/bath/shower on floor below) is not enough.

My very first house had a combined header tank and cylinder in the airing
cupboard alongside the bathroom. The head of water was about 3 feet. The
flow rate of hot water, even with the regulation 22 mm pipes for HW, was
dire. It was also heated *only* by immersion heaters: one element on Economy
7 heated the whole tank, the other one half-way down heated just the top
half of the tank and could be switched on manually at any time. The bottom
element blew up at 1 AM when E-7 turned it on, and I had to manhandle the
whole *full* cylinder of water on its support blocks through 180 degrees to
get at the drain valve which was on the *rear* of the cylinder. I'd drained
the header tank but you can never get the water out of the cylinder via the
hot taps. Once I'd got access to the drain cock it was easy enough to drain
the cylinder through a hosepipe.


We are lucky in our present house that we have a gas-fire Aga and a
wood/peat-burning stove as well as the gas-fired CH/HW, so if the power went
off for 6 days (and counting) as some people have suffered due to the recent
storm, we'd have the ability to keep at least the kitchen and lounge warm,
so we'd need to move mattresses into the lounge to sleep there.

It's one thing that worries me about the demonisation of combustible fuel
(gas, oil, solid) for CH/HW: the thought that you will be utterly dependent
on your electricity supply for the basics of heating and hot water, with no
back-up if it fails for a long period of time.

Michael Chare

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Dec 2, 2021, 3:06:10 PM12/2/21
to
On 02/12/2021 15:05, Fredxx wrote:
> On 02/12/2021 14:41, www.GymRats.uk wrote:
>> Trying to decide as and when our boilers finally die (both are at the
>> "care-home" stage of their lives) whether to try and find space for a
>> pressurised HW tank rather than stick with the inherent fragility of
>> combi's.
>>
>> Obviously the downside of stored HW is reduced flue heat recovery from
>> condensing (that can't make much of a difference though can it?) but I
>> feel the advantages outweigh the disadvantages.
>
> I would say time to getting hot water should also be factored in with a
> combi. They can waste an awful lot of water if used constantly.
>
> A HW tank loses an awful lot of heat in the day. So if HW is drawn only
> occasionally a combi wins hands down. Also if space is at a premium.
>

Mine has a 2" thick covering of foam so I expect it to retain heat. I
have an immersion heater controlled by a timer which I use to provide
hot water if the boilr is not in use for some reason.


newshound

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Dec 2, 2021, 3:49:26 PM12/2/21
to
On my last overhaul, I decided to stick with a vented cylinder and
replaced the Combi with a system boiler for simplicity and convenience
(slightly odd loft geometry in 1780's cottage). To get decent flow, I
already had a two impeller Stuart Turner for the shower and added a
single impeller Stuart Turner for the rest of the DHW. Just an observation.

Theo

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Dec 2, 2021, 4:41:17 PM12/2/21
to
Robin <r...@outlook.com> wrote:
> Is another possible factor now the ease of conversion to heat pump? (A
> cylinder seems a plus for that but I've not looked at the likelihood of
> that making much difference in the overall scale of the buggeration
> involved.)

You'd need a cylinder with a coil designed for a heat pump, due to the lower
flow temperature needing a larger coil. Not to say you couldn't get a heat
pump compatible cylinder and plumb it into a gas boiler if you wanted - not
a problem to have a higher flow temp.

Theo

John Rumm

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Dec 2, 2021, 6:46:27 PM12/2/21
to
On 02/12/2021 18:44, NY wrote:

> It's one thing that worries me about the demonisation of combustible
> fuel (gas, oil, solid) for CH/HW: the thought that you will be utterly
> dependent on your electricity supply for the basics of heating and hot
> water, with no back-up if it fails for a long period of time.

There are times we have had "all day" power cuts. I have a camping style
dual gas ring portable cooker + grill that runs from a cylinder of LPG
that is quite handy in those cases for making hot drinks at least.

Plus one real open fire and a good stash of logs.

RJH

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Dec 2, 2021, 11:28:27 PM12/2/21
to
On 2 Dec 2021 at 18:41:25 GMT, "John Rumm" <see.my.s...@nowhere.null>
wrote:

> On 02/12/2021 18:06, Tim+ wrote:
>> NY <m...@privacy.invalid> wrote:
>>
>>>
>>> Having had various types of CH and HW systems in different houses I would
>>> rate them (least preferable first, most preferable last)
>>>
>>> - cylinder/tank-fed
>>> - cylinder/mains-fed
>>> - combi/mains-fed
>
> My own experience is that the unvented cylinder trounces the combi in
> every way... Massively faster delivery of water, at higher temperatures,
> and people can have long showers without turning off the house heating.
>

I'd have thought it costs more to run the HW side of things with the heat
losses from the cylinder.

But in other respects I tend to agree that HW using a combi is a mixed
experience. Takes a good 20s for hot water to come through, although when it
does it's a decent amount and plenty hot enough. I don't use the preheat
option - it's pretty quick if I do. Multiple showers etc. are not going to
happen, which would be more of a problem for larger households. As it is, on
balance, a combi suits me better overall.

