Glossary · Boilers and boiler types
What is a central heating pump?
A central heating pump is the electric pump that pushes hot water from the boiler round the pipes and radiators and back again. Without it the water would sit in the boiler. On most modern boilers the pump is built in behind the casing, and it often runs for a short time after the burner stops.
Key points
- The pump circulates water round the heating circuit. It is separate from the burner that heats the water.
- On the Worcester Greenstar combi we checked, the pump is built in and has three speeds with an automatic air vent.
- Pump run-on keeps water moving after the burner stops. On that model the engineer can set it from 1 to 10 minutes, and the standard setting is 3 minutes.
- Worcester says dirty system water can block pumps, heat exchangers and valves. A system filter is the usual defence.
- The pump is inside the boiler on most models, so replacing it is work for a Gas Safe registered engineer.
How a central heating pump works
The boiler heats the water. The pump moves it. Water leaves the boiler hot, travels through the pipes to each radiator, gives up some of its heat and returns to the boiler to be heated again. Without a pump that circuit would barely move.
On most modern boilers the pump is part of the boiler. The Worcester Greenstar 24/28/30 combi manual lists a three-speed heating circulating pump with an automatic air vent as one of the boiler’s built-in features, along with pump anti-seize protection.[1]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: Installation, commissioning and servicing instructions The automatic air vent lets trapped air out of the boiler. Other makes and models will differ in the detail.
The pump is only for the heating circuit. Ideal Heating explains that on a combi the water for your taps is already under mains pressure, so it flows naturally and there is often no need for a pump.[3]Ideal Heating, What is a combi boiler? The pump’s work is the closed loop through the radiators. On a combi the diverter valve decides when that loop is in use.
Pump run-on
After the burner stops there is still heat in the boiler. Many boilers keep the pump running for a while to carry it away into the system. The same Worcester manual has a setting for the heating pump run-on time that an engineer can choose from 1 to 10 minutes, and gives the standard setting as 3 minutes.[1]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: Installation, commissioning and servicing instructions So a pump that hums for a few minutes after the radiators go quiet is working as designed. It is not a fault.
The pump speed and the way it is controlled are also set on that model. The manual lists several pump control modes, including one where the pump runs continuously.[1]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: Installation, commissioning and servicing instructions These are installer settings and not for the householder to change.
What shortens a pump’s life
The enemy is dirty water. Worcester says contaminated central heating water can cause pumps, heat exchangers and valves to block, and that it can also leave cold spots on radiators.[2]Worcester, Bosch Group, Greenstar System Filter: Installation and servicing instructions The same document says the system filter should be cleaned each year during the annual boiler service.[2]Worcester, Bosch Group, Greenstar System Filter: Installation and servicing instructions You can read about the filter on the magnetic system filter page and about the dirt itself under central heating sludge.
Signs and next steps
| What you notice | What it may point to | What to do |
|---|---|---|
| Radiators cold but the boiler fires | Circulation problem, which can come from the pump among other causes | Book an engineer to diagnose |
| Pump noise or humming | Could be air, dirty water or a worn pump | Book a service and mention the noise |
| Pump runs on after heating stops | Normal run-on | Nothing |
The pump is inside the boiler casing on most models, so a replacement is gas appliance work. Anyone employed to work on gas appliances in a home must be Gas Safe registered and competent in that area of gas work.[4]Health and Safety Executive, Domestic gas: frequently asked questions BASI covers this under boiler repair. If a single radiator is cold while the others are hot, the cause is usually local to that radiator, and the page on a radiator not heating up covers it.
Example
Sandeep in Pudsey has a combi boiler fitted a few years ago. After a cold morning he hears the boiler pump humming for a few minutes after the heating has switched off. That is run-on and is expected. Weeks later, two upstairs radiators stay cold while the others warm up. That is a separate problem, and an engineer will check the water quality and flow before deciding whether any part needs replacing.
What people get wrong
A common assumption is that a noisy or cold system always means the pump has failed. Pumps are blamed often, but the cause can be air, sludge or a valve. Replacing the pump without finding the cause means the same fault comes back.
Another is thinking the pump pushes the hot water to the taps on a combi. It does not. That water comes direct from the mains, and the pump serves the radiators.
The bottom line
The central heating pump keeps hot water moving round the radiator circuit. On a modern boiler it is built in, it runs on for a short time after the burner stops and it is the part most at risk from dirty system water. Keep the water clean, have the system serviced every year and leave any pump work to a Gas Safe registered engineer.
Common questions
What does a central heating pump do?
It circulates hot water from the boiler through the pipes and radiators and returns it for reheating. Without circulation the radiators would not warm up properly.
Where is the central heating pump?
On most modern boilers it is inside the boiler casing. Older or regular boiler systems may have a separate pump fitted on the pipework, often near the hot water cylinder.
Why does my boiler pump keep running after the heating stops?
That is pump run-on. It moves residual heat out of the boiler after the burner switches off. On the Worcester model we checked, the standard run-on is 3 minutes and an engineer can change it.
What makes a heating pump fail?
Worcester says contaminated system water can cause pumps, heat exchangers and valves to block. A magnetic system filter and proper water treatment are the guards against that.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Worcester, Bosch Group. Greenstar 24/28/30 RSF combination boiler: Installation, commissioning and servicing instructions. Checked 11 November 2026. ↩
- Worcester, Bosch Group. Greenstar System Filter: Installation and servicing instructions. Checked 