Hot water cylinder heat exchanger efficiency - reheat time?

[MJA]

Anyone have any pointers to hot water cylinder heat exchanger
efficiency data?

I will probably have a new boiler installed in the spring. I will
retain a vented hot water cylinder. I am looking at dual temperature
boilers with weather compensation, which adjust the flow temperature
and have hot water priority over central heating.

I am trying to estimate what the worst case time is to reheat the hot
water cylinder during which there will be no central heating. A naive
calculation with 18Kw output boiler and 100 litre hot water cylinder
raising temperature by 50 degrees suggests:

reheat time = 100 * 50 * 4190 / 18,000 seconds

(the specific heat capacity of water is 4190 J/Kg/K)

which is about 20 minutes -- probably acceptable considering only a
bath would use the whole lot.

The reheat time will be longer as the heat exchanger in the hot water
cylinder is not perfect. In practice as the return temperature
increases the boiler will modulate down and will not achieve 18Kw
output, particularly at the end of the reheat cycle when the
temperature difference across the heat exchanger is low.

I cannot find any manufacturer documentation to calculate the effect
of hot water cylinder heat exchanger efficiency on reheat time. Is it
significant in the case of a part L compliant hot water cylinder? Is
there any point in paying for a premium cylinder such as the Albion
CF80 (other makes exist)? Could I leave the old cylinder (which uses
gravity circulation) unchanged except for conversion to fully pumped
primary?

Regards,

MJA

[Martin Bonner]

On Sunday, November 17, 2013 1:59:31 PM UTC, MJA wrote:
> The reheat time will be longer as the heat exchanger in the hot water
> cylinder is not perfect. In practice as the return temperature
> increases the boiler will modulate down

But surely, as the hot water starts to use less heat from the boiler,
the central heating pump can start up and use the extra.

Thus it may take longer than 20 minutes before the hot water is hot
enough to run a second large bath, but you won't be without central
heating for much longer.

[Andrew Gabriel]

In article <5288cbc3$0$1227$5b6a...@news.zen.co.uk>,

One of the systems I've computerised uses a hot water cylinder, so
I can give you some data from its logs. The cylinder is probably
from 1990, and so not a fast recovery type, although it appears to
be well insulated. The boiler is a Potterton Profile, adjusted for
18kW input, which gives 14kW output.

Heating the cylinder up from cold, the max heat it can initially
absorb is half the boiler output, which is 7kW i.e. the boiler
burner (fixed power) runs at 50% duty cycle if I subtract the
initial excess for heating up the boiler's low capacity cast
iron heat exchanger and the direct hot water loop.

As the cylinder heats, the differential temperature will drop,
and consequently the ability to absorb heat will drop too. It
seems that the max heat it can absorb shortly before the cylinder
stat cuts off (2 hours after starting from cold in this case) is
1.25kW (8.9% burner duty cycle).

It looks like the cylinder requires about 8kWhr to completely
heat it up (setpoint is about 58C on the cylinder stat, but
I've not measured how accurately it achieves this). Of course,
this doesn't heat the whole cylinder, because the cylinder stat
is never positioned at the bottom - it has to be well above the
bottom of the coil.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

[The Natural Philosopher]

On 17/11/13 13:59, MJA wrote:
> Anyone have any pointers to hot water cylinder heat exchanger
> efficiency data?
>

100% usually.

--
Ineptocracy

(in-ep-toc’-ra-cy) – a system of government where the least capable to
lead are elected by the least capable of producing, and where the
members of society least likely to sustain themselves or succeed, are
rewarded with goods and services paid for by the confiscated wealth of a
diminishing number of producers.

[harryagain]

"Martin Bonner" <martin...@yahoo.co.uk> wrote in message
news:e70242c0-dfba-4142...@googlegroups.com...

You are asking the wrong question.
Efficiency is not the issue.

The words you want to google are "hot water recovery time"

[John Rumm]

On 17/11/2013 13:59, MJA wrote:

> Anyone have any pointers to hot water cylinder heat exchanger
> efficiency data?

Yup...

