Your boiler keeps turning on and off repeatedly, sometimes every few minutes. Here are some quick checks you can do:
DIY Checks You Can Do
Check system pressure – Should be 1–1.5 bar when cold. Low pressure can cause short cycling.
Check thermostat location – Is it in direct sunlight or near a radiator? This can cause false readings.
Check thermostat batteries – If wireless, flat batteries can cause erratic behaviour.
Listen for unusual noises – Kettling, banging, or pump noises indicate problems.
Feel radiators – Check if they’re all heating evenly.
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What Is Short Cycling?
Short cycling is when your boiler fires up, runs for a few minutes, then switches off, only to fire up again shortly after.
Normal operation: Boiler runs 10–20 minutes, reaches temperature, stays off, then fires again when needed.
Short cycling: Runs 2–5 minutes, switches off, then immediately fires up again.
Common Causes of Short Cycling
Oversized Boiler
If your boiler is too powerful for your home, it heats water too quickly, reaches temperature, then cycles repeatedly.
This is common when boilers are replaced without proper sizing calculations.
Low System Pressure
If pressure is too low, the boiler may fire briefly then shut down for safety.
How to check: Look at the pressure gauge – it should read 1–1.5 bar when cold.
Solution: Repressurise your boiler using the filling loop (check your manual).
Faulty Thermostat
A thermostat in the wrong location (near radiators, in sunlight) or one that’s faulty can cause short cycling.
Check wireless thermostat batteries and consider relocation.
Blocked Heat Exchanger
Sludge or limescale buildup restricts water flow, causing overheating and shutdown.
Signs include kettling noises or reduced hot water flow. Professional cleaning or power flushing is required.
Pump Problems
If the pump isn’t working properly, water doesn’t circulate, causing overheating.
Radiators may heat slowly or unevenly. The pump may need replacing or adjusting.
Blocked Condensate Pipe
A frozen or blocked condensate pipe can cause the boiler to shut down.
Listen for gurgling sounds or check for error codes. Thaw or clear the blockage.
When to Call an Engineer
After checking basics – If pressure is correct and thermostat seems fine.
Error codes showing – Professional diagnosis needed.
Unusual noises – Kettling, banging, or grinding sounds.
Boiler is old – Older boilers develop multiple issues.
Problem getting worse – Cycling becoming more frequent.
No heating or hot water – When it affects comfort.
Why Short Cycling Damages Your Boiler
Short cycling increases wear on components, raises fuel bills, stresses ignition parts, and reduces boiler lifespan.
It’s worth fixing promptly to avoid bigger problems later.
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