--
Cheers, Rob

The Natural Philosopher

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Dec 3, 2021, 4:41:26 AM12/3/21
to
On 03/12/2021 04:28, RJH wrote:
> On 2 Dec 2021 at 18:41:25 GMT, "John Rumm" <see.my.s...@nowhere.null>
> wrote:
>
>> On 02/12/2021 18:06, Tim+ wrote:
>>> NY <m...@privacy.invalid> wrote:
>>>
>>>>
>>>> Having had various types of CH and HW systems in different houses I would
>>>> rate them (least preferable first, most preferable last)
>>>>
>>>> - cylinder/tank-fed
>>>> - cylinder/mains-fed
>>>> - combi/mains-fed
>>
>> My own experience is that the unvented cylinder trounces the combi in
>> every way... Massively faster delivery of water, at higher temperatures,
>> and people can have long showers without turning off the house heating.
>>
>
> I'd have thought it costs more to run the HW side of things with the heat
> losses from the cylinder.

Since the cost of DHW is minuscule compared with heating a house, who
henstly cares?

>
> But in other respects I tend to agree that HW using a combi is a mixed
> experience. Takes a good 20s for hot water to come through, although when it
> does it's a decent amount and plenty hot enough. I don't use the preheat
> option - it's pretty quick if I do. Multiple showers etc. are not going to
> happen, which would be more of a problem for larger households. As it is, on
> balance, a combi suits me better overall.
>
Takes longer than that for hot water to get to some of my taps with a
pressurised tank. So there is always a slight waste, but would be just
the same with a combi. a pipe fill of cold water is a pipe full of cold
water.

At some level the size of combi needed to heat a decent water flow
exceeds the cost of a system boiler and pressurised tank.

I'd only contemplate a combi in a small 1 or two person house.

--
“It is hard to imagine a more stupid decision or more dangerous way of
making decisions than by putting those decisions in the hands of people
who pay no price for being wrong.”

Thomas Sowell

RJH

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Dec 3, 2021, 5:19:51 AM12/3/21
to
On 3 Dec 2021 at 09:41:22 GMT, "The Natural Philosopher" <t...@invalid.invalid>
wrote:

> On 03/12/2021 04:28, RJH wrote:
>> On 2 Dec 2021 at 18:41:25 GMT, "John Rumm" <see.my.s...@nowhere.null>
>> wrote:
>>
>>> On 02/12/2021 18:06, Tim+ wrote:
>>>> NY <m...@privacy.invalid> wrote:
>>>>
>>>>>
>>>>> Having had various types of CH and HW systems in different houses I would
>>>>> rate them (least preferable first, most preferable last)
>>>>>
>>>>> - cylinder/tank-fed
>>>>> - cylinder/mains-fed
>>>>> - combi/mains-fed
>>>
>>> My own experience is that the unvented cylinder trounces the combi in
>>> every way... Massively faster delivery of water, at higher temperatures,
>>> and people can have long showers without turning off the house heating.
>>>
>>
>> I'd have thought it costs more to run the HW side of things with the heat
>> losses from the cylinder.
>
> Since the cost of DHW is minuscule compared with heating a house, who
> henstly cares?
>

I like to avoid waste. So I care.

>>
>> But in other respects I tend to agree that HW using a combi is a mixed
>> experience. Takes a good 20s for hot water to come through, although when it
>> does it's a decent amount and plenty hot enough. I don't use the preheat
>> option - it's pretty quick if I do. Multiple showers etc. are not going to
>> happen, which would be more of a problem for larger households. As it is, on
>> balance, a combi suits me better overall.
>>
> Takes longer than that for hot water to get to some of my taps with a
> pressurised tank. So there is always a slight waste, but would be just
> the same with a combi. a pipe fill of cold water is a pipe full of cold
> water.
>

No, mine's much worse. The longest pipe run in my house is about 3m
(bath-boiler).

> At some level the size of combi needed to heat a decent water flow
> exceeds the cost of a system boiler and pressurised tank.
>
> I'd only contemplate a combi in a small 1 or two person house.

Which is what mine is most of the time. Hence why it suits me.

--
Cheers, Rob

The Natural Philosopher

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Dec 3, 2021, 5:53:54 AM12/3/21
to
On 03/12/2021 10:19, RJH wrote:
> On 3 Dec 2021 at 09:41:22 GMT, "The Natural Philosopher" <t...@invalid.invalid>
> wrote:
>
>> On 03/12/2021 04:28, RJH wrote:
>>> On 2 Dec 2021 at 18:41:25 GMT, "John Rumm" <see.my.s...@nowhere.null>
>>> wrote:
>>>
>>>> On 02/12/2021 18:06, Tim+ wrote:
>>>>> NY <m...@privacy.invalid> wrote:
>>>>>
>>>>>>
>>>>>> Having had various types of CH and HW systems in different houses I would
>>>>>> rate them (least preferable first, most preferable last)
>>>>>>
>>>>>> - cylinder/tank-fed
>>>>>> - cylinder/mains-fed
>>>>>> - combi/mains-fed
>>>>
>>>> My own experience is that the unvented cylinder trounces the combi in
>>>> every way... Massively faster delivery of water, at higher temperatures,
>>>> and people can have long showers without turning off the house heating.
>>>>
>>>
>>> I'd have thought it costs more to run the HW side of things with the heat
>>> losses from the cylinder.
>>
>> Since the cost of DHW is minuscule compared with heating a house, who
>> henstly cares?
>>
>
> I like to avoid waste. So I care.

Ah. Dont do Sums then?

Waste, to you, is a binary concept. I care about MASSIVE waste, like
renewable energy, long before I care about filling the kettle up beyond
halfway...

