Central heating pump seized or humming — how to free it

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.

Once the shaft rotates freely, refit the bleed screw tightly. Restore the electrical supply and observe the pump through one heat cycle — listen for normal running (a gentle hum, no rattling) and check that all radiators are heating.

If the pump cannot be freed by the above method, or if it is over 10 years old, replacement is the sensible option. A Grundfos or Wilo pump replacement costs £60–£120 for the pump; a plumber will charge 1–2 hours labour. A new pump is a worthwhile investment — modern variable-speed pumps are also significantly more energy-efficient than older fixed-speed models.