Your boiler heats water, but the pump pushes it round the house. If the pump slows or stops, the boiler overheats or locks out and the radiators stay cold even though the burner lights.
What the heating pump does and where it sits
A central heating pump is an electric motor driving an impeller, a small set of vanes that spins inside the pump body and drives water along the flow pipe, through the radiators and back to the boiler. Where you find it depends on the boiler:
- Combi and system boilers: the pump is built in, behind the front casing and usually near the bottom. Baxi's EcoBlue System installation and service manual (document 7219708-03) shows a "pump with automatic air vent" inside the boiler, and Ideal's Logic Combi2 installation and servicing instructions (UIN 236183) list a circulating pump among the built-in parts.
- Regular (heat only) boilers on open-vented systems: the pump is often a separate unit on the pipework, commonly in the airing cupboard beside the hot water cylinder, with an isolating valve either side.
Modern boilers also run the pump briefly to stop it sticking. Baxi's EcoBlue System manual says the pump runs for 30 seconds once a week if there has been no demand, and Worcester Bosch's Greenstar i ErP user instructions (6 720 806 946) say the system pump runs "for a few seconds" after 24 hours without a heating demand.
Signs a central heating pump is not working properly
Pump problems usually build up over weeks. Watch for:
- Radiators cold or slow to warm while the boiler is firing, and pipes hot only near the boiler.
- New noises: humming, grinding, rattling or knocking, or gurgling from trapped air.
- No sound at all. If the burner lights but you cannot hear the usual faint whirr, the pump may not be turning.
- Water below the boiler or around an external pump, often from a seal or from the air vent on the pump body.
- Short cycling or lockouts with an overheat or circulation code.
On a combi, the same pump carries boiler water to the plate heat exchanger for your taps, so a pump that has stopped completely usually affects hot water too, while a weak one may only leave the radiators struggling.
Should the boiler pump be hot?
Yes. A pump carrying heating water is warm to hot, because the water inside it is often 60°C or more, so heat alone is not a fault. A hot flow pipe with cold radiators and a cold return pipe suggests the water is not circulating. On a combi or system boiler, judge it by the radiators and the display, not by reaching behind the casing.
Why pumps fail: sludge, air, seizing and electrics
Sludge and magnetite
Radiators corrode from the inside and shed black iron oxide (magnetite), which wears the pump bearings and can jam the impeller. Vaillant's ecoTEC plus installation and maintenance instructions (document 0020238195) tell installers to test for magnetite with a magnetic rod and, if they find it, clean the system and fit a magnetic filter.
Air in the pump
A pump full of air spins without moving much water. Worcester Bosch's Greenstar i System ErP manual (6 720 806 945) words fault A1 281 as "pump stuck or running dry", and the Greenstar 8000 Life manual describes 281 as "pump stuck or running in air". Air gets in after draining, repeated top ups or through a faulty automatic air vent.
A seized impeller
A pump that has stood still for months, or one packed with debris, can stick fast, humming with no circulation.
Electrical faults
The motor, its wiring or the output from the boiler's control board can fail. Ideal's Logic Combi2 manual lists F 12 as "Pump Rotor Blocked" and F 13, F 15 or F 17 as "Pump Fault", telling the engineer to check the pump wiring and replace the pump if the wiring is sound.
Fault codes that point to the pump
These codes can be caused by the pump, although each has other causes too. The wording comes from the manufacturers' documents.
| Brand and code | Manufacturer wording | Document |
|---|---|---|
| Vaillant ecoTEC F75 | "No pressure change detection when starting pump" | ecoTEC plus installation and maintenance instructions (0020239015) |
| Glow-worm F75 | Pressure change "too low when the heating pump is started up" | Glow-worm boiler fault codes page |
| Worcester Bosch A1 281 | "Pump stuck or running dry" | Greenstar i System ErP manual (6 720 806 945) |
| Baxi E125 | "Circulation Fault (Primary Circuit)" | Duo-tec Combi HE A installation and servicing instructions |
| Ideal F 12 | "Pump Rotor Blocked" | Logic Combi2 installation and servicing (UIN 236183) |
| Viessmann F2 | "Temperature limiter has responded" | Vitodens 100-W B1HC/B1KC instructions (5697 787 GB) |
Vaillant's manual lists a defective water pressure sensor, air in the system and a lack of water alongside the pump as causes of F75, so read our Vaillant F75 guide before assuming the worst. Baxi's Megaflo System HE IE manual explains that E125 appears when the boiler temperature has not changed by 1 degree within 15 seconds of lighting, or has twice overshot the set temperature by 30 degrees within 10 minutes. Our Baxi E125 page covers it in detail. Viessmann's measures for F2 include checking the fill level, checking the circulation pump and venting the system. For Worcester, see A1 281.
Before calling anyone, check the pressure gauge reads 1 to 1.5 bar with the system cold, bleed any radiators that are cold at the top, and press reset as your manual describes, no more than 2 or 3 times.
Pump speed, size and how long a pump lasts
Older stand-alone pumps have a three-position speed selector, while newer high-efficiency pumps adjust themselves. Pumps built into combi and system boilers are set by the installer through the boiler's menu: Ideal's Logic Combi2 manual lets the installer set the minimum heating pump speed to 100% or 70%, and Vaillant's ecoTEC plus manual says to set the pump so the flow and return differ by no more than 20°C at full flow temperature.
Turning a pump up rarely cures cold radiators: sludge, air and poor balancing are more common culprits, and an over-speeded pump can be noisy. A bigger pump only makes sense if an engineer calculates that the system needs more flow, for example after radiators are added.
No manufacturer document we checked gives a fixed lifespan. Ten years or more is commonly quoted for a pump in a clean system, and sludge is what shortens it most. A built-in pump is covered by the boiler's warranty, which usually depends on yearly servicing.
Why replacing a boiler pump is engineer work, and what it costs
On a combi or system boiler the pump sits inside the sealed boiler casing, next to the gas and combustion parts. Freeing a seized pump, bleeding it or replacing it means opening that casing, so it must be done by a Gas Safe registered engineer. Worcester Bosch's user instructions put it plainly: "Do not remove the appliance case."
Swapping an external pump on an open-vented system still means draining or isolating it and mains wiring, so use a qualified heating engineer.
The engineer will usually confirm power reaches the pump, check it turns, release trapped air, test the pressure and look at the water for sludge. For wider context, see our boiler repair cost guide.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £60 to £150 |
| Pump replacement, part and labour | Typically £150 to £400 |
| Chemical flush | Typically £150 to £350 |
| Powerflush | Typically £400 to £800 |
| Magnetic filter, fitted | Typically £120 to £260 |
A new pump on a dirty system will fail early, so clean water is the best protection. A magnetic filter catches magnetite before it reaches the pump, and inhibitor slows the corrosion that creates it. Baxi's EcoBlue System manual says the inhibitor concentration should be checked "at every service", following BS 7593.