>
>>>
>>> But in other respects I tend to agree that HW using a combi is a mixed
>>> experience. Takes a good 20s for hot water to come through, although when it
>>> does it's a decent amount and plenty hot enough. I don't use the preheat
>>> option - it's pretty quick if I do. Multiple showers etc. are not going to
>>> happen, which would be more of a problem for larger households. As it is, on
>>> balance, a combi suits me better overall.
>>>
>> Takes longer than that for hot water to get to some of my taps with a
>> pressurised tank. So there is always a slight waste, but would be just
>> the same with a combi. a pipe fill of cold water is a pipe full of cold
>> water.
>>
>
> No, mine's much worse. The longest pipe run in my house is about 3m
> (bath-boiler).
>
>> At some level the size of combi needed to heat a decent water flow
>> exceeds the cost of a system boiler and pressurised tank.
>>
>> I'd only contemplate a combi in a small 1 or two person house.
>
> Which is what mine is most of the time. Hence why it suits me.
>
Its a substandard solution with a price that reflects that.

People who add heatbanks and pumps and so on would be better off simply
nuying a system boiler and a pressurised tank.

My cylinder hardly loses any heat at all. It an insulated unit and well
insulated at that.

--
“Some people like to travel by train because it combines the slowness of
a car with the cramped public exposure of 
an airplane.”

Dennis Miller

www.GymRats.uk

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Dec 3, 2021, 5:54:15 AM12/3/21
to
On 02/12/2021 20:06, Michael Chare wrote:

>> A HW tank loses an awful lot of heat in the day. So if HW is drawn
>> only occasionally a combi wins hands down. Also if space is at a premium.
>>
>
> Mine has a 2" thick covering of foam so I expect it to retain heat. I
> have an immersion heater controlled by a timer which I use to provide
> hot water if the boilr is not in use for some reason.

That's been the other issue with both Combi's when it breaks which both
have done numerous times of the last 15 to 20 years there is no
showering until it's fixed.
Fortunately one combi is in the flat above our shop and the other at
home so it wasn't quite as bad.
I still have the bathroom at home to put together as it's been gutted
since 2012 so was going to put an electric shower in the shower room and
the new bathroom fed from the boiler just in case, but a HW tank negates
this requirement.

Cheers
Pete

www.GymRats.uk

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Dec 3, 2021, 6:16:41 AM12/3/21
to
On 02/12/2021 15:17, John Rumm wrote:
> On 02/12/2021 14:41, www.GymRats.uk wrote:
>
>> Trying to decide as and when our boilers finally die (both are at the
>> "care-home" stage of their lives) whether to try and find space for a
>> pressurised HW tank rather than stick with the inherent fragility of
>> combi's.
>
> System boilers these days share much of the design and parts list of a
> combi - just lack a diversion valve and secondary plate heat exchanger.

The diversion valve and actuator on my German boiler at home was the
cause of many problems but so far since the updated version was fitted
it's not been a problem.

Plate H/Ex on the work boiler AND diverter valve have been problematic
but we don't need hotwater in the shop so it wasn't such a problem.


> With a modern fast recovery cylinder (which all unvented ones will be)
> they can swallow the full output of a modest sized boiler. (my system
> boiler is 24kW, and the cylinder coil will take up to 22kW). So the
> boiler will be in condensing mode for much of the reheat.

I wasn't aware of that. Thanks John.


> Yup additional coils can be specced for a cylinder for solar thermal
> input. (and immersion heaters can be used to mop up solar PV).
>
> About the only heat source you can't easily integrate with an unvented
> cylinder is solid fuel.

I've got this other slightly hare-braned scheme on the cards... using
the 4m x 8m space under the balcony as a thermal store. Initially the
idea was to duct hot air from a south facing solar furnace just to
create a semi warm "greenhouse/garden room" for delicate plants but then
went on to thinking of digging deeper and insulating etc to super heat
in summer and bleed off in winter... No doubt I'll never live long
enough to see that one through though. LOL


>> Any pros or cons I've missed?
>
> Chapter and verse here:
>
> http://wiki.diyfaq.org.uk/index.php/Unvented_DHW

Excellent. Thankyou

> Specifically when compared to a combi (of the "normal" type rather than
> ones with a large internal store of water):
>
> You get high pressure water to all taps from both.
> You get higher flow rate from the cylinder (mains supply being good
> enough), and that can also be at a higher temp to allow mixing down to
> final use temp.
> Neither need header tanks
>
> Both need reliable cold mains supply.

We're pretty good on that front.

> Combi will be simpler and cheaper and take less space. Heating
> performance ought to be similar. Hot water performance will be
> significantly poorer, but can still be adequate for most users if the
> boiler is sized large enough.
>
> System boilers at the posh end of the market can do more advanced tricks
> somewhat more easily like use weather compensation or multiple heating
> zones.

Combi at home is weather compensated which once I finally found the
perfect heating/temp. curve works really well so that's a must-have.


> Unvented hot water cylinders need servicing, but that is an easily
> DIYable job (check strainer is clear, check emergency valves operate and
> reseal, check pressure in expansion vessel).

Thanks John.
Greatly appreciated.

Cheers - Pete


Andrew

unread,
Dec 3, 2021, 6:27:36 AM12/3/21
to
But you do have a large tank of water at ?much more than 60C, so
is there a possible safety issue ?.

