Two thermostat new build

[mjthorne]

Afternoon,

I live in a new build that has a thermostat downstairs to control downstairs radiators. And upstairs thermostat that controls the upstairs heating.

Previously they have worked independently, so can set different temperatures to upstairs and downstairs.

I haven't done anything differently to the system etc. but over the last few days the heating upstairs will only come on (even if the display on the thermostat upstairs is indicating the heating should be on upstairs), if the heating downstairs comes on first.

Any thoughts on why this might have changed and how I can fix it? Thanks

 

[stem]

Usually each of the two zones are controlled by a dedicated two port motorised valve that is opened by the associated thermostat. Inside each of the valves is a small microswitch that when the valve opens the switch operates and starts the boiler.

Most likely the microswitch in the valve controlling the upstairs heating has failed, so the valve opens but the boiler doesn't start, then when the downstairs heating comes on, its microswitch starts the boiler and the upstairs radiators come on as well.

If you have a multimeter and can use it safely it's fairly easy to test. The microswitch uses the grey and orange wires. Usually the motorised valve powerhead can be replaced without needing to drain the system