What does the central heating pump do, and why is mine hot and noisy?

What the pump actually does, what its speed setting changes, and how to read the difference between normal running noise, an air problem and a failing bearing.

Difficulty: intermediate
Time: 20 min
Read: 5 min
Safety: warning

Short answer

The pump pushes hot water round the radiator circuit — without it heat only creeps by convection. It runs warm in normal service, but a hot casing, a loud hum, a rattling or gurgling noise, or radiators that never get hot at the far end of the house are all signals. Air, sludge and worn bearings account for nearly every pump complaint.

The circulating pump is the component that makes a modern heating system work, and it is the one people notice only when it starts making a noise. It sits on the pipework — usually near the boiler or in the airing cupboard — with two large nuts either side and a small electrical box on top. Understanding what it does makes several otherwise baffling heating symptoms obvious.

Safety

A circulating pump is a mains-powered device in wet pipework. Loosening its bleed screw releases hot system water and is only done with the system cool and the electrics isolated — and even then it is a job many householders should leave alone. Do not open the pump's electrical terminal box, change its speed setting or replace it: that is heating engineer and electrician work.

Step by step

Find it and identify it.A cylindrical body about the size of a fist on a horizontal pipe run, with big nuts either side, an electrical box on top, and usually a speed selector with settings one, two and three. Modern high-efficiency pumps have a small display or an indicator instead.
Understand what it is for.Water carries heat from the boiler to the radiators and back. The pump is what moves it. Its job is to circulate enough water for the furthest radiator to receive properly hot water without over-circulating so much that the return comes back too hot for a condensing boiler.
Understand the speed setting.Higher speed means more flow — better circulation to the far corners of the house, but a smaller temperature drop across each radiator, more noise, and a hotter return that hurts condensing efficiency. Lower speed is quieter and better for the boiler but may not reach the end of a long circuit. It is a balance and it belongs to whoever commissioned the system.
Expect it to be warm.The pump body sits in the flow and is in contact with system water at heating temperature, so warm is normal. What is not normal is a casing significantly hotter than the pipe it is on, which points at a motor working too hard or an obstruction.
Read the noise.A low steady hum is normal running. A gurgling, tinkling or trickling sound is air in the pump or the circuit. A harsh rattle or a rumble that changes with speed is a bearing on its way out. A rhythmic knocking usually means debris in the impeller.
Deal with air first.Air is the most common and the most fixable cause. Bleed the radiators properly, starting downstairs and working up, then check whether the noise has gone. On a sealed system, top the pressure back up afterwards. Persistent air is a symptom of its own and points at a leak or a badly vented high point.
Recognise the sludge picture.A pump straining against thick magnetite sludge runs hot, is noisier than it was, and coincides with radiators cold at the bottom and black water when bleeding. The pump is the victim rather than the cause, and replacing it without cleaning the system just puts a new pump into the same slurry.
Know when it has failed.No noise at all with the boiler firing, no circulation, and radiators that stay cold while the boiler heats and cycles is a stopped pump. A boiler that repeatedly cuts out on overheat with cold radiators is the classic pattern — no flow means the heat has nowhere to go.

Tips

A pump that has run for fifteen years and starts making a noise is usually just old. Modern high-efficiency replacements use a fraction of the electricity and are markedly quieter.
Never replace a pump on a sludged system without cleaning the system. The new pump inherits the same problem and often fails early.
The two large nuts either side of the pump are its isolating valves on most installations, which is what makes replacement a contained job rather than a full drain-down.
Noise that arrived suddenly is worth investigating; noise that has crept up over years is usually wear.

If it doesn't work

Gurgling or trickling from the pump

Cause: Air in the circuit, often after refilling or topping up — Fix: Bleed the whole system methodically, then repressurise. Repeated air means water is escaping and being replaced — look for a leak or a failing automatic air vent.

Loud hum or vibration through the pipework

Cause: The pump running at a higher speed than the system needs, or a rigid mounting transmitting vibration into the structure — Fix: Speed changes are a commissioning decision, not a householder adjustment — but it is exactly the right thing to raise with an engineer. Vibration can also be reduced with proper pipe clipping.

Pump hot, radiators barely warm at the far end

Cause: Restricted circulation from sludge, a partly closed valve, or a pump losing performance — Fix: Check the obvious valves first, then treat it as a system-water problem. A magnetic filter and a chemical clean often restore circulation without touching the pump.

Boiler cuts out on overheat, radiators cold

Cause: No circulation at all — a stopped pump, a seized impeller or a fully closed valve — Fix: Switch the heating off; running a boiler with no circulation stresses it. This needs an engineer promptly.

Pump works but only when tapped

Cause: A seized impeller freeing momentarily — usually after a long period standing still — Fix: It is on its way out. Tapping is a stopgap to get heat, not a repair. Running the heating briefly in summer is what prevents this.

Keeping on top of it

Run the heating for ten minutes to keep the pump and valves moving (Monthly through summer)
Check the pump and its unions for weeping (Annually, with the boiler service)

Questions people ask

Should I change the pump speed myself?

No. The setting was chosen to balance circulation against return temperature and noise, and changing it affects every radiator in the house. It is a legitimate thing to ask an engineer to review, particularly if the system has been altered.

How long does a pump last?

Commonly ten to fifteen years, much less in a system with dirty water. Clean system water is by far the biggest factor in pump life.

Is the pump the same as the boiler pump?

On a combi and most modern system boilers the pump is inside the boiler casing rather than on the pipework. On older heat-only systems it is a separate component in the airing cupboard. Anything inside the boiler casing is Gas Safe territory.