Central heating pump replacement EN1

What the central heating pump does

The central heating circulator pump moves hot water around the heating circuit from the boiler to the radiators and back. Without a working pump the boiler fires but the water does not circulate — the boiler overheats and trips on the limit thermostat. Modern condensing boilers have an internal pump inside the boiler casing. Older system and heat-only boilers often have an external pump in the airing cupboard or close to the boiler.

Internal and external pump types

An internal pump is fitted inside the boiler casing and replacement requires a Gas Safe engineer or boiler-competent heating engineer. The Grundfos UPS2 series is the market standard inside many Worcester Bosch and Ideal boilers. An external pump is a wired pump unit typically in the airing cupboard that a plumber can replace without touching gas. External pumps are commonly Grundfos, Wilo, or DAB brand with 28 mm or 22 mm connections.

Signs of pump failure

The boiler fires but radiators are cold — the pump has seized or failed. A grinding or humming noise from the pump indicates bearing failure. The pump runs but there is no flow — the impeller has seized with limescale, which is common in London hard water. A pump that overheats indicates air in the system or restricted flow. Always bleed the pump before condemning it — trapped air causes cavitation and a high-pitched whine.

Pump replacement process

Isolate the electrical supply, close the pump isolation valves (quarter-turn service valves on each side of the pump body), drain down a small section, remove the pump body from the two 28 mm or 22 mm unions, fit the new pump body with new fibre washers, open isolation valves, vent the pump, restore electrical supply, and test. Internal boiler pump replacement requires opening the boiler casing and is carried out by a Gas Safe engineer.

System sludge and magnetite

A seized pump impeller is almost always caused by magnetite sludge — black iron oxide — the same sludge that causes cold radiator bottoms. At pump replacement, always fit an inline magnetic filter (Magnaclean or Fernox TF1) if not already present to protect the new pump. Fitting a filter without powerflushing the system first leaves the existing sludge in circulation.

Pricing guide

External pump replacement (standard airing cupboard pump) from £220 supply and fit. Internal boiler pump replacement from £300. Magnaclean magnetic filter supply and fit from £150. System powerflush recommended at the same time from £400. All prices include labour and materials.