Andrew

unread,
Dec 3, 2021, 6:30:24 AM12/3/21
to
Then when you built your house you should have insulated all the
hot water pipe runs properly and not just with a roll of horsehair
lagging.

Andrew

unread,
Dec 3, 2021, 6:31:56 AM12/3/21
to
Unlike the pipes connecting it to your bath and sinks

Andrew

unread,
Dec 3, 2021, 6:37:03 AM12/3/21
to
On 02/12/2021 18:44, NY wrote:

> at any time. The bottom element blew up at 1 AM when E-7 turned it on,
> and I had to manhandle the whole *full* cylinder of water on its support
> blocks through 180 degrees to get at the drain valve which was on the
> *rear* of the cylinder. I'd drained the header tank but you can never
> get the water out of the cylinder via the hot taps. Once I'd got access
> to the drain cock it was easy enough to drain the cylinder through a
> hosepipe.
>

I don't see why the tank would not have drained out to the
point where the level in the tank matched the height of a
hot tap though.

If you had disconnected all the pipework to move it,
then good old-fashioned syphon it out with a bit of
hosepipe might have done the trick.

Unknown

unread,
Dec 3, 2021, 6:42:31 AM12/3/21
to
It happens that www.GymRats.uk formulated :
> Obviously the downside of stored HW is reduced flue heat recovery from
> condensing (that can't make much of a difference though can it?) but I
> feel the advantages outweigh the disadvantages.

I doubt there is much difference at that stage, but reheating a tank
from cold at the colder stages, will be much more condensing efficient.
Stored hot water means that if there is a boiler issue an immersion
heater can still be available to supply hot, if water goes off and you
have a cold water storage tank, you still have HW for a while.

A combi has to fire up everytime even a tiny amount of HW is needed, so
much more where and tear, plus less efficient. A combi has all the
failure prone bits inside it, so a specialist is needed for every
failure. The combi puts the system under slightly higher pressure so
more likelihood of leaks and the slightest leak will involve lots of
regular topping up.

We had an open vented system installed 40 years ago and now on our
third boiler. Each time replacement was due, they wanted to move us to
a combi, but I insisted we should remain open vented - which allows me
to easily fix most of the issues which arise. HW flow is also much
faster, than provided by most combi and a bath can be filled much
quicker with both hot and cold running at the same time.

Remember - an open vented system is much more expensive to install than
a combi, once the open vented is removed it will cost a lot of money to
reinstall.

The big advantage of a combi, is space saving - great for compact flats
and having instant HW water, rather than having to wait 20 minutes for
the cylinder to heat.

Unknown

unread,
Dec 3, 2021, 6:47:23 AM12/3/21
to
John Rumm explained :
> Unvented hot water cylinders need servicing, but that is an easily DIYable
> job (check strainer is clear, check emergency valves operate and reseal,
> check pressure in expansion vessel).

Isn't that the type which needs a G3 qualification?

www.GymRats.uk

unread,
Dec 3, 2021, 7:38:24 AM12/3/21
to
On 02/12/2021 16:14, Fredxx wrote:

>> With a modern fast recovery cylinder (which all unvented ones will be)
>> they can swallow the full output of a modest sized boiler. (my system
>> boiler is 24kW, and the cylinder coil will take up to 22kW). So the
>> boiler will be in condensing mode for much of the reheat.
>
> I've gone the other way, using a thermal store. That really swallows the
> boiler output!
>
> It has the advantage of being at low pressure and doesn't require
> servicing.

That was the other thought but I'd forgotten about that option.

Would be nice to get a wood burner in the lounge (also on the to-do
list) but plumbing would be tricky as the house is upsidedown and I have
UFH in all the upstairs rooms so no direct route downstairs to the
boiler area.

Cheers - Pete

www.GymRats.uk

unread,
Dec 3, 2021, 8:19:22 AM12/3/21
to
On 02/12/2021 18:41, John Rumm wrote:

>> For that reason alone, I won’t have the “eggs all in one basket “
>> approach
>> of combis in my house.
>
> You could always go for the Dr Drivel solution, two combis!

But they have to be Japanese boilers if I remember correctly...

In fairness to Dr.Evil I did go with his suggestion of boiler at home.
"EcoHometec" AKA M.A.N /Micromat which has been superb and to date
everything has been repairable, and the more I'm forced to repair the
simpler it becomes. So far the last repair of pin-holes in the back
plate patched up with repair putty is still holding up. That was the
first time I've taken the boiler off the wall but all went well.
:)

www.GymRats.uk

unread,
Dec 3, 2021, 8:32:31 AM12/3/21
to
On 03/12/2021 11:42, Harry Bloomfield wrote:
> It happens that www.GymRats.uk formulated :

> We had an open vented system installed 40 years ago and now on our third
> boiler. Each time replacement was due, they wanted to move us to a
> combi, but I insisted we should remain open vented - which allows me to
> easily fix most of the issues which arise. HW flow is also much faster,
> than provided by most combi and a bath can be filled much quicker with
> both hot and cold running at the same time.