> I will probably have a new boiler installed in the spring. I will
> retain a vented hot water cylinder. I am looking at dual temperature
> boilers with weather compensation, which adjust the flow temperature
> and have hot water priority over central heating.
>
> I am trying to estimate what the worst case time is to reheat the hot
> water cylinder during which there will be no central heating. A naive
> calculation with 18Kw output boiler and 100 litre hot water cylinder
> raising temperature by 50 degrees suggests:
>
> reheat time = 100 * 50 * 4190 / 18,000 seconds
>
> (the specific heat capacity of water is 4190 J/Kg/K)
>
> which is about 20 minutes -- probably acceptable considering only a
> bath would use the whole lot.

Much depends on your choice of cylinder. Old school cylinders frequently
can't absorb heat faster than around 5 - 7kw - which makes them not idea
in DHW priority systems. Modern part L compliant "fast recovery" ones
are better and will often do twice that. Something like the unvented one
I fitted can absorb at a maximum of 22kW which works quite well on a
split temperature system.

> The reheat time will be longer as the heat exchanger in the hot water
> cylinder is not perfect. In practice as the return temperature
> increases the boiler will modulate down and will not achieve 18Kw
> output, particularly at the end of the reheat cycle when the
> temperature difference across the heat exchanger is low.

This is true, however with a split temperature system you can set a much
higher set point water temperature for the cylinder reheat without
compromising the efficiency for running the heating. Also if the
cylinder can absorb a resonable power, you will only be running at a
lower efficiency toward the end of the heating cycle.

I find a full reheat of my 250L cylinder is around 35 - 40 mins.

> I cannot find any manufacturer documentation to calculate the effect
> of hot water cylinder heat exchanger efficiency on reheat time. Is it
> significant in the case of a part L compliant hot water cylinder? Is

Many cylinders will state the transfer rate of the indirect coil - so
you can work out what you need from that.

> there any point in paying for a premium cylinder such as the Albion
> CF80 (other makes exist)? Could I leave the old cylinder (which uses
> gravity circulation) unchanged except for conversion to fully pumped
> primary?

You could obviously keep the old one, but it won't be well suited to a
DHW priority setup, and the boiler / weather compensation won't be
suited to any form of paralleled operation of the cylinder and the heating.

Having said that, depending on your DHW demands (and the size of
cylinder you spec) you may find that a full reheat early in the morning
before the CH kicks in, then a couple of refreshes during the day are
adequate anyway, and they can be scheduled during heating setback periods.

More detail on the calcs etc here:

http://wiki.diyfaq.org.uk/index.php?title=Sizing_a_hot_water_cylinder



--
Cheers,

John.

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

[John Rumm]

On 17/11/2013 14:06, Martin Bonner wrote:
> On Sunday, November 17, 2013 1:59:31 PM UTC, MJA wrote:
>> The reheat time will be longer as the heat exchanger in the hot water
>> cylinder is not perfect. In practice as the return temperature
>> increases the boiler will modulate down
>
> But surely, as the hot water starts to use less heat from the boiler,
> the central heating pump can start up and use the extra.

The difficulty there is the split temperature operation... normally the
boiler and weather compensator will run the rads at a much lower flow
temperature than it will use for the cylinder reheat. So if you allow
the control system to run the rads during a cylinder reheat, then you
will suddenly have the rads running at 70 - 80 degrees flow temp.

> Thus it may take longer than 20 minutes before the hot water is hot
> enough to run a second large bath, but you won't be without central
> heating for much longer.

If you go for a cylinder that can swallow the full output of the boiler
(or near enough), then you are usually better off getting that out of
the way first and then switching back to heating after. A 20 min
interruption to heating is not likely to be that noticeable.

[Dudley]

replying to John Rumm , Dudley wrote:


To answer the original question, the efficiency is about 30%, so it's
probably as cheap to use the electric immersion on economy 7
Unless you actually use stored water, install an on-demand boiler. We
only use hot water for showering. Every other appliance is cold fill.
A good on-demand boiler will save a lot of money; a hot tanks wastes 2-3kW
overnight. You could even consider fitting on-demand to the kitchen and
bath taps and keeping the cylinder for the showers.

