wrote in message
...
On Thu, 19 Feb 2004 12:52:40 -0800, "John Stumbles"
wrote:

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 :-)

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.

Does that help at all?

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.