"Tony Bryer" wrote in message
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On Thu, 29 Dec 2005 18:23:24 -0000 Doctor Drivel wrote :
Not so rare. Look at the Eco-Homtec site as they have a boiler
dedicated for the task. It really improves combi performance
having the inlet raised to 25C and above.

You could do much the same at less cost by having a do-nothing store
in the combi feed. The water would gradually come up to the house
ambient (of course this heat is being provided by the combi) so go in
to the combi at 15-20 instead of the winter 5 or whatever.

It would have to be tall thin cylinders with no insulation, and depends on
frequency if use. They take a time to get to ambient if cooled quickly to
5C will quite a time rise.

You could use the heat stored in radiators and use then radiators as a store
of heat. they would cool rapidly and when switched to CH re-heat rapidly
too. They will give 20-25C in summer and maybe even cool the house.

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 a plate
heat exchanger, pump, flowswitch and two check valves. The system should
not have thermostatic rad valves, or few of them. On a combi system:

- a by-pass pipe between the flow and return at the boiler
- on this pipe a re-heat plate heat exchanger is fitted
- The pump is fitted between the plate and the return by-pass pipe tee to
the return.
- A check valve between the pump and the plate
- A check valve on the boiler flow before the tee to the by-pass pipe.
- A flow switch on the cold mains water before the pre-heated plate heat
exchanger.

The check valve on the flow pipe ensure no flow back into the boiler,
although the internal 3-way valve should do this.
The check valve on the by-pass pipe ensure no short circuit in normal CH
operation.
The cold mains water runs through the pre-heat plate heat exchanger. This
pre-heated main water then runs into the combi as normal. When calling for
DHW the combi diverts to DHW only to heat the incoming cold water.
The flow of mains water is detected by the flow switch and switches on the
by-pass pump. This pumps water from the rads into the pre-heat plate heat
exchanger. This will raise the mains water substantially and the combi tops
up.

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 combi 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 20 -25C when the CH
is off. This stored 20C plus 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.

A simple and cheap way to vastly improve the output of a combi.