Heating Not Working but Hot Water Is? Common Causes and Fixes

A surprising number of call-outs turn out to be a thermostat set too low or a timer in the wrong mode. Turn the thermostat up a few degrees above the room temperature and listen for the boiler and pump starting. See heating controls explained and boiler pressure explained.

A combi boiler uses a diverter valve to send hot water either to your taps or to the radiators. If the valve sticks in the hot water position, the taps work perfectly but the heating never comes on. Sometimes radiators get warm only when a hot tap is running, which is another sign of a diverter fault.

Diverter valves can sometimes be freed, but usually the valve or its actuator needs replacing by a Gas Safe engineer. See combi vs system vs regular boiler.

Homes with a hot water cylinder use motorised valves (zone valves) to switch between heating and hot water. If the heating zone valve fails – a motor burns out, or it sticks shut – hot water still works but radiators don't. Some valves have a manual lever on the head you can move to 'open' as a temporary fix. Engineers can usually replace the motor head without draining the system.

Homes with more than one heating zone may find only one zone has failed. See heating zones explained.

If the pump has failed or seized, water won't circulate to the radiators, though in some systems hot water may still be produced. You may hear the boiler firing briefly and cutting out. Airlocks can also stop circulation, especially after work on the system. Bleeding radiators sometimes helps; airlocks in pipes may need an engineer.

See how to bleed a radiator and noisy central heating.

If the boiler is old and parts are becoming hard to find, a repair may still be worthwhile, but it's a good time to compare with signs you need a new boiler. To book help, contact a local Gas Safe engineer.