On Mon, 15 May 2006 20:15:35 +0100, Keith D wrote:

"John Stumbles" wrote in message
news

... before going further may I ask:
1: is your house already draught-proofed and insulated to the highest
standards
2: does your heating system already have thermostatic radiator
valves, a programmable thermostats and - if the house is not absolutely
tiny - separate heating zones
3: is your combi already a high-efficiency condensing type?

yes to all of them

OK, good boy ;-)

I that case you may want to investigate thermal store/heat bank
systems to store the heat from the solar system. One arrangement that
suggests itself to me it to have the store supply hot water (at mains
pressure) directly to taps when the water is hot enough, and to use it
as input to the combi when it's too cool to use directly.

However, depending on the design of the combi you may still be limited to
the lower rate of flow that it can provide even though it's having to do
less work, given pre-heated incoming water. A configuration that might
give better flow might be to combine the output of the thermal store
with the output of the combi via a thermostatic mixing valve: combi to hot
input and combi to cold input of the valve. Again you'd only want this
in-circuit when the store can't supply hot enough water.

In each case I guess you might use a 2-port or 3-port motorised valve to
bypass the combi, controlled by a thermostat sensing the temperature of
the store. Although these valves are only intended for closed systems with
corrosion inhibitor they usually have brass bodies and some sort of rubber
(EPDM?) actuator ball which should be OK (if not strictly kosher in terms
of water regs and WRAS approval) for DHW.

Is your solar system going to be low or hi tech? E.g. old radiators in
glass boxes or evacuated tubes. Depending on how much you're spending
there are ways of DIY-ing thermal stores at low cost.