I'm not a plumber, as just about anyone will tell in a minute.
Thermal Stores provide mains pressure hot water, by keeping a large amount
in a tank, and passing water through sophisticated heat exchangers.
In my house, which has a conventional fully pumped system, the radiator in
the bathroom is always on when the central heating is, I guess this is so
when the house reaches temperature the water has somewhere to go if one
of the zone valves fails.
It's occurred to me, that you could design a system which uses the radiators
as the thermal store. (Provided the pump was going.)
Has anyone done this?
I think that normally you would keep the thermal store up to temperature
all the time, so that you can get hot water whenever you want it. Using
radiators to do this would mean running your central heating full pelt
all day everyday, even in the height of summer, which would not be
You could insulate the rads to reduce the heat loss, but this would be
an expensive way of stopping the radiators from heating your house in
the depths of winter.
Some systems with thermal stores are arranged rather as the reverse of what
you're suggesting: the thermal store has a heat exchanger from which the
space heating is run. This arrangement tends to be implemented more where
the space heating is true radiators i.e. radiant emitters e.g. underfloor
heating which require lower temperatures of circulating water than so called
'radiators' which are mostly convectors and want higher temperatures. (You
with me so far? :-)
Other conventional ('radiator' based) systems dispense with the always-on
bathroom rad in various ways (which have been discussed at length not to say
ad nauseam in uk.d-i-y, as Mr Google will no doubt tell you if you ask
On Thu, 19 Feb 2004 12:52:40 -0800, "John Stumbles"
I'm not familiar with the systems Mr Stumbles describes, but it sounds
interesting. I may look it up later.
From your initial question it sounds like you are trying to apply the
logic of compressed air systems to heating systems. No doubt you are
aware that when rotary air compressors began to be introduced into
industry the theory was that there would be no need for a central
storage unit. The theory was that the piping in the system would act
as the storage, and since the the compressor would be running
constantly in either loaded or unloaded state, the pressure in the
system would be constant. In practise it didn't really work out that
All the shops I have worked in prefer rotary to reciprocating
compressors, but they all eventually incorporated a central storage
tank such as comes with a reciprocating compressor. It just works
Applying this to heating, I would suggest that even with the water
being circulated constantly through the system with a pump you are not
going to be able to get away from a central storage system. Every
large system I've ever seen has a circulating pump (or several) in the
system so that the water at the tap is hot as soon as you turn it on.
The problem with that is the inevitable heat loss through the pipes,
even when they are insulated. It just wouldn't be practical or cost
effective trying to keep the water hot with just the pipes to store
it. Just like they told you in 8th grade biology, the nearer a bodys
volume to area ratio is to 1:1, the faster that body loses heat. A
central storage (or multiple storage) system works better for the
purpose, and the bigger the better.
In a situation where you use the radiators in a heating system as
storage this problem would be compounded. I get the impression that is
what Mr Stumbles was driving at. I've only ever worked on the one
floor heating system, but that also had a separate storage tank where
the glycol was heated.
Does that help at all?
firstname.lastname@example.org wrote in message
This is true it seems from the replies, however, since the tax isn't
going to be at any pressure there's a further possibility.
Someone would make a tank that is the shape of the underside of a bath.
This way it would take up useless space, and there'd be no need for
an airing cupboard.
In fact the tank could be any shape.
The heat in a full heating system can be used to pre-heat cold mains
pressure hot water. This can be done. I have seen this done using two plate
heat exchangers and a system boiler. A heating engineer relative of mine did
this, with my assistance. The system should not have thermostatic rad
valves, or few of them.
- A plate heat exchanger is on the return of the heating system,
- A plate heat exchanger is on the DHW only pipe loop.
- The DHW and CH have their own pumps.
- A 3-way "diverter valve is on the CH return to the boiler, and when DHW is
called this moves to divert the return water back to the rads by-passing the
- Just before the 3-way valve the pre-heat plate heat exchanger is located.
The cold mains water runs through the plate heat exchanger on the CH return
(the heating pump is on) pre-heating the mains water using the stored heat
from the rad circuit. This pre-heated main water then runs through the DHW
plate heat exchanger, which is being heated directly from the boiler, taking
the full power of the boiler.
You can fill a bath up in a few minutes doing it this way. The rads cool
down a lot. This doesn't matter as when the system switches over to CH, the
boiler re-heats the rads ASAP, with loss in room temp so small it is not
noticeable to the occupants. The control system has to be designed to suit
The flow rate in summer, when the CH is off is better than an average
flowrate combi as the water in the rads will be around 20C when the CH is
off. This stored 20C heat is used to pre-heat the cold mains water, which is
around 10-12C. Depending on the efficiency of the plate heat exchanger and
power of the boiler, the flow rate may be very good, even in summer.
Cooling the rads also helps to cool the house in summer too.
So, a normal system boiler and no cylinder and in winter very fast bath fill
So in effect you create one pumped circuit of radiators plus first
heat exchanger and another of boiler and second heat exchanger?
.. and probably quite quickly since the first heat exchanger will have
very cold mains water and have quite a transfer rate. A typical
domestic system will have 60-100 litres in the primary circuit so
there is a reasonable amount of stored energy but at the likely
temperature, less than a thermal store. Interesting nonetheless.
Much less of a pre-heating effect.
Not significant. Radiators are not good collectors of heat so the
main effect would be when the water has been stationary in them for a
while and is then circulated through the first heat exchanger. They
will then cool, but the heat extracted from the room is not going to
be a lot.
The chilled water in the radiators is likely to result in condensation
on them as well.
