Boiler Not Heating Radiators

Faulty diverter valve (combi boilers)

On combi boilers, the diverter valve switches between heating and hot water. If stuck on the hot water setting, radiators get no heat. A common fault on older Vaillant and Worcester Bosch boilers.

Motorised zone valve failure

System and heat-only boilers use motorised zone valves to direct hot water to heating or the cylinder. A failed valve means an entire zone receives no heat.

Pump failure

If the circulating pump fails, heat cannot distribute from the boiler. The boiler itself fires, but rooms stay cold. The pump can usually be replaced on the same visit.

Sludge blockage

Magnetite and sludge from corroded radiators can block the pump and heat exchanger, restricting or stopping circulation. Power flushing restores flow.

Incorrect thermostat or programmer settings

Check your programmer, room thermostat, and TRVs before calling. A TRV stuck closed or a programmer on "off" is a surprisingly common cause.

Air lock in system

Air trapped in the pipework or pump can prevent circulation. Bleeding radiators and checking pump operation resolves most air lock issues.

Why does my hot water work but not my heating?

On a combi boiler, this is almost always a stuck diverter valve. On a system boiler, it's usually a failed heating zone valve. Both are one-visit repairs.

My boiler is on but all radiators are cold — what do I check first?

Check: 1) programmer is set to 'heating on', 2) room thermostat is set above room temperature, 3) boiler pressure is above 1 bar, 4) any TRVs are open. If all check out, call an engineer.

How long does it take to fix a heating circulation problem?

Most diverter valve and zone valve replacements are completed within 1–2 hours. If a power flush is needed, allow 4–6 hours for a typical London property.

Can I bleed my radiators myself?

Yes — use a radiator bleed key (available at any hardware store) to release trapped air. Hold a cloth under the bleed valve, open slowly until water drips out, then re-tighten. Re-pressurise the boiler after bleeding.