well a very low flow would be needed not to upset the stratification and boilers supply at much higher figures...

You go straight to he heart of the non existent debate as to what is the most energy efficient way to heat a house... what I call "panic" heating, that is the I feel cold model lets light a fire and get nice hot radiators , or a "trickle" heating model that seeks to maintain a controlled temperature and at the same time using gas or other sources of heat efficiently.

Gas boilers are at their most efficient at low flow temperatures so it makes sense to use them as they are designed with of course compensation controls.

wood burners on the other hand have to be very high temperatures and must be less efficient than gas by nature of their high temperatures in the flue.

combining the two would (without knowing the loads) mean a buffer tank for the stove that can get as hot as it needs, and trickle heating through mixing valves the flow temperature to a compensated radiator circuit. the gas boiler would kick through the same control system.

alternatively the wood burner could preheat one of two dhw cylinders...

PV for immersions would presumably displace electricity pulled from the grid. Much better to reduce the load on a gas boiler heating the hot water with solar thermal...selling the PV back to the grid...

Thanks Alec, I understand what you are saying now, interesting problem, never brought up by the TS suppliers, they all show nice pic with lots of inputs including gas boiler :-) Kind of disappointing as had my heart set on the simply concept of one big tank!

One thing that is bugging me though, in a traditional setup with a gas boilers and DHW tank the boiler is heating DHW to high temps though? I guess the difference is that the DHW coil in a normal tank ensures the return temp is low, unlike a TS where the stratification may have been disturbed so its difficult to ensure a low return temp.

Could you expand on how you would solve the problem then not sure I got what you were saying and if anyone else has any info / diagrams on a configuration to combine gas boiler, solar thermal for DHW and contributing to space heating, as well as wood stove that would be great?

Posted By: Phil.Chaddah-DukeOne thing that is bugging me though, in a traditional setup with a gas boilers and DHW tank the boiler is heating DHW to high temps though? I guess the difference is that the DHW coil in a normal tank ensures the return temp is low, unlike a TS where the stratification may have been disturbed so its difficult to ensure a low return temp.

Yes gas boilers have what is called hot water priority so they heat the dhw cylinders at max out put and high temperature as required. The rest of the time they will be following the compensated demand for space heating at the lowest temperature necessary. Given that space heating is usually the biggest demand it makes sense to use the heat source as efficiently as possible.

Posted By: Phil.Chaddah-DukeWhy do you say "export it where it can be used more productively" is heating water with electric very inefficient?

Not at all. Resistive heating is very very efficient. I think his point was more that grid electricity is very dirty. A kWh from your PV array used to displace a kWh of gas in your home only saves about 180g of CO2 emissions. If you sent it to your neighbours and it displaced a kWh of grid juice it would save 520g of CO2.

So the altruistic thing to do would be export, although the financial incentive points strongly towards exporting as little as possible. The problem is the pricing structure of the FIT discourages people from being green. Bit of an own goal there.

Posted By: Phil.Chaddah-DukeAh, now I see the point, the CO2 figures make it VERY clear :-) Thanks.

Precisely. (Thanks to Seret for beating me to the answer!)

well a very low flow would be needed not to upset the stratification and boilers supply at much higher figures...

A system boiler doesn't get to control it's own flow rate - that depends on the pump settings. I donlt know if the boiler will simply turn off, rater than modulating correctly if the pump rate is too low? Sadly they don't give us knobs to twiddle for that.

An alternative is to use a mixing valve and recirculate some of the water. That trades off flow rate into the TS against somewhat increased return temp. I think it can be done (and intend to try) but it is quite complicated to analyse.