To be better able to assess your system it will be necessary for you to include pumps/valves etc. on the schematic.

I would expect the pipework from the gas boiler to the thermal store to be opposite to that shown.

What is the make and model of your thermal store?

The tank has a plate that says:  "Chromagen,  Model: BSF750V2M085". There's also lots of other information on the plate.

I've added the pumps and valves to the diagram.

I also wondered about the position of the pipes from the gas boiler, but I checked them several times and I'm sure that's the way they are.

@peterm

Which part of your home has UFH and which part uses radiators?

The ground floor has UFH (there are two separate UFH loops - it's a big house and I didn't put all the details on the diagram). Each UFH circuit is controlled by a room thermostat controlling a valve in the return pipe from the loop. The feed pipe goes through a mixer valve, which I assume mixes the feed and return in order to achieve the correct temperature.

The first floor uses radiators. There are 8 radiators, divided into two circuits, each circuit is controlled by a room thermostat.

@peterm

From the information provided this is how I assume your system is designed to work.

On sunnier days the solar thermal system heats the water in the thermal store, which I believe also acts as the hot water cylinder.

If the UFH systems operates, water is passed through a heating coil within the thermal store and absorbs thermal energy which is supplied to the UFH loops. As you have stated the water temperature may be reduced by the mixing valve.

Having given some thought on the subject, I believe that the pipework from the gas boiler is reversed from the normal arrangement for a reason. When solar energy has been collected within the heat store this can then be used in preference to burning gas. I am assuming that the water pump is within the gas boiler, so to get water to the radiators it is necessary for the gas boiler to operate, but by heating this water in a heating coil within the thermal store before it goes as return water to the gas boiler, the gas boiler may not need to fire or fires at a lower output.

In the Winter months when solar thermal energy may be limited, the gas boiler will not only supply the radiators, but also warm the thermal store and hence supply the UFH and DHW.

I think that the thermal store must therefore contain three individual heating coils, one for solar thermal, one for UFH and one associated with the gas boiler.

Thanks, Derek, that gives me a lot to think about. I'll try to absorb your analysis and get to grips with the system!