One zone not heating — upstairs or downstairs cold
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Most likely cause & what to check
Zone-based heating systems use motorised zone valves (typically two-port valves, 22mm or 28mm) to direct hot water to upstairs and downstairs independently. A failed zone valve is the most common cause of one zone not heating.
Go to the boiler cupboard and listen when the affected zone calls for heat — you should hear a click from the zone valve as it opens. No click suggests the valve motor has failed.
Check the thermostat for the affected zone — confirm it is set above the current temperature.
On some zone valves there is a manual lever on the side. Pushing this open bypasses the motor and allows you to confirm if the valve mechanics are working. Do this as a temporary test only.
A replacement zone valve motor head (e.g. Honeywell V4043, Danfoss ABQM) costs £25–£60 and a plumber or heating engineer can replace it in under an hour.
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