The central heating system only seems to work if the hot water is also switched on. The control unit is a Siemens RW829 and the thermostat (in the hall) is a Horstmann HRT3. We have a Solar Twin (water) solar panel on the roof and want to use this to boost the domestic hot water. When we switch the CENTRAL HEATING control "ON" and turn the thermostat up so that the LED glows for a high temperature, the radiators do NOT get warm. As soon as the HOT WATER switch is turned on at the control unit the radiators start to warm up and if the HOT WATER switch is turned off (even though the central heating is ON) the radiators cool down. It appears that the central heating is "tied" to the hot water and not independent. This is confirmed when we have a tank full of very hot water the central heating system will NOT work.
Older heating systems required the hot water to be on before the heating to the radiators would work. The programmers would automatically switch the water on whenever the heating was selected. If the timer programmer has been replaced without modifying the pipework this would cause the problem you describe.
Search for Sundial heating plans and identify which type of system you have installed.
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