CH only works when calling for HW

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Canaryman
Canaryman ✭
in General Questions & Topics

I need some help to diagnose what might be causing my CH to only work when HW is being called for.

To be clear, the 'S-plan' system has been working fine for ages, but after a recent power failure my CH now only works when HW is being called for. Does this indicate a jammed valve? Other ideas?

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  • GrilledCheese2
    GrilledCheese2 ✭✭✭
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    You might have a faulty zone valve, but it could be electrical rather than a sticky valve.

    When the room thermostat calls for heat the motor in the zone valve is energised and drives the valve open to allow water to pass through to the radiators. When the valve is fully open, a micro switch is operated and makes the boiler switch on.

    Under normal operation the motor energises for a few seconds to open the water valve. If you can hear the motor working then the problem could be with the micro switch. The switch can fail, or if your valve is sticking and not opening fully then the micro switch will never activate.

    There should be a manual lever on the valve you can use to help find the problem. When the room thermostat is calling for heat move the lever and lock it in place. This should force the water valve open and activate the internal micro switch, causing the boiler to switch on.

    If the power head is faulty you can get someone to replace this without replacing the valve body. Avoids having to drain down, refill and vent all the pipework.

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