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How to replace a central heating pump

Updated: 22 October 2026

The new circulator is on the doormat, the boiler is off, and you are wondering how much water will end up on the airing-cupboard floor if you undo the wrong union first. This page sequences a UK wet-system pump swap for readers who can isolate electrics and heating water, from manufacturer install themes: flow arrow, gaskets, and venting. It does not rank products. Choose the 130 mm match on the [heating pump comparison](/hub/heating/heating-pump) after the checks on the [heating pump buying guide](/hub/heating/heating-pump/sub/heating-pump-buying-guide). Peter F. set the sequence against public UK guidance. There was no lab test. A qualifying purchase can earn a commission at no extra cost to you.

The new circulator is on the doormat, the boiler is off, and you are wondering how much water will end up on the airing-cupboard floor if you undo the wrong union first.

Our top picks

Trident Trident TP25-60-130

Best overall

Trident Trident TP25-60-130
9.2/10about £42.99
View offer
Anesty Anesty

Best value

Anesty Anesty
8.6/10about £35.90
View offer
JASSFERRY JASSFERRY A-Rated-AC

Alternative

JASSFERRY JASSFERRY A-Rated-AC
8.4/10about £48.50
View offer
On this page
  1. 1.Before you start
  2. 2.Isolate and drain
  3. 3.Swap the pump
  4. 4.Refill, bleed, and pressure
  5. 5.When to stop and call someone

Before you start

Gather towels, a drain tray, adjustable spanners, a flat screwdriver for the vent screw, and a phone photo of the old label and wiring. Confirm 130 mm length and union size. The Grundfos pump replacement page decodes an old UPS plate. The new arrow must match the old orientation. Plan new seals. Some kits include connectors, others only washers. Gas-train work belongs to a Gas Safe Register engineer. Unfamiliar wiring at the fused spur is Part P territory for an electrician. Do not open the boiler combustion case. This sequence assumes the circulator is on heating pipework you can isolate.

Isolate and drain

Switch the boiler off at the programmer and isolate the fused spur. Prove the terminals dead with a non-contact tester before you touch them. Close isolating valves on both sides of the pump if they are fitted. Without them, drain part of the circuit through a drain-off and a hose. Good isolators usually mean litres, not a full-system flood. Still keep buckets ready. When dripping stops, mark the flow pipe. Photograph the terminals before you disconnect, and only after isolation. If the pump is seized, gentle heat on the unions, kept off nearby plastic, and penetrating fluid beat force that twists copper.
Trident Trident TP25-60-130

Testsieger

Trident Trident TP25-60-130
9.2/10about £42.99
View offer

Swap the pump

Support the old body while you undo the union nuts. Its weight strains soldered joints. Clean the flange faces and fit new seals dry, or with a thin smear of jointing compound only if the manufacturer allows it. Never put thread sealant in the impeller cavity. Offer the new pump with the arrow in the flow direction and tighten the unions until snug, not crushed. Reconnect live, neutral, and earth as before. Many UK pumps are not polarity-sensitive. Earth must stay continuous. Fit the cover before you fill. If reducers came in the box, confirm 1 in or 1½ in before refill.

Refill, bleed, and pressure

Open the isolating valves slowly. On a sealed system, use the filling loop to the cold band in the boiler manual. Around 1.0 to 1.5 bar cold is a common combi band, and only a rule of thumb until that manual confirms it. Bleed upstairs radiators first, then downstairs, then the pump bleed screw until water runs without spluttering. Call for heat and start a three-speed pump on speed 1. Step up only if lag remains after bleeding. Add inhibitor if you lost a lot of water. Give the system an hour. Air can take several passes. If downstream radiators stay cold, partially close TRVs on the hot ones rather than cranking speed. Keep the old pump for a day in case a union weeps.

When to stop and call someone

Call Gas Safe for a gas smell, a burner fault, or any task inside the boiler gas section. Call an electrician if the spur, flex, or earth path is damaged. Call a heating engineer if unions leak twice after a re-seat, if the pump runs hot but the pipes stay cold, or if one radiator stays cold while the others heat. That last case may be balance or a blockage. A screech that remains after bleeding may be speed or duty, on central heating pump noisy. Do not force a shaft that will not turn.

Common questions

Do I need to turn off the boiler and electrics?
Yes. Boiler off, fused spur isolated, and the pump terminals proved dead before you disconnect. A programmer set to off is not the same as an isolated spur. Use a non-contact tester on the terminals you are about to handle.
How much water will I lose?
It depends on the valves and the system volume. With isolators that hold, it is often a bucket. Without them, you are into a partial drain-down through a hose. Plan buckets either way, because a valve that looks shut can still pass.
Which way does the flow arrow point?
From the flow pipe toward the return, through the pump. Match the old unit and the leaflet diagram. A reversed arrow reduces performance and can add noise even when the unions are tight.
How do I bleed after refill?
Radiators from the top floor downward, then the pump vent screw, until water runs without spluttering. Keep a cloth under the screw. On a sealed system, note the pressure before and after, and top up to the boiler manual’s cold band if it has fallen.
When must I call Gas Safe?
Whenever the work would disturb gas pipes, the burner, or the flue. A pump on wet heating pipes you can isolate is a different scope. If you are unsure whether the pump sits inside the gas section, stop. Do not open the combustion case to find out.
Should I lubricate the pump shaft before install?
Follow the manufacturer leaflet. Most sealed circulators ship dry and must not be flooded with oil by the installer. Lubricant in the wrong place can contaminate the motor chamber. Do not improvise a grease routine the leaflet does not describe.

Related pages

Best central heating pump

Heating pump

Heating pump buying guide: 130 mm fit, head and EEI

Grundfos pump replacement: 130 mm UPS footprint fit

Central heating pump noisy: air, speed and replace

Sources

Public UK sources named below. amazon.co.uk prices, stars and review counts on these ten listings were taken from the listing files read on 20 October 2026. Page updated October 2026.

  1. Wilo UK, replacing circulating pumps (retrieved 20 October 2026)
  2. Electrical Times, Wilo compliance checker (retrieved 20 October 2026)
  3. Ideal Heating, Part L and flow temperature (retrieved 20 October 2026)
  4. Approved Document L, Volume 1 (dwellings) (retrieved 20 October 2026)
  5. Grundfos UPS3 installation guidance (retrieved 20 October 2026)
  6. Grundfos, how to find a pump replacement (retrieved 20 October 2026)