The boiler heats the domestic hot water cylinder but does not supply central heating, even when central heating is calling.

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

Possible Causes

Stuck 3-port mid-position valve, Failed 2-port Central Heating zone valve, Faulty microswitch in the valve head, Wiring centre fault

Safety: Isolate power before removing valve heads or accessing wiring centres.

User checks:

Confirm that the room thermostat is turned up and the programmer is set to Central Heating ON.
Listen near the motorised valve(s) for movement when Central Heating is selected.

Engineer steps:

Identify the 3-port or 2-port valves controlling Central Heating and Domestic Hot Water circuits.
Check the valve actuator for movement and test the end-switch output that signals the Grant boiler to fire.
Replace the valve head or the complete valve if it is seized or if the microswitch has failed.
Verify correct wiring in the wiring centre to ensure Central Heating demand is passed to the boiler.

Medium 3/5

3-port motorised valve

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