Central Heating Pump Failure: How to Tell If Your Pump Has Failed in London

What the Central Heating Pump Does

The central heating circulator pump is the component responsible for moving hot water around your heating circuit. It sits between the boiler and the radiators, drawing water through the flow pipe from the boiler heat exchanger and pushing it around the circuit to each radiator before returning it to the boiler via the return pipe. Without a functioning pump, the boiler heats the water in the heat exchanger but that water does not travel anywhere — the radiators remain cold and the boiler overheats and shuts down on the limit thermostat.

In modern condensing combi and system boilers, the pump is housed inside the boiler casing and forms part of the sealed boiler assembly. In older system and heat-only boilers, the pump is typically an external unit mounted in the airing cupboard or immediately adjacent to the boiler, wired into the boiler or the heating controls independently.

The Primary Symptom: Boiler Fires but Radiators Are Cold

The most common and definitive symptom of a failed central heating pump is a boiler that fires correctly — you can hear the burner ignite, the boiler reaches its set temperature on the flow thermometer — but the radiators throughout the house remain cold. There is no water circulating. The boiler fires, overheats at the heat exchanger, and trips on the high limit thermostat. You may see a fault code on the boiler display related to overheating or flow temperature fault.

This is distinct from a boiler that does not fire at all, which indicates a different fault — ignition, gas supply, or controls. If the boiler fires and reaches temperature but nothing heats up, the pump is the primary suspect.

Grinding or Humming Noise

A pump that is running but failing will often produce a grinding or humming noise from the pump body. A grinding noise indicates bearing failure — the pump motor bearings have worn and the shaft is no longer running true. The pump is still running (the motor is turning) but the bearings are failing and the pump will seize completely in the near future. A low humming noise from the pump body, louder than the normal operating hum, can indicate the same thing.

In London hard water areas, bearing failure is accelerated by magnetite sludge circulating through the pump — the iron oxide particles act as an abrasive. A pump making grinding noises should be replaced promptly, before the bearings seize completely and the motor shaft locks up.

Pump Running but No Flow

A common failure mode in London properties is a pump that is electrically running — the motor is turning and you can feel a faint vibration from the pump body — but there is no water flow through the circuit. The radiators are cold even though the pump appears to be operating. This is caused by a seized impeller. The impeller is the internal spinning component of the pump that creates the pressure differential driving water circulation. In London hard water areas, limescale and magnetite sludge deposit on the impeller and inside the pump volute, eventually seizing the impeller solid.

When the impeller is seized, the motor continues to run (it is not mechanically connected directly to the shaft in many pump designs) but no water is moved. The pump body will be warm or hot because the motor is dissipating heat, but there is no flow. The diagnosis is confirmed by the absence of any temperature difference between the flow and return pipes — both will be the same temperature when there is no circulation.

High-Pitched Whine: Air in the Pump

A high-pitched whine or a cavitation noise from the pump — a rapid rattling or chattering — almost always indicates trapped air in the pump body. Air in the system causes cavitation (the formation and collapse of vapour bubbles around the impeller), which both reduces pump efficiency and causes premature bearing and impeller wear. Before condemning a pump that is making a high-pitched whine, bleed the pump using the bleed screw on the pump head.

Most external circulation pumps have a bleed screw on the pump head — a small slot-headed screw that, when turned slightly, allows air to escape from the pump body. With a cloth ready to catch any water, turn the bleed screw a quarter turn anti-clockwise and wait for a hiss of air. When water appears steadily at the screw, re-tighten. This takes 30 seconds and may resolve a noise problem entirely. If the noise returns after a short period, there is a persistent air entry point in the system.

Pump Overheating

A pump body that is excessively hot to touch — hotter than the adjacent pipework — indicates restricted flow through the pump. The pump motor generates heat in normal operation, but this heat is carried away by the water flowing through the pump body. If flow is restricted — by a partially closed isolation valve, a sludge blockage, or an undersized bypass — the pump body overheats. Check that both pump isolation valves (the quarter-turn service valves on each side of the pump body) are fully open. If the isolation valves are open and the pump is still overheating, the flow restriction is within the system.

What to Do Next

If the boiler fires but radiators are cold, and the pump body is either silent, grinding, or warm with no circulation, the pump needs replacement. Before booking a pump replacement in London, check two things: first, confirm both pump isolation valves are fully open (quarter-turn valves should be in-line with the pipe); second, bleed the pump to rule out air. If these checks do not resolve the problem, contact Prestige Engineers for central heating pump replacement. We replace both external airing cupboard pumps and internal boiler pumps across all London boroughs.