--

[harryagain]

"Dudley" <caedfaa9ed1216d60e...@example.com> wrote in
message news:2f32e$528a43d2$cf3aab60$77...@news.flashnewsgroups.com...

There is no such thing as efficiency with a heat exchanger.
There is only heat losses.

[Tim+]

You are Dribble AICMFP.

Tim

[Vir Campestris]

Absolutely. The fact that he can't tell a Joule from a Watt is telling :)

BTW, when people quote efficiency figures on combis are they allowing
for the short-cycling that happens every time you turn a hot tap on to
wash your hands?

Andy

[John Rumm]

On 18/11/2013 16:44, Dudley wrote:
> replying to John Rumm , Dudley wrote:
>
>
> To answer the original question, the efficiency is about 30%, so it's

While plausible for an old cast iron lump boiler and low transfer rate
cylinder, that seems a little too pessimistic for a modern boiler /
cylinder combination. Although not in place yet, the SAP ratings system
used to provide the SEDBUK figures will be split in future to show the
system efficiency separately for heating and hot water production. From
some of the examples I have seen, the hot water only performance is
often about 2/3rds that of the space heating performance with
efficiencies around the 60 - 70%

> probably as cheap to use the electric immersion on economy 7
> Unless you actually use stored water, install an on-demand boiler. We
> only use hot water for showering. Every other appliance is cold fill.
> A good on-demand boiler will save a lot of money; a hot tanks wastes 2-3kW

I presume you mean kWh? Again this is excessive for a properly insulated
cylinder. However its worth noting that if you want a warm airing
cupboard, then the heat is not "wasted" as such.

> overnight. You could even consider fitting on-demand to the kitchen and
> bath taps and keeping the cylinder for the showers.

Not sure why you would want on demand heating for bath fill - its the
application that its poorest suited to. Showers on the other hand are
well suited.

Using a combi boiler and a cylinder makes sense with the cylinder being
used for bath fills, and the combi for showers and kitchen taps.

[John Rumm]

Sort of, the current ratings system factors in hot water usage, but
there are some assumptions made about the usage ratio of space to water
heating. So in very well insulated properties the figures will somewhat
optimistic.

Its worth noting that some combis are capable of condensing operation
while in DHW mode, and some are not.

[harryagain]

"Tim Streater" <timst...@greenbee.net> wrote in message
news:181120131902052344%timst...@greenbee.net...
> In article <181120131756205592%timst...@greenbee.net>, Tim Streater
> <timst...@greenbee.net> wrote:
>
>> In article <l6djdd$pkf$9...@dont-email.me>, harryagain

>> yes there is. I expect it's something like:
>>
>> eff = ( ( heat in - heat out ) / heat in ) x 100%
>
> or even:
>
> eff = ( ( heat out ) / ( heat in ) ) x 100%
>

Nope. NOT true.
There is no energy conversion.

Heat out after zero hours lapsed=100%
Heat out after say 24hrs = zero. (Due to heat LOSSES).
But varies depending on INSULATION

Go to the bottom of the class. Stupid boy.

[John Rumm]

Where does the heat come from harry?

> Heat out after zero hours lapsed=100%
> Heat out after say 24hrs = zero. (Due to heat LOSSES).
> But varies depending on INSULATION
>
> Go to the bottom of the class. Stupid boy.

Harry missing the big picture as usual.

The efficiency that matters is that of the whole system. If you use a
boiler to heat a cylinder via its indirect coil, then I can guarantee
that you will not convert all the energy contained in the gas, into the
heat contained in the cylinder. Therefore the system efficiency is less
than 100%.

Well designed coil in tank HEs are reasonably effective these days,
however they are not able to transfer all of the available heat from the
input side to the output side in a single pass[1]. So whichever way look
at it, there will be a less than perfect system efficiency as described
by Mr Streater.