To email, substitute .nospam with .gl
Enough to fill a bath very quickly, and have two high pressure showers
easily enough, or three average showers. It bumped an approx 12 litre.min
performance to approx 20 litres/min in winter. Most people,bath in winter
rather than summer, so perfect. Even in summer the bath fill up is very
Of course as the water in the rad circuit is only around 20C. Nevertheless,
the pre-heat does contribute. 10-12C mains water can be taken up to approx
20C by pre-heat, just taking heat from the rad circuit.
It does assist in cooling the house.
There has been no condensation on these rads yet. The temperature of the
rads doesn't get cool enough, also in summer there are more windows open and
ventilations takes away moist air from the house.
It works. We did it.
It could be honed to be more efficient, here and there. One way is to have
the cold water mains that supplies the DHW laid under a concrete floor of a
southish facing conservatory in 22mm pipe. This also acts as a pre-heat. So,
1st stage pre-heat, the conservatory. 2nd stage the heat stored in the
system, the third the boiler. It is extracting heat from around the building
to do something useful, rather than waste space with stored water cylinders.
The heat gained from the conservatory is free via solar gain.
Heat can be stored in large piped solar collectors too; let the collectors
have lots of large pipes and that be the water store. Once again free solar
gained heat too.
He wanted a secondary circulation loop in this system as some taps are quite
way giving a long lag. This was easily done by taking the hot draw-off to
the taps back to the cold mains after the first plate heat exchanger. From
where this loop tees back into the cold water mains, from this tee to the
2nd plate heat exchanger was 28mm pipe with heavy insulation. This acted a
water store too, to avoid any lags in response to the taps. This can be
done with any combi boiler too; the loop is maintained to the temperature
set by the boiler and a 28mm pipe before the boiler is the water store.
In short, the system did act as a water store; a store that would have been
laying idle. A powerful boiler assisted this store of water, and replenished
the "system" store very quickly.
I like the look of this one. Well done to IMM. Assorted quotes.
But that isnt a problem in itself. When you're filling a bath it makes
little difference if the first half of the fill is faster or hotter
than the second, its the total fill that matters. Extracting the CH
heat will improve fill speed relative to simple boiler output alone.
I find this an odd comment though :)
In summer, taking just 10C of heat from the rads is going to reduce
summer HW heating costs a little: the heating temp rise will be about
20-65 = 45C instead of 10-65 = 55C. Thats in the region of 20%
reduction, and thus also 20% more heat / flow at the tap than you
would get by just running the cold mains through the boiler.
Now onto the conservatory bit:
floor of a
While this could be done, it is a particularly inefficient way of
doing solar heating, and would have no real hope of paying for its
not at all, it would be especially expensive. Piping and plumbing is
There are other ways to improve the heat delivery to the bath and cut
energy use as well. Another heat exchanger under the bath, operating
between the cold bath tap /shower and the bath waste pipe will recycle
much of the heat that goes down the drain during showers, and will
thus enable a HW system of limited capacity to deliver much more
performance, since the HW is mixing with a warmer cold supply. And it
looks like thats just whats needed on this system. And from what I've
seen the payback looks good for these.
I have done it. See the clip below froma previous post.
The solar gain is free. It is cheap enough to do on a conservatory
installation. A retro would be more expensive. See clip below.
Do a Google on subject "Designing DHW... Fuel costs - electric is cheaper !"
This is covered. A dedicated unit is available. Having the drain water
heat recovery unit on the shower is the most cost effective.
So, doing what I suggested above: by having a conservatory floor pre-heat
cold mains water, recover heat from a shower drain, use the heating system
as a thermal store and the power of a largish boiler great economies can be
had and decent flow rates.
Regarding UFH in a conservatory. A neighbour recently had a conservatory
fitted. I suggested UFH in this, as the back wall of the conservatory was
the garage. I also recommended insulating the garage wall on the inside
with Cellotex too, to reduce heat loss of the conservatory in cold weather.
This he did. He has a combi boiler delivering 13 litres/min. I also
suggested installing alongside the UFH pipes 22mm plastic pipes too. In
fact far more of it than the UFH, as much as could be installed in a large
loop. This 22mm UFH pipe was to take in the cold water mains that supplies
the combi. He fitted it all in 28mm to get more water volume under the
floor. The advantage was that in winter the UFH would heat the floor and
the cold water mains would gain heat from the floor, pre-heating the combi
water giving a higher flowrate. In summer, the sun playing inside the
conservatory would pre-heat the 22mm mains pipe under the floor. Also the
cold water entering would cool the floor cooling the conservatory.
The total volume of mains water in the pipes under the floor is about 30
litres. The conservatory came on line in August and immediately the rise in
mains water temp was apparent just by the sun playing on the floor. The
floor was notably cool, especially in periods when water was being drawn
off. Also water absorbs 3-4 times more heat than masonry.
Now it is in winter the mains water is pre-heating by the UFH to well over
25C. Not bad. He fills his baths far faster, not to mention the savings on
DHW bills too. Also the sun plays inside the conservatory, even when 3C
outside and heats it up, heating the floor too, which in turn pre-heats the
water supplying the combi. Very simple and very effective so far, and
nothing to run and never goes wrong.
An integrated thermal store or heat bank. Conventional rads can, and are,
easily run from an integrated thermal store. The whole store can be
maintained at a high temperature. Also sections of the store can be
maintained at different temperatures for differing applications: top section
at 80C for DHW, middle sections at 75C for upstairs rads, bottom section at
a lower temperature, 45C, for UFH. The bottom section of a thermal store,
that supplies the heating, can have the temperature varied, dictated by an
outside temperature weather compensator.
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