Extract from 'Central Heating - Fault Finding & Repair'

Introduction
On every occasion the boiler fires up the circulating pump should run and when the boiler turns off the circulating pump will either stop immediately or will run for a short time and then stop. When the pump runs on for a short while it will remove latent heat from the boiler. This is called pump ‘over-run.’ Circulating pump faults will either be:-

1. Pump failure
2. Pump speed low
3. Pump noise

Failure
There are four ways a circulating pump can fail to start:

1. No electrical supply
2. Worn out bearing
3. Rotor seized
4. Burnt out motor windings

Electrical Supply
If the pump does not start it may be there is no electrical supply going to the pump.

1. Remove terminal cover
2. CAUTION - using a multi-meter check the voltage at the pump terminals. It should read 230V across the brown or red (live) and blue or black (neutral)
3. If there is no voltage the problem is at the boiler (check terminals for voltage), cylinder thermostat,
programmer, room thermostat etc
4. You can check the circulating pump has failed by:
    a) Turning off the boiler and the electrical supply to the pump
    b) Remove the three wires from the pump terminals
    c) Using a test lead fitted with a plug and 3 Amp fuse, connect the three wires to the pump terminals        (Brown-Live, Blue-Neutral, Green/Green and Yellow-Earth)
    d) Plug the test lead into a socket
    e) Switch the socket on and if the pump runs then the fault is not with the pump but with another
        system component.

Note: If the pump does not start immediately turn off the electrical supply or the pump motor (windings) may
burn out.

Worn Out Bearings
If the bearings are worn out then the pump will need to be replaced. When checking for this be prepared for water spillage.

1. Turn off the electricity supply to the boiler and pump
2. Close the isolating valves on either side of the circulating pump
3. Remove the vent plug in the face of the circulating pump. Mop up any water spillage
4. Slowly open one of the isolating valves
5. If water pours out then the bearings are worn out. Mop up the spillage
6. Close isolating valves
7. Replace vent plug
8. Fit new pump

Rotor Seized
The rotor may have seized due to particles of rust/sludge or scale preventing it from rotating. When checking for this be prepared for water spillage.

1. Turn off the electricity supply to the boiler and pump
2. Close the isolating valves on either side of the circulating pump
3. Remove the vent plug in the face of the circulating pump. Mop up any water spillage
4. Slowly open one of the isolating valves
5. If just a few drops of water seep out fit a 3mm screwdriver through the vent plug opening and gently turn the     shaft anti-clockwise
6. If the rotor shaft turns freely (anti-clockwise) replace the vent plug, open the isolation valves, reconnect
    the power, turn the boiler on and check the pump is working correctly
7. If the rotor shaft feels ‘rough’ as it spins or it will not move a new circulating pump is required
8. Close the isolating valve
9. Replace vent plug
10. Overhaul the existing pump or fit a new circulating pump

Burnt Out Motor Windings
Having checked electrical supply and the rotor has been freed and now turns freely (anti-clockwise), but the pump does not work then the motor has burnt out due to the rotor seizing and causing overheating in the motor windings. Fit new pump.

Extract from 'Central Heating - Fault Finding & Repair'