Central heating pump seized or humming — how to free it
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →⚠️ Isolate the pump electrically before attempting to free the impeller. The pump body contains pressurised hot water — open the bleed screw carefully with a container underneath.
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Most likely cause & what to check
The central heating pump circulates hot water around the radiator circuit. A pump that hums but does not produce flow has an impeller (the spinning rotor inside) that is seized — usually from a summer shutdown where sediment has settled and bonded the impeller to the pump housing.
Turn off the boiler and the pump electrics. Allow the system to cool for 30 minutes — the water inside is under pressure and hot.
Locate the pump bleed/access screw: this is a large slot-head screw on the pump body (usually a 6mm wide slot). Place a cloth underneath and a small container nearby. Unscrew the bleed screw slowly — water will drip out, which is normal. Once the dripping stops, the internal pressure is relieved.
With the bleed screw removed, insert a large flat-head screwdriver into the slot — you should be able to reach the end of the pump shaft. Turn the shaft with the screwdriver clockwise and anti-clockwise. If it is seized, apply gentle but firm turning pressure — often a seized impeller can be freed with a bit of persistent effort.
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Frequently asked questions
How do I know if my heating pump has seized?
Classic signs: boiler fires and makes its normal flame noise, but radiators stay cold after 20 minutes. The pump body may be very hot to the touch if the motor is running but the impeller is stuck. You may also hear a humming sound from the pump without the normal slight vibration of water movement.
Can a seized pump be fixed or does it need replacing?
A recently seized pump (usually due to summer inactivity) can sometimes be freed by the technique described in this guide. A pump that seizes repeatedly, makes grinding noises, or leaks from the shaft seal needs replacing. Pump replacement costs £150–£350 fitted and is a standard job for any Gas Safe engineer.
How long do central heating pumps last?
A quality central heating pump (Grundfos, Wilo, Salmson) should last 10–15 years. Pumps on heavily sludged systems fail sooner as debris damages the impeller. Fitting a magnetic filter protects the pump alongside the boiler heat exchanger.