[1] The implication of this is that as the cylinder temperature rises,
the rate of transfer of heat from the primary water to the cylinder
water falls. The result is there will come a point where you push the
boiler into less efficient modes of operation (e.g. cycling or running
with a return temperature above the dew point)

[Chris J Dixon]

John Rumm wrote:

>On 18/11/2013 16:44, Dudley wrote:

>> probably as cheap to use the electric immersion on economy 7
>> Unless you actually use stored water, install an on-demand boiler. We
>> only use hot water for showering. Every other appliance is cold fill.
>> A good on-demand boiler will save a lot of money; a hot tanks wastes 2-3kW
>
>I presume you mean kWh? Again this is excessive for a properly insulated
>cylinder.

I'm not sure it is. I have a Range Supercal double-lagged (which
seems to mean 50 mm foam thickness) 206 litre cylinder, which,
according to the label, has a maximum stored heat loss of 3.16
kWh per 24 h.

Chris
--
Chris J Dixon Nottingham UK
ch...@cdixon.me.uk

Plant amazing Acers.

[harryagain]

"John Rumm" <see.my.s...@nowhere.null> wrote in message
news:JuKdnX5fG5fDsxbP...@brightview.co.uk...

If you want, you can go all the way back to Gazprom Siberia for the energy
source

But the OP is about hot water cylinders with heat exchanger.
Efficiency is not relevant.
There is no energy consversion, no "work" is done.
Taking the instantaneous value, it must always be 100%.

The factor to consider are:-
Rate of heat exchange. (Many variables.)
Amount of water stored.
Heat loss. (Many variables)
Time taken to heat the store from income temp to desired temp. (Recovery
time) Which depends upon the above factors.

This is all very difficult to calculate so one goes on the manufacturers
figures and conventions/empirical methods.
Somewhere there will be charts/graphs showing the performance under various
conditions.
Even this is only a general guide which is where the "funk factor" comes in.

[dennis@home]

On 19/11/2013 06:46, harryagain wrote:

Heat exchangers have an efficiency, simple coils will transfer less of
the energy available than a well designed flat plate exchanger.

The energy isn't lost if its recirculated through the heat source and
then back through the exchanger.

[dennis@home]

On 19/11/2013 11:41, Chris J Dixon wrote:
> John Rumm wrote:
>
>> On 18/11/2013 16:44, Dudley wrote:
>
>>> probably as cheap to use the electric immersion on economy 7
>>> Unless you actually use stored water, install an on-demand boiler. We
>>> only use hot water for showering. Every other appliance is cold fill.
>>> A good on-demand boiler will save a lot of money; a hot tanks wastes 2-3kW
>>
>> I presume you mean kWh? Again this is excessive for a properly insulated
>> cylinder.
>
> I'm not sure it is. I have a Range Supercal double-lagged (which
> seems to mean 50 mm foam thickness) 206 litre cylinder, which,
> according to the label, has a maximum stored heat loss of 3.16
> kWh per 24 h.
>
> Chris
>

That's poor, I have a 210l one and its less than 2 kWh/24 hours.
Its worth it when the alternative is less than 24 l/min flow through the
shower.

[John Rumm]

On 19/11/2013 16:27, harryagain wrote:

> But the OP is about hot water cylinders with heat exchanger.
> Efficiency is not relevant.

His question specifically mentions recovery time as being the salient
point. So the "efficiency" he is interested in is that of how quickly
heat can be transferred from the boiler to the tank. You may not like
the word, but it should be very clear what he means (i.e. rate of energy
uptake by the cylinder compared with the potential maximum rate
deliverable by the boiler)

> There is no energy consversion, no "work" is done.
> Taking the instantaneous value, it must always be 100%.

Rather meaningless, and not particularly relevant.

> The factor to consider are:-
> Rate of heat exchange. (Many variables.)
> Amount of water stored.

Yes, and generally information that is available from the cylinder
manufacturer.

> Heat loss. (Many variables)

Not significant enough have any bearing - assuming you have some lagging
on your primary loop, the cylinder may be loosing 100W, which for these
purposes is negligible compared with an input rate of many kW.

> Time taken to heat the store from income temp to desired temp. (Recovery
> time) Which depends upon the above factors.

you don't say...