Mums house built early 60's had a huge floor standing gas boiler,
gravity fed, hot water tank and header tanks in the airing cupboard. We
had to replace the indirect HW tank once (might have been late 70's)
when internals rotted out and the boiler was replaced about 25 years ago
with a little wall mounted thing. Never missed a beat. The HW tank was
a pain in the days before we had an electric shower though as it would
only do one bath and the next had to wait an hour or more to bath.
:)


Paul G

unread,
Dec 3, 2021, 10:29:34 AM12/3/21
to
On 02/12/2021 20:49, newshound wrote:

> On my last overhaul, I decided to stick with a vented cylinder and
> replaced the Combi with a system boiler for simplicity and convenience
> (slightly odd loft geometry in 1780's cottage). To get decent flow, I
> already had a two impeller Stuart Turner for the shower and added a
> single impeller Stuart Turner for the rest of the DHW. Just an observation.

I too have a Stuart Turner pump (hot & cold feeds) for our showers.
Unfortunately the pressure to our kitchen hot tap (downstairs) is poor
and I'm under spousal pressure to improve it. It could be simply at the
end of a long pipe run.

Are there any gotchas in adding another pump for the DHW ?

Dave Plowman (News)

unread,
Dec 3, 2021, 10:53:27 AM12/3/21
to
In article <socvr4$okb$1...@dont-email.me>,
Harry Bloomfield wrote:
> The big advantage of a combi, is space saving - great for compact flats
> and having instant HW water, rather than having to wait 20 minutes for
> the cylinder to heat.

I discovered with my system boiler and fast recovery cylinder - having
come back off holiday where it was switched off - that I got an acceptable
shower very shortly after switching it on. Might have not been in the
depths of winter, though, where the header and storage tanks would have
had colder water.

--
*If a parsley farmer is sued, can they garnish his wages?

Dave Plowman da...@davenoise.co.uk London SW
To e-mail, change noise into sound.

Fredxx

unread,
Dec 3, 2021, 11:02:34 AM12/3/21
to
On 02/12/2021 16:50, John Rumm wrote:
> On 02/12/2021 16:14, Fredxx wrote:
>> On 02/12/2021 15:17, John Rumm wrote:
>>> On 02/12/2021 14:41, www.GymRats.uk wrote:
>>>
>>>> Trying to decide as and when our boilers finally die (both are at the
>>>> "care-home" stage of their lives) whether to try and find space for a
>>>> pressurised HW tank rather than stick with the inherent fragility of
>>>> combi's.
>>>
>>> System boilers these days share much of the design and parts list of
>>> a combi - just lack a diversion valve and secondary plate heat
>>> exchanger.
>>>
>>>> Obviously the downside of stored HW is reduced flue heat recovery from
>>>> condensing (that can't make much of a difference though can it?) but I
>>>> feel the advantages outweigh the disadvantages.
>>>
>>> With a modern fast recovery cylinder (which all unvented ones will
>>> be) they can swallow the full output of a modest sized boiler. (my
>>> system boiler is 24kW, and the cylinder coil will take up to 22kW).
>>> So the boiler will be in condensing mode for much of the reheat.
>>
>> I've gone the other way, using a thermal store. That really swallows
>> the boiler output!
>
> OOI how are you extracting the DHW from it, with a coil or an external
> plate heat exchanger and a pump?

This has a big coil in it, I'm afraid I don't know much more. It does
mean you have a lot of primary water and a bigger expansion tank.

>> It has the advantage of being at low pressure and doesn't require
>> servicing.
>
> Yup thermal store is another good way to go. Easy to get heat into it,
> and can take solid fuel as well as heat from a heat pump, and can be
> direct rather than indirect so full boiler output and condensing
> efficiency as well. Slightly more complicated getting the heat out, but
> has the advantage you could run rads of UFH from it as well if its big
> enough.

The hot enters a mixing valve, with cold to reduce the temperature to a
constant safe level.

> I did look at doing one for here, but in the end it worked out a bit
> more expensive than going unvented if buying a COTS solution, and more
> hassle if going homebuilt.

I suppose I save an expansion vessel and all the pressure relief
hardware, drain etc, but then add a header tank with all it's issues and
hardware.

Fredxx

unread,
Dec 3, 2021, 11:04:04 AM12/3/21
to
Yes, but that's the same as an unvented tank, except mine operates at a
lower pressure and doesn't need the pressure relief valve hardware.

SH

unread,
Dec 3, 2021, 12:07:04 PM12/3/21
to

>
> You could always go for the Dr Drivel solution, two combis!
>

A half decent combi like a Vaillant 838 will take 38 kW of gas...

So two of these is 76 kW which is far more than a domestic gas meter can
supply, that being around 66kW...

and thats before you add in gas hobs, gas ovens, gas fires etc.

You could have 2 off 30 kW combis but the water won't be quite as good
as on a 38 kW boiler.

Fredxx

unread,
Dec 3, 2021, 1:08:29 PM12/3/21
to
On 02/12/2021 18:41, John Rumm wrote:

<snip>

> You could always go for the Dr Drivel solution, two combis!

I remember those days and his posts!

John Rumm

unread,
Dec 3, 2021, 1:25:49 PM12/3/21
to
On 03/12/2021 11:27, Andrew wrote:
In my case the unvented cylinder heats to 60 most days, and 70 once per
week (anti legionella cycle). I put a blending valve on the output so it
can't feed the house at more than about 50 or so.

Unvented cylinders have a number of over pressure and over temperature
safety valves that will dump water to a waste pipe in an emergency.

Heat banks are typically at atmospheric pressure anyway. Extraction of
heat usually being by means that allow the DHW maximum temperature to be
limited.

