Boiler Short Cycling — Keeps Turning On and Off
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →Do not attempt to adjust burner settings or internal boiler components yourself. If resetting and basic checks do not resolve short cycling, call a Gas Safe engineer.
Short cycling — where a boiler fires for 1–3 minutes, shuts down, then fires again — wastes fuel, increases wear, and significantly shortens boiler lifespan. It is caused by one of several issues: oversizing, a faulty thermostat, low system pressure, a blocked heat exchanger, or a pump problem.
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Most likely cause & what to check
Check the boiler pressure gauge. If pressure is below 1 bar, repressurise the system using the filling loop (usually a flexible silver hose under the boiler). Low pressure causes the boiler to shut off on a safety cut-out.
Check the boiler is not "overfiring" for the load required. If the boiler is a high-output combi (e.g. 35kW) in a small property with short pipe runs, it will short cycle by design. The fix is to have an engineer reduce the output via the boiler's service menu.
Check the thermostat location — if it is near a heat source (sunny window, oven, other radiator), it will reach temperature quickly and cut the boiler off early. Move the thermostat or add a remote sensor.
Bleed all radiators to remove trapped air. Air in the system reduces heat transfer, causing the boiler to overheat and cut out on a high-limit stat.
Check if the pump is running when the boiler fires — put your hand on the pump body (near the boiler) and feel for vibration. If the pump is not running, the boiler will overheat and short cycle. A seized pump needs replacing (£100–200 for parts + labour).
If none of the above resolves it, the heat exchanger may be partially blocked with scale or sludge, causing rapid temperature rise. A Gas Safe engineer can check the flue gas temperature differential to confirm this — a power flush or descale may be needed.
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