> This is all very difficult to calculate so one goes on the manufacturers
> figures and conventions/empirical methods.
> Somewhere there will be charts/graphs showing the performance under various
> conditions.
> Even this is only a general guide which is where the "funk factor" comes in.

If all you need is an indication of how long before it can run another
bath etc, then its relatively easy to calculate ballpark figures IME.

For example if you take figures for a tank such as the Unistor 210, the
spec from the manufacturer gives you:

Primary HE flow rate required : 23.3 lpm
Heat up time to EN12897 : 28 mins
Recovery (70%) : 20 mins
Primary HE performance : 22.6 kW

(which alone would give the OP a good enough indication if he can find
similar data for his), however, say you just had the HE transfer rate
and the volume of water?

Lets say you have drawn off 150L of water and now want to reheat that
amount from 10 to 60 degrees. If you do a crude calc such as:

150 x 50 x 4200 / 22,600 / 60 = time, you get about 23 mins

Now in reality, there are other factors that come into it (when the
boiler starts reheating being the main one), but as an indication is
pretty serviceable.

[John Rumm]

On 19/11/2013 11:41, Chris J Dixon wrote:

> John Rumm wrote:
>
>> On 18/11/2013 16:44, Dudley wrote:
>
>>> probably as cheap to use the electric immersion on economy 7
>>> Unless you actually use stored water, install an on-demand boiler. We
>>> only use hot water for showering. Every other appliance is cold fill.
>>> A good on-demand boiler will save a lot of money; a hot tanks wastes 2-3kW
>>
>> I presume you mean kWh? Again this is excessive for a properly insulated
>> cylinder.
>
> I'm not sure it is. I have a Range Supercal double-lagged (which
> seems to mean 50 mm foam thickness) 206 litre cylinder, which,
> according to the label, has a maximum stored heat loss of 3.16
> kWh per 24 h.

That's quite high by modern standards. My 210L unistor is rated at 1.89
kW/24h, i.e. a loss rate of just under 80W

(even the 310L version is only 2.26 kW/24h)

[harryagain]

"John Rumm" <see.my.s...@nowhere.null> wrote in message

news:su6dnXK09s-htRHP...@brightview.co.uk...

Even a small heat loss is relevant because in many cases it's there 24/7.

[harryagain]

"dennis@home" <den...@killspam.kicks-ass.net> wrote in message
news:528bd038$0$10663$c3e8da3$3388...@news.astraweb.com...

Dennis, as usual you haven't a clue. Go back to sleep.

[Chris J Dixon]

John Rumm wrote:

>On 19/11/2013 11:41, Chris J Dixon wrote:

>> I have a Range Supercal double-lagged (which
>> seems to mean 50 mm foam thickness) 206 litre cylinder, which,
>> according to the label, has a maximum stored heat loss of 3.16
>> kWh per 24 h.
>
>That's quite high by modern standards. My 210L unistor is rated at 1.89
>kW/24h, i.e. a loss rate of just under 80W

Yes, that is a little over half mine, which is disappointing. :-(

What insulation does yours have?

Mine is about 9 years old now. It sits, together with a pump, in
the corner of my inset garage, and there isn't a lot of room to
add further insulation.

[John Rumm]

On 20/11/2013 08:20, harryagain wrote:
> "John Rumm" <see.my.s...@nowhere.null> wrote in message
> news:su6dnXK09s-htRHP...@brightview.co.uk...
>> On 19/11/2013 16:27, harryagain wrote:

>>> Heat loss. (Many variables)
>>
>> Not significant enough have any bearing - assuming you have some lagging
>> on your primary loop, the cylinder may be loosing 100W, which for these
>> purposes is negligible compared with an input rate of many kW.

[snip]

>> For example if you take figures for a tank such as the Unistor 210, the
>> spec from the manufacturer gives you:
>>
>> Primary HE flow rate required : 23.3 lpm
>> Heat up time to EN12897 : 28 mins
>> Recovery (70%) : 20 mins
>> Primary HE performance : 22.6 kW
>>
>> (which alone would give the OP a good enough indication if he can find
>> similar data for his), however, say you just had the HE transfer rate and
>> the volume of water?
>>
>> Lets say you have drawn off 150L of water and now want to reheat that
>> amount from 10 to 60 degrees. If you do a crude calc such as:
>>
>> 150 x 50 x 4200 / 22,600 / 60 = time, you get about 23 mins
>>
>> Now in reality, there are other factors that come into it (when the boiler
>> starts reheating being the main one), but as an indication is pretty
>> serviceable.

