solar preheating a combi boiler

harryh38

Help on gas saving
Would there be savings if the mains water was solar preheated in an unvented indirect cylinder prior to the combi boiler?
What are the criterior of the boiler and why are not all combi boilers suitable for preheating where the boiler has a gas modulation facility?
Over heat protection by mixing valves could limit the input water temperature to the boiler on the very sunny day.
The boiler manufacturers are not very helpful when asked these question when relating to there older combi boilers ( two years old)
and recommend an upgrade from a "D" rated unit to an "A" rated unit only to improve efficiency by less than 10%
Can someone explain why its not on to just preheat the water, turn the boiler control to it summer setting? Surely there most be a reasonable saving in gas if the boiler only has to heat the mainswater from warm? or have I missed some thing?
Retired Plumber.

Pincher

They totally agree with your logic:

http://www.rinnaiuk.com/rinn/index.php?id=36

See page 8:

http://www.rinnaiuk.com/rinn/assets/templates/shop/Downloads/3_Corporate_Downloads_PDFs/2_Commercial_Hot_Water_Solutions_Brochure.pdf

Note that Rinnai is using a water heater, not a combi.

It's possible pre-heated water would trigger some kind of failsafe response that actually shuts off the gas, to prevent superheating and scalding. The poor combi is already juggling between central heating and hot water delivery, so unless it's designed to cope with pre-heated water, you are only confusing it.

Solar pre-heating is not a problem most of the time, it's the one hot day of the year that can boil your water. I suppose the pressure relief valve should take care of it, but I think you'll find you are supposed to use a open vented cylinder, which just overflows if the store water is overheated. As page 8 shows, you should use an open vented thermal store. The mains cold water picks up the heat by flowing through a coil, but still under mains pressure, which is what you want.

Using an unvented pressurised cylinder means you will live in dread of going on holiday in July, and having nightmares of coming home to an exploded cylinder. How did you manage to fit the one pressure relief valve in a million that fails?