Central heating circulator pumps: what they do and signs of failure

A central heating system needs something to push hot water from the boiler round the radiators and, on many systems, through the coil in the hot water cylinder. That job is done by the circulator pump, usually just called the heating pump or central heating pump.

This guide explains what the pump does, how to spot the signs that it may be failing, and what you can safely check yourself. It gives no method for freeing, bleeding or replacing a pump. That is work for a plumber or heating engineer, and where the pump is inside or next to a gas boiler it needs a Gas Safe registered engineer.

What the pump does

Hot water does not move round a modern heating system fast enough on its own. The pump keeps it moving so each radiator gets a steady supply of hot water and the cooler water returns to the boiler to be reheated.

Most domestic heating pumps are glandless pumps. The motor and the spinning part (the rotor) are sealed together with the system water, so there is no shaft seal to leak. Many newer pumps are variable speed or electronically controlled and adjust themselves to the system.

The pump is not the same as a shower pump. A shower pump boosts the hot and cold water to a shower. A heating pump only moves water round the closed heating circuit. If your problem is a weak shower, see our shower pump guides instead.

Where to find it

Where the pump is depends on the type of system. Our guide to combi, system and regular boilers explains the three common setups.

• Combi and system boilers: the pump is usually built into the boiler, behind the casing. You will not see it.
• Regular (heat-only) boilers: the pump is usually separate, on the pipework, often in the airing cupboard near the hot water cylinder. It is a small motor housing, often with a label, between two valves.

If the pump is inside the boiler, it is part of a gas appliance. Do not open the boiler casing to look for it.

Signs the pump may be failing

None of these proves the pump is the problem. They are symptoms a plumber or engineer will want to hear about.

• The boiler fires but the radiators stay cold, or only the pipes near the boiler get hot.
• Radiators warm up slowly or unevenly, even after bleeding and balancing. Our radiator bleeding and balancing guide covers those routine steps.
• Hot water works but the heating does not, or the other way round, on a system with a cylinder. Valves can cause this too.
• New noises: a grinding, rattling, humming or whining from the pump or boiler.
• The pump casing is very hot to touch, or you can feel no vibration at all when the heating should be running.
• A leak or crusty deposits at the pump unions or around the pump body.
• The boiler shows a fault code about circulation or overheating. Note the code and check the boiler manual.
• On an open-vented system, water pumping over from the vent pipe into the loft tank. See feed and expansion tanks.

Sludge in the system is a common cause of pump trouble. Black magnetite can wear or jam the moving parts. Our guide to heating inhibitor and sludge explains why, and a magnetic filter is one way plumbers protect a new pump.

What you can safely check

These checks do not involve opening anything:

1. Is the heating actually calling for heat? Check the programmer, room thermostat and any radiator thermostats are set to ask for heat.
2. Is there power? Check the boiler display and any fused switch for the heating are on. Do not take anything apart to check wiring.
3. On a sealed system, is the pressure in the normal range? Read the gauge and compare it with the boiler manual. Our boiler pressure guide covers this.
4. Have the radiators been bled? Trapped air can stop circulation.
5. Listen and feel at the pump, if it is separate and you can reach it safely. Note any noise, vibration or heat to describe later.

If none of that explains the problem, stop. That is a job for a plumber or heating engineer.

Who replaces a pump

Replacing a heating pump involves isolating or draining the heating, breaking into the circuit, refilling, removing air, restoring any inhibitor and checking the controls. The pump also has an electrical supply.

• Pump inside or attached to a gas boiler: this is work on a gas appliance. It must be done by a Gas Safe registered engineer. Our guide on when to call a Gas Safe engineer explains the law and how to check an engineer's card.
• Separate pump on the pipework, away from the boiler: a competent plumber or heating engineer can usually do it. If the work touches the boiler or its controls in any way, use a Gas Safe registered engineer.
• The electrical connection: a plumber or heating engineer will normally reconnect a like-for-like pump. Any new wiring or change to the controls is work for someone competent in electrics, such as a qualified electrician.

Our guide to hiring a plumber covers finding someone suitable and what to ask.

A replacement should suit the system. The engineer will check the pump's fit, size and settings, and whether the old pump failed because of sludge, air or a fault elsewhere. Replacing a pump without fixing the cause can mean the new one fails too.

We do not link to heating pumps on this page. We do not give a method for changing one.

How do I know if my central heating pump has failed?

Common signs are cold radiators while the boiler is firing, new grinding or humming noises, a pump that is very hot or completely still, a leak at the pump, or a boiler fault code about circulation. A plumber or heating engineer can confirm it.

Can I replace a central heating pump myself?

We do not recommend it and do not give a method. If the pump is inside or next to a gas boiler, the work must be done by a Gas Safe registered engineer. A separate pump on the pipework is a job for a competent plumber or heating engineer.

Is the heating pump the same as a shower pump?

No. A heating pump moves water round the closed heating circuit. A shower pump boosts the water supply to a shower. They are different products for different systems.

Why does my heating pump keep failing?

Sludge, air in the system and pumps that do not suit the system are common causes. Ask the engineer to look for the cause, not just fit another pump. A system clean, inhibitor and a filter may be part of the fix.