> Even a small heat loss is relevant because in many cases it's there 24/7.

Its relevant in some discussions, but not this one about recovery time.

Take the above example, let's say that ~23 mins calculated was the time
including 100W of loss from the tank. Do the sums again with an
effective additional 100W of input resulting from using a "perfectly
lagged" cylinder, and you get 23.13 mins instead of 23.23 mins - less
than 10 seconds.

[John Rumm]

On 20/11/2013 08:47, Chris J Dixon wrote:
> John Rumm wrote:
>
>> On 19/11/2013 11:41, Chris J Dixon wrote:
>
>>> I have a Range Supercal double-lagged (which
>>> seems to mean 50 mm foam thickness) 206 litre cylinder, which,
>>> according to the label, has a maximum stored heat loss of 3.16
>>> kWh per 24 h.
>>
>> That's quite high by modern standards. My 210L unistor is rated at 1.89
>> kW/24h, i.e. a loss rate of just under 80W
>
> Yes, that is a little over half mine, which is disappointing. :-(
>
> What insulation does yours have?

Truth be told I don't know. The spec does not say, and you can't tell
from looking at it since its all fully encased in an outer rigid skin
over the insulation.

> Mine is about 9 years old now. It sits, together with a pump, in
> the corner of my inset garage, and there isn't a lot of room to
> add further insulation.

Yup, tricky one. Although if its an inset garage, then presumably most
of the waste heat will end up in the room above it?

Mine is in the airing cupboard, so some heat loss is actually useful
(and also why I did not bother lagging the flow and return pipework in
there, since the losses from the cylinder itself are so low)

[Chris J Dixon]

John Rumm wrote:

>On 20/11/2013 08:47, Chris J Dixon wrote:

>> What insulation does yours have?
>
>Truth be told I don't know. The spec does not say, and you can't tell
>from looking at it since its all fully encased in an outer rigid skin
>over the insulation.
>
>> Mine is about 9 years old now. It sits, together with a pump, in
>> the corner of my inset garage, and there isn't a lot of room to
>> add further insulation.
>
>Yup, tricky one. Although if its an inset garage, then presumably most
>of the waste heat will end up in the room above it?

Some of it, certainly, though the up and over metal door can't
help matters. I did vaguely wonder about trying to glue some
insulation onto it, but the counterbalance spring just about
copes at the moment. I fear the extra mass would overload it, and
trying to increase the tension looks like a task for someone who
doesn't value their fingers.

At least I don't have a problem with my tools rusting. ;-)

[MJA]

On 2013-11-17, Andrew Gabriel <and...@cucumber.demon.co.uk> wrote:
> In article <5288cbc3$0$1227$5b6a...@news.zen.co.uk>,

> MJA <m...@nospam.co.uk> writes:
>> Anyone have any pointers to hot water cylinder heat exchanger
>> efficiency data?
>

> One of the systems I've computerised uses a hot water cylinder, so
> I can give you some data from its logs. The cylinder is probably
> from 1990, and so not a fast recovery type, although it appears to
> be well insulated. The boiler is a Potterton Profile, adjusted for
> 18kW input, which gives 14kW output.
>
> Heating the cylinder up from cold, the max heat it can initially
> absorb is half the boiler output, which is 7kW i.e. the boiler
> burner (fixed power) runs at 50% duty cycle if I subtract the
> initial excess for heating up the boiler's low capacity cast
> iron heat exchanger and the direct hot water loop.
>
> As the cylinder heats, the differential temperature will drop,
> and consequently the ability to absorb heat will drop too. It
> seems that the max heat it can absorb shortly before the cylinder
> stat cuts off (2 hours after starting from cold in this case) is
> 1.25kW (8.9% burner duty cycle).
>
> It looks like the cylinder requires about 8kWhr to completely
> heat it up (setpoint is about 58C on the cylinder stat, but
> I've not measured how accurately it achieves this). Of course,
> this doesn't heat the whole cylinder, because the cylinder stat
> is never positioned at the bottom - it has to be well above the
> bottom of the coil.
>

That is really interesting, thanks. Is that a fully pumped system, or
one with gravity circulation for the hot water cylinder?