John Rumm

unread,
Dec 3, 2021, 1:28:26 PM12/3/21
to
No reason why you can't. Only downside is a bit of added noise when you
run the tap.

John Rumm

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Dec 3, 2021, 1:32:17 PM12/3/21
to
On 03/12/2021 11:36, Andrew wrote:
> On 02/12/2021 18:44, NY wrote:
>
>> at any time. The bottom element blew up at 1 AM when E-7 turned it on,
>> and I had to manhandle the whole *full* cylinder of water on its
>> support blocks through 180 degrees to get at the drain valve which was
>> on the *rear* of the cylinder. I'd drained the header tank but you can
>> never get the water out of the cylinder via the hot taps. Once I'd got
>> access to the drain cock it was easy enough to drain the cylinder
>> through a hosepipe.
>>
>
> I don't see why the tank would not have drained out to the
> point where the level in the tank matched the height of a
> hot tap though.

Because the hot feed out of a cylinder it at the top of the cylinder.
Once the supply of water pressure at the inlet falls, there is nothing
to push the water out of the cylinder.

John Rumm

unread,
Dec 3, 2021, 1:46:26 PM12/3/21
to
On 03/12/2021 04:28, RJH wrote:
> On 2 Dec 2021 at 18:41:25 GMT, "John Rumm" <see.my.s...@nowhere.null>
> wrote:
>
>> On 02/12/2021 18:06, Tim+ wrote:
>>> NY <m...@privacy.invalid> wrote:
>>>
>>>>
>>>> Having had various types of CH and HW systems in different houses I would
>>>> rate them (least preferable first, most preferable last)
>>>>
>>>> - cylinder/tank-fed
>>>> - cylinder/mains-fed
>>>> - combi/mains-fed
>>
>> My own experience is that the unvented cylinder trounces the combi in
>> every way... Massively faster delivery of water, at higher temperatures,
>> and people can have long showers without turning off the house heating.
>>
>
> I'd have thought it costs more to run the HW side of things with the heat
> losses from the cylinder.

The losses from the cylinder itself are pretty small (couple of hundred
watts perhaps) - it is well lagged with the lagging contained in an
outer skin. Keep in mind that in many cases you want some of the heat
loss from the cylinder, since it can then heat an airing cupboard. (I
deliberately chose not lag some of the pipework so as to lose some extra
heat into the airing cupboard)

Combi heatloss is in theory lower - but you do end up running off more
partially heated water waiting for the output to get to temperature. Not
all combis can run in condensing mode when heating the DHW. Also combis
typically need to be near an outside wall, so pipe runs can be longer. A
cylinder is often best near the middle of a property.

> But in other respects I tend to agree that HW using a combi is a mixed
> experience. Takes a good 20s for hot water to come through, although when it
> does it's a decent amount and plenty hot enough. I don't use the preheat
> option - it's pretty quick if I do. Multiple showers etc. are not going to
> happen, which would be more of a problem for larger households. As it is, on
> balance, a combi suits me better overall.

I installed a combi in my last place when I converted the loft. Went for
a 35kW model. I found that it could do one really good shower or two
"adequate" ones at once - at least in summer. (the 1/2" mains feed in
that place could only muster about 17 lpm at best). All in all it was
ok, and did better than many combis I had experienced in the past. (and
the vented system it replaced was particularly poorly done, so I hardly
lost any bath filling performance).

In this place there is a decent MDPE mains feed that can do 30 lpm or
better, so the cylinder will do two reasonably good showers at once.

From the DHW point of view I was designing it for performance, and gave
no consideration to cost of running TBH. However I did focus on cost and
efficency for the system as a whole - going from a single heating zone
off the back of an old Ideal Mexico floor stander that was probably less
than 70% efficent, to a modern condensing system boiler with 95%+
efficiency, split heating zones, external weather compensation, and
split temperature operation (so the CH can run at 45 degrees when its
mild out, but the DHW still gets heated to full temperature)

John Rumm

unread,
Dec 3, 2021, 1:49:17 PM12/3/21
to
Since the building regs were updated some years back (for the purposes
of heat and energy conservation) all hot water storage systems now come
under part G and would need signing off by building control or by a
suitably competent installer when installing or servicing.

John Rumm

unread,
Dec 3, 2021, 2:01:19 PM12/3/21
to
Old cylinders typically only had coils large enough to reheat less than
~5kW. This made some sense on old gravity circulated (i.e.
thermosyphon) cylinders, or when running fully pumped on old Y plan
installs with a fix rate output boiler (where the cylinder load could be
added to the CH and reduce cycling of the boiler)

With a modern one on S Plan or W plan, you can reheat a fairly standard
sized cylinder in 15 to 20 mins.

John Rumm

unread,
Dec 3, 2021, 2:02:24 PM12/3/21
to
On 03/12/2021 15:39, Dave Plowman (News) wrote:
> In article <socvr4$okb$1...@dont-email.me>,
> Harry Bloomfield wrote:
>> The big advantage of a combi, is space saving - great for compact flats
>> and having instant HW water, rather than having to wait 20 minutes for
>> the cylinder to heat.
>
> I discovered with my system boiler and fast recovery cylinder - having
> come back off holiday where it was switched off - that I got an acceptable
> shower very shortly after switching it on. Might have not been in the
> depths of winter, though, where the header and storage tanks would have
> had colder water.

