"Andy Hall" wrote in message
...

On Sat, 28 Feb 2004 14:17:05 -0000, "IMM" wrote:

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.

So in effect you create one pumped circuit
of radiators plus first heat exchanger and
another of boiler and second heat exchanger?

yes.

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.

.. 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.

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.

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.

Much less of a pre-heating effect.

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.

Cooling the rads also helps to cool
the house in summer too.

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.

It does assist in cooling the house.

The chilled water in the radiators is likely
to result in condensation
on them as well.

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.

So, a normal system boiler and no cylinder
and in winter very fast bath fill ups.

Interesting idea.

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.