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.)
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 desirable.
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.
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 way. 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 better.
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.
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 nicely :-)
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 boiler.
- 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 of course.
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 ups.
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.
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 acceptable.
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.
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.
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
pre-heat. So,
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 installation cost.
not at all, it would be especially expensive. Piping and plumbing is not free.
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.
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