Boiler keeps firing and cutting off repeatedly
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →⚠️ Short-cycling causes excessive wear on the boiler and should be investigated by a Gas Safe engineer. Do not ignore it.
Short-cycling — where your boiler fires up, reaches temperature almost instantly, then shuts down only to fire up again a few minutes later — is one of the most common heating complaints we see in UK homes. It's particularly frustrating because your heating feels like it's working, but inefficiently, noisily, and at the cost of premature wear on expensive components. The problem can stem from several causes, ranging from simple adjustments you can make yourself (like TRV settings) to more serious issues like a failed expansion vessel that absolutely requires a Gas Safe engineer. This guide walks you through the most likely culprits in order of frequency, so you can identify and fix the issue before short-cycling damages your boiler beyond repair. If you're not confident adjusting your boiler settings or suspect a pressure or pump fault, it's well worth calling a qualified heating engineer rather than risk further damage.
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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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Frequently asked questions
Is short-cycling dangerous?
Short-cycling itself isn't immediately dangerous, but it causes serious long-term damage to your heat exchanger and other components, leading to expensive repairs or replacement. If you suspect a pressure fault (expansion vessel), that does need urgent professional attention to prevent system damage.
Why does my boiler cycle on and off when heating is set to maximum?
When your flow temperature is set very high (80°C+), the boiler reaches that temperature so quickly that the heating circuit has nowhere left to distribute the heat, so the boiler shuts off. Reducing the flow temperature to 60–70°C on a combi boiler allows the system to heat more gradually and run for longer, more efficient cycles.
Can closed radiator valves cause short-cycling?
Yes — if most of your TRVs or radiator valves are closed, there's very little load for the boiler to heat, so it reaches temperature almost instantly and shuts down. Opening more radiators creates greater demand and longer, steadier burn cycles, which is much better for your boiler and your heating.
What does a faulty expansion vessel have to do with short-cycling?
An expansion vessel absorbs the pressure surge when water heats up; if it fails or is too small, pressure spikes rapidly each cycle, triggering the pressure relief valve to close the boiler down prematurely. This is a Gas Safe job because it involves the pressurised heating circuit, and getting it wrong can damage your system further.