Perhaps it is time for me to order a Raspberry Pi to use as a data
logger for my system!

MJA

[Vir Campestris]

On 18/11/2013 21:45, John Rumm wrote:
> Using a combi boiler and a cylinder makes sense with the cylinder being
> used for bath fills, and the combi for showers and kitchen taps.

Having been using a combi for the last 6 months - I'll disagree on
kitchen taps. It fires up very time you touch the tap, heats its
exchanger, then goes off. Can't possibly be efficient.

Andy

[John Rumm]

My comment was not so much about efficiency as convenience. i.e most
people have the boiler near the kitchen, so less wait for hot water, and
also the hot water from a combi is drinkable, so it can make sense to
fill the kettle / pan etc from the hot tap.

[MJA]

Thanks for all the useful responses.

On 2013-11-17, John Rumm <see.my.s...@nowhere.null> wrote:

> On 17/11/2013 13:59, MJA wrote:
>
>> Anyone have any pointers to hot water cylinder heat exchanger
>> efficiency data?
>

> Yup...

I should have written heat exchanger effectiveness -- efficiency was a
poor choice of word and I have been chastised. I am primarily
interested in reheat time with a hot water priority system.

<useful and interesting response clipped>

>> I cannot find any manufacturer documentation to calculate the effect
>> of hot water cylinder heat exchanger efficiency on reheat time. Is it
>> significant in the case of a part L compliant hot water cylinder? Is
>
> Many cylinders will state the transfer rate of the indirect coil - so
> you can work out what you need from that.

I find it difficult to find manufacturers information. For example,
Range <www.range-cylinders.co.uk/pdfs/sales/copper.pdf> quote reheat
times of 25mins for Hercal cylinders, 20mins for Supercal cylinders
and 15mins for Ultracal cylinders. I am suspicious about these
figures because they are specified with 82C flow temperature at 15
litres/min (18 for Ultracal).

This is unrealistic -- a small (18Kw) boiler will produce a
temperature rise of about 17C at 15 litres/min between return and
flow. Hence the flow temperature will start low (10+17 = 27C) and
slowly increase.

The brochure smacks too much of Hi-Fi amplifier makers giving peak
music power output (PMPO) rather than continuous sine wave power into
a resistive load at onset of clipping.

When it gets nearer to spring and time to do the work I will try
contacting the technical departments of one or two copper cylinder
manufacturers to see if they can help.

>> Could I leave the old cylinder (which uses
>> gravity circulation) unchanged except for conversion to fully pumped
>> primary?
>
> You could obviously keep the old one, but it won't be well suited to a
> DHW priority setup, and the boiler / weather compensation won't be
> suited to any form of paralleled operation of the cylinder and the heating.
>
> Having said that, depending on your DHW demands (and the size of
> cylinder you spec) you may find that a full reheat early in the morning
> before the CH kicks in, then a couple of refreshes during the day are
> adequate anyway, and they can be scheduled during heating setback periods.

I will probably do the work in stages -- get a gas fitter in to change
the boiler, then DIY the hot water cylinder change. I can see what
the cylinder reheat time is like during the summer, and change it if
necessary before the next heating season.

Regards,

MJA

[John Rumm]

On 22/11/2013 00:28, MJA wrote:
> Thanks for all the useful responses.
>
> On 2013-11-17, John Rumm <see.my.s...@nowhere.null> wrote:
>> On 17/11/2013 13:59, MJA wrote:
>>
>>> Anyone have any pointers to hot water cylinder heat exchanger
>>> efficiency data?
>>
>> Yup...
>
> I should have written heat exchanger effectiveness -- efficiency was a
> poor choice of word and I have been chastised. I am primarily
> interested in reheat time with a hot water priority system.