As long as you don't draw heat at a faster rate than the boiler can
supply, you should be able to get constant delivery of DHW in much the
same was as you do from a combi. (it does help if the controls are set
to fire the boiler soon after it detects a drop in temperature though)

John Rumm

unread,
Dec 3, 2021, 2:04:11 PM12/3/21
to
On 03/12/2021 13:19, www.GymRats.uk wrote:
> On 02/12/2021 18:41, John Rumm wrote:
>
>>> For that reason alone, I won’t have the “eggs all in one basket “
>>> approach
>>> of combis in my house.
>>
>> You could always go for the Dr Drivel solution, two combis!
>
> But they have to be Japanese boilers if I remember correctly...

Sometimes - it seemed to change fairly frequently depending on what he
was plugging at the time. Rianni and Reemha seemed to be favourites in
later years.

> In fairness to Dr.Evil I did go with his suggestion of boiler at home.
> "EcoHometec" AKA M.A.N /Micromat which has been superb and to date
> everything has been repairable, and the more I'm forced to repair the
> simpler it becomes. So far the last repair of pin-holes in the back
> plate patched up with repair putty is still holding up. That was the
> first time I've taken the boiler off the wall but all went well.
> :)

So far I have been impressed with how easy the Vaillant designs are to
work on as well - tightly packaged but well though out and serviceable.

John Rumm

unread,
Dec 3, 2021, 2:08:01 PM12/3/21
to
On 03/12/2021 17:06, SH wrote:
>
>>
>> You could always go for the Dr Drivel solution, two combis!
>>
>
> A half decent combi like a Vaillant 838 will take 38 kW of gas...
>
> So two of these is 76 kW which is far more than a domestic gas meter can
> supply, that being around 66kW...

Indeed - one of the many ways his "solutions" fell down!

> and thats before you add in gas hobs, gas ovens, gas fires etc.
>
> You could have  2 off 30 kW combis but the water won't be quite as good
> as on a 38 kW boiler.

The moral being if you need a delivery rate of more than ~15 lpm of DHW,
then you need a storage system of some kind (even if its a big floor
standing combi with built in cylinder)

David

unread,
Dec 3, 2021, 2:59:50 PM12/3/21
to
On Thu, 02 Dec 2021 14:41:23 +0000, www.GymRats.uk wrote:

> Trying to decide as and when our boilers finally die (both are at the
> "care-home" stage of their lives) whether to try and find space for a
> pressurised HW tank rather than stick with the inherent fragility of
> combi's.
>
> Obviously the downside of stored HW is reduced flue heat recovery from
> condensing (that can't make much of a difference though can it?) but I
> feel the advantages outweigh the disadvantages.
>
> Adding the potential of a solar-thermal HW in the summer and looking at
> my mothers "Ideal" system boiler that is only the second boiler ever
> fitted to the house in 55 years has me thinking.
>
> And... depending on whatever other "green agenda" nonsense is forced
> upon us it may well be more suitable for utilising solar p.v. if my
> backwards-running electricity meter ever has to be replaced.
>
> Any pros or cons I've missed?
>
> Cheers - Pete

Cold mains water flow is the vital thing.
Applies both to combi and pressurised systems.

Tale of woe(ish):

House (-2) had an unvented hot water system with a system boiler.
Upgrade to the plumbing was a shower pump (hot and cold) which turned out
to need a bigger cold tank and a bigger hot tank to fill the spanky new
extra large bath.
However the pump was strong enough to service a second shower/bath when we
put in a second upstairs bathroom, and would even do both at once.

House (-1) has a Baxi back boiler on the gas fire, and a vented system in
the airing cupboard in the hall behind the fire.
Whole thing was shagged so we replaced it with a Worcester Bosch combi of
modest proportions in the loft.
This filled a bath full at a good rate, and also provided a good flow rate
on the shower.

Child has own house with a combi and gets a really good flow rate to the
shower.

This house we have a larger Worcester Bosch sized to run two showers
simultaneously but a puny cold water flow which is really irritating.
However not irritating enough to take the plumbing apart to try and find
out why we don't have a decent flow.
Downstairs has a better flow than upstairs but it would anyway given the
difference in pressure.

So before you commit to a mains pressure system double check that you get
enough flow.

If in doubt go with a vented system because you can always increase
pressure and flow with one or more pumps.

Cheers


Dave R

--
AMD FX-6300 in GA-990X-Gaming SLI-CF running Windows 7 Pro x64

--
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

critcher

unread,
Dec 4, 2021, 11:36:17 AM12/4/21
to
On 02/12/2021 15:00, Dave Plowman (News) wrote:
> In article <soalum$j1l$1...@dont-email.me>,
> www.GymRats.uk <0845.86...@GymRatZ.Gym.Equipment> wrote:
>> Trying to decide as and when our boilers finally die (both are at the
>> "care-home" stage of their lives) whether to try and find space for a
>> pressurised HW tank rather than stick with the inherent fragility of
>> combi's.
>
>> Obviously the downside of stored HW is reduced flue heat recovery from
>> condensing (that can't make much of a difference though can it?) but I
>> feel the advantages outweigh the disadvantages.
>
>> Adding the potential of a solar-thermal HW in the summer and looking at
>> my mothers "Ideal" system boiler that is only the second boiler ever
>> fitted to the house in 55 years has me thinking.
>
>> And... depending on whatever other "green agenda" nonsense is forced
>> upon us it may well be more suitable for utilising solar p.v. if my
>> backwards-running electricity meter ever has to be replaced.
>
>> Any pros or cons I've missed?
>
> I stuck with a cold water header tank and stored hot water when I changed
> boilers not that long ago. Means things can go on pretty well as normal
> for a day or so if the water is cut off. Much longer if I was careful. And
> like my high flow shower, and the ability to fill a bath quickly.
>
> But your choice if you could put the room these things take up to better
> use.
>
+1

