Home › Boilers › Parts › Circulating pump

Boiler Circulating pump: What It Does and Signs It's Failing

The pump moves heating water from the boiler around the radiators (or the hot water cylinder) and back. In combis and system boilers it's built in, low down inside the casing. Some boilers also have a non-return valve in the circuit to stop water flowing the wrong way.

How it works

An electric motor spins an impeller in the water, and many modern pumps change speed to suit demand. The boiler needs steady flow through its heat exchanger, so the board watches flow and return temperatures to judge whether the pump is doing its job. Sludge and air are what usually stop it.

Signs it is failing

• Burner lights then cuts out fast, sometimes with banging or kettling
• Radiators staying cold while the boiler runs
• Loud humming or grinding from low in the boiler
• Hot water works but the heating doesn't (on combis it can be the diverter instead)
• Overheat lockouts

Temperature-rising-too-fast, flow and return gap, dry-fire and overheat codes often come down to the pump or air in the boiler. Vaillant also lists a stuck or wrongly fitted non-return valve.

Can you replace it yourself?

The built-in pump is behind the casing and wired to mains power, so freeing, venting or replacing it is a job for a Gas Safe engineer or, in Ireland, an RGI. What you can do is make sure the pressure is right and bleed the radiators the way your user manual describes, starting nearest the boiler as Vaillant suggests. Tell the engineer if noise started after a top-up or radiator bleed.

Buying the right part

Boiler pumps are usually a model-specific head or complete assembly, not a standard pump off the shelf, so order by model and GC number. Ask about a magnetic filter if sludge caused the failure.

Error codes linked to the circulating pump

• E110 / E130Overheat lockout (primary water or flue)Baxi
• F.22Water pressure in the heating system is too lowVaillant
• F.20Overheat safety cut-out has trippedVaillant
• F.23Too big a gap between flow and return temperatureVaillant
• F.24Temperature rising too fastVaillant
• F.75No pressure rise when the pump startsVaillant
• F.83Dry fire: no temperature change when the burner litVaillant
• E9 / E9 224Overheat limiter trippedWorcester Bosch
• A1Pump has run dryWorcester Bosch

Last reviewed 2026-10-21.