With the combi boiler this is an odd one. What may work is to put a small mains flow hot water tank in the pipe work loop of the central heating. This would be heated with an immersion heater with its own thermostat. So when CH is off the water tank is heated by PV, and would turn off when water at max temperature. When the CH is ON the boiler will operate normally.
This is the tricky bit, if the water tank is on the outward side of the boiler then when the boiler starts it is pumping all the cold water in the CH system before it gets boiler gets up to temperature. During that time the hot water tank will not cool down, and the boiler will always see the cold return water first.
If the hot water tank was on the return side of the boiler then very quickly the boiler will be seeing hot water so the boiler turns off. Eventually the hot water tank will get colder and the boiler will see colder return water and start automatically.
If the solar PV was not enough to power the CH then the boiler will switch on normally
Unfortunately it doesn’t work in real life. The water tank temperature will not go below the CH return temperature, which at the end of a central heating period will be fairly warm. The insulation is so good that only a small PV heating top up is needed. Nice idea starting from cold but not daily.
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A traditional hot water tank is at low pressure, but a mains flow hot water tank Like Santon would work.
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