Heating Engineers Fault Finder – Component testing pumps

Reasons a pump will fail to start:

  • No 240V supplying (L) & (N) terminals
  • Faulty capacitor
  • Rotor shaft seized or debris preventing impeller rotation
  • Burnt out motor windings
  • Worn out bearings
240v Warning Sign


Always check across red or brown and black or blue with multimeter set on correct voltage scale.

(If no voltage is evident at pump terminals, check for demand via programmer/timer, room thermostat or cylinder thermostat).

Burnt out motor windings

  • Electrically isolate boiler / system controls
  • Remove E N L wires from pump terminals
  • Set multimeter on ohms scale for resistance readings, across L N at pump terminals:
    • less than 100 ohms = replace
    • No resistance/infinity = replace
    • Approx. 120 ohms – 250 ohms = ok


A good indication when 3 amp fuses and or “quick – blow” glass type fuses repeatedly fail is when the motor windings have burnt out.

When 240V is present at the pump terminals and the rotor shaft is free moving (anti-clockwise), but still refusing to mechanically spin, then you know the motor windings have burnt out. (This is due to the rotor shaft seizing at some point and causing the pump motor windings to overheat).

Rotor shaft seized

  • Electrically isolate boiler
  • Remove vent plug on pump head
  • Manually, turn rotor shaft (ant-clockwise) several times (Resistance being felt and or not spinning or spinning but with resistance donates a new pump head).

Worn out bearings

  • Electrically isolate boiler / system controls
  • Remove vent plug from pump head:
    • If water severely escapes, bearings are worn out (new pump needed)
    • If a small amount escapes, this is to be expected and is quite normal.