Boiler keeps firing and cutting off repeatedly
Check the steps below first — if you\'re not confident, get it fixed safely today.
Find a Gas Safe engineer near you →⚠️ Short-cycling causes excessive wear on the boiler and should be investigated by a Gas Safe engineer. Do not ignore it.
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Most likely cause & what to check
Short-cycling means the boiler fires briefly, reaches temperature quickly, shuts off, then fires again within minutes. It is noisy, inefficient and damages the heat exchanger over time.
Check the boiler flow temperature setting — if set very high (above 75°C), combi boilers will cycle quickly because the system reaches temperature before heat can be distributed. Try reducing to 60–70°C on a combi.
Check the system has adequate radiators open — if most TRVs are closed, there is no load for the boiler to heat and it reaches temperature almost instantly. Open more TRVs to create greater demand.
A failed or undersized expansion vessel causes short-cycling — pressure spikes rapidly each time the boiler fires. This requires a Gas Safe engineer to diagnose and replace.
On older systems, a failing pump can cause short-cycling by not circulating water away from the boiler fast enough. A plumber or heating engineer should assess.
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