Yeah don't worry, we knew what you meant... don't mind harry.

> <useful and interesting response clipped>
>
>>> I cannot find any manufacturer documentation to calculate the effect
>>> of hot water cylinder heat exchanger efficiency on reheat time. Is it
>>> significant in the case of a part L compliant hot water cylinder? Is
>>
>> Many cylinders will state the transfer rate of the indirect coil - so
>> you can work out what you need from that.
>
> I find it difficult to find manufacturers information. For example,
> Range <www.range-cylinders.co.uk/pdfs/sales/copper.pdf> quote reheat
> times of 25mins for Hercal cylinders, 20mins for Supercal cylinders
> and 15mins for Ultracal cylinders. I am suspicious about these
> figures because they are specified with 82C flow temperature at 15
> litres/min (18 for Ultracal).
>
> This is unrealistic -- a small (18Kw) boiler will produce a
> temperature rise of about 17C at 15 litres/min between return and
> flow. Hence the flow temperature will start low (10+17 = 27C) and
> slowly increase.

That assumes that the cylinder HE will extract all the heat per pass,
which it won't - especially at such a low differential. (in fact if the
cylinder is partly hot, you may even find the return temp to the boiler
is warmer than the flow for a short while as the cylinder gives up heat
to the primary water).

After a few trips through the boiler, you will be up to the expected
flow temp.

Some of the cylinder specs (its worth downloading the installation
manual with the technical stuff in it rather than just using the sales
brochures) will have proper graphs of power inputs vs reheat time for
one or more flow temps. Generally these are produced from test measurements.

> The brochure smacks too much of Hi-Fi amplifier makers giving peak
> music power output (PMPO) rather than continuous sine wave power into
> a resistive load at onset of clipping.

The normal element of salesmanship is that many of the figures will be
quoted for flow temps of 80 degrees - which are far less commonly used
for modern boilers.

> When it gets nearer to spring and time to do the work I will try
> contacting the technical departments of one or two copper cylinder
> manufacturers to see if they can help.
>
>>> Could I leave the old cylinder (which uses
>>> gravity circulation) unchanged except for conversion to fully pumped
>>> primary?
>>
>> You could obviously keep the old one, but it won't be well suited to a
>> DHW priority setup, and the boiler / weather compensation won't be
>> suited to any form of paralleled operation of the cylinder and the heating.
>>
>> Having said that, depending on your DHW demands (and the size of
>> cylinder you spec) you may find that a full reheat early in the morning
>> before the CH kicks in, then a couple of refreshes during the day are
>> adequate anyway, and they can be scheduled during heating setback periods.
>
> I will probably do the work in stages -- get a gas fitter in to change
> the boiler, then DIY the hot water cylinder change. I can see what
> the cylinder reheat time is like during the summer, and change it if
> necessary before the next heating season.

It its an older "normal" gravity cylinder, then obviously it will still
work, although the reheat times will probably be two or three times what
you would expect with a modern one with a high power HE.

I used to find my old setup (cast iron lump boiler running 80 degree
flow, through a normal 900 x 450 indirect copper cylinder (max coil
transfer rate about 5kW) would take over an hour for a full reheat.
There was not much point running just the cylinder on its own since it
could not consume the 20+ kW the boiler could produce. The new setup
with a cylinder of twice the capacity, is very much quicker, simply
because it can consume the full output of the boiler for most of the
reheat.

I did contemplate doing the boiler swap and cylinder etc separately, but
decided it was much easier to get all the changes (i.e. boiler, weather
comp / controls, conversion to unvented, new cylinder, radiator
juggling, extra TMVs etc) done at once to save going through all the
hassle more than once.

[Andrew Gabriel]

In article <528d1ec7$0$1226$5b6a...@news.zen.co.uk>,

This is fully pumped.

> Perhaps it is time for me to order a Raspberry Pi to use as a data
> logger for my system!

Yes. This system predates raspberry pi's by 10+ years, but I have
been playing with a raspberry pi for the next system I do.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]