Replacing a central heating pump

Summary: Learn how to remove a central heating pump and replace a central heating pump.

If you need to replace a central heating pump make a note of the old pump’s specifications, as this will assist your supplier to provide you with a suitable replacement. The details you will require are:

  • The length of the old pump
  • The type and diameter of its connections.
  • The setting of the output regulator (domestic pumps can have different ratings)

Today domestic pumps are a standard size but if the pump you are replacing is quite old it may be longer than the new models. This means that adaptors will have to be fitted on the ends of the pipes in order for them to reach the connections on the pump. The majority of new pumps have 38mm (1½ in) threaded connections.

Removing a pump

  • Before you attempt to remove the old pump, turn off the power to the relevant circuit at the consumer unit.
  • Make a note of which cores (Live, Neutral and Earth) connect to which terminals in the back of the pump. Making a diagram or labelling the cores is also a good idea. Having done this, disconnect the cores from the terminals with a screwdriver.
  • The old pump may have isolating valves either side of it. These will either have a wheel handle or a spindle which should be turned with an adjustable spanner. By closing these down the pump can be removed without draining the system. If there are no valves fitted, you will have to drain the central heating system.
  • Place towels and a bowl or bucket under the pump to catch any water that leaks from the pump when it is removed.
  • Using a wrench and holding the pump in place, unscrew the union nuts either side of the pump, turning them anti-clockwise. The pump will become loose and you will be able to remove it.

Replacing a pump

  • Position the new pump between the pipes with new sealing washers in place to prevent leaks.
  • Tighten both union nuts clockwise until the pump is held securely in position.
  • Turn the water supply back on either by opening the isolation valves or refilling the central heating system. Check the union valves are watertight and that there are no leaks.
  • Make sure the pump is totally dry before reconnecting the cores to the terminals. Consult the diagram or notes you made earlier and connect the cores to the correct terminals.
  • To test the pump is working efficiently, restore the power at the consumer unit and turn on the central heating at the programmer or timer. To get the system working quickly it is also a good idea to turn up the room thermostat.
  • With the system running it is important to check that the open safety-vent pipe over the feed-and-expansion tank is not discharging water every time the pump starts and stops. If this does happen, turn off the system and call in a professional heating engineer.
  • If you have had to add water to the tank, bleed the system to remove any airlocks, which will help prevent corrosion and protect the pump. This is achieved by bleeding the radiators starting from the lower floor and working up to the top floor.
  • You should also bleed the new pump. With the pump switched off, open the screw-in bleed valve located on the pump’s outer casing. Use a screwdriver or valve key to open the valve slightly until you hear air hissing. As soon as the hissing stops and water appears, close the valve. 

Tools needed

  • Wrench
  • Screwdriver OR valve key

Materials needed

  • New central heating pump
  • Sealing washers

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