Chris B

unread,
Dec 5, 2021, 5:36:05 AM12/5/21
to
On 03/12/2021 16:02, Fredxx wrote:

>
> This has a big coil in it, I'm afraid I don't know much more. It does
> mean you have a lot of primary water and a bigger expansion tank.
>
>>> It has the advantage of being at low pressure and doesn't require
>>> servicing.
>>
>> Yup thermal store is another good way to go. Easy to get heat into it,
>> and can take solid fuel as well as heat from a heat pump, and can be
>> direct rather than indirect so full boiler output and condensing
>> efficiency as well. Slightly more complicated getting the heat out,
>> but has the advantage you could run rads of UFH from it as well if its
>> big enough.
>
> The hot enters a mixing valve, with cold to reduce the temperature to a
> constant safe level.
>

These probably work best in soft water areas. A whole estate round here
built in I guess the early to mid 90's was fitted with a thermal store
with a cold water mains double coil running through it to extract DHW.
The problem was that each coil was only about 5mm dia (good I guess for
surface area to volume ratio) and although all of the houses were fitted
with this type of water softener

https://www.amazon.co.uk/PERMUTIT-INHIBITOR-REPLACEMENT-CARTRIDGE/dp/B01MSJSKOT

the hard water round here was a constant bug bear. I think very few of
the original installations were left 10 years after the new build.


--
Chris B (News)

Fredxx

unread,
Dec 5, 2021, 8:01:36 AM12/5/21
to
I would say that the water here is is pretty soft.


www.GymRatZ.co.uk

unread,
Dec 5, 2021, 9:02:49 AM12/5/21
to
On 03/12/2021 19:59, David wrote:

>
> Cold mains water flow is the vital thing.
> Applies both to combi and pressurised systems.

When we had the extension added I re-routed the mains water pipe in 25mm
in a straight run from the pavement to the boiler room, about 8 metre
run so flow is no longer a problem.
:)


In follow up to your tales of woe, my first house was "interesting" to
say the least.

Coal fired Parkray back boiler in the lounge with vented H/W tank above
and a single pipe central heating system with a pump under the floor in
the lounge.

I added a 2nd pipe to make a proper flow/return C/H but to completely
automate everything I added 3 tank/pipe thermostats, one near the bottom
of the tank, one near the top and one on the 1 1/8" rising pipe from the
back boiler, a 3-way zone valve and new circulation pump.

With everything connected and switched on the rising H/W from the boiler
would trigger the pipe stat that switched the circulating pump on and
the 3-way valve was controlled by the lower tank 'stat so HW tank was
always priority before the valve switched to C/H circuit but the
cylinder heated up very quickly.

The 3rd cylinder stat at the top of the tank was connected in series
with the immersion heater for more accurate control of immersion.

Oh I also added a twin impeller Stuart Turner pump too for a proper
"power shower" . All worked flawlessly and when gas came to the
village I replaced the coal back burner with a living-flame gas fire
that also had a gas back boiler and then of course added a timer.
Extremely simple and very effective. :)


> So before you commit to a mains pressure system double check that you get
> enough flow.
>
> If in doubt go with a vented system because you can always increase
> pressure and flow with one or more pumps.


Pressure is around 3.5 bar so flow is pretty good especially with the
upgraded water main pipe.

Cheers - Pete

John Rumm

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Dec 6, 2021, 9:35:23 AM12/6/21
to
On 03/12/2021 10:53, The Natural Philosopher wrote:
> On 03/12/2021 10:19, RJH wrote:


>> No, mine's much worse. The longest pipe run in my house is about 3m
>> (bath-boiler).
>>
>>> At some level the size of combi needed to heat a decent water flow
>>> exceeds the cost of a system boiler and pressurised tank.
>>>
>>> I'd only contemplate a combi in a small 1 or two person house.
>>
>> Which is what mine is most of the time. Hence why it suits me.
>>

> Its a substandard solution with a price that reflects that.

If it fits the requirement, then it's not substandard. The fact that it
might not fit *your* requirements matters not a jot! Price may
frequently be one of the requirements.

> People who add heatbanks and pumps and so on would be better off simply
> nuying a system boiler and a pressurised tank.

You are conflating different things there. Heatbanks typically provide
mains pressure hot water (and it indeed may be mains pressure rather
than reduced to 3 - 4 bar by the typical control valve arrangements on
an unvented cylinder).

Pumps are not necessarily required for a heat bank at all unless you are
using one to circulate primary water to an external plate heat exchanger
for DHW rather than having a purpose made tank with a DHW coil.

> My cylinder hardly loses any heat at all. It an insulated unit and well
> insulated at that.

As would be a good heat bank. Heat banks are also good if you want to
bring in solid fuel to the mix and can do split temperature outputs nice
and easily with multiple tapings. So you draw heat for DHW from near the
top, for rads from the middle, and for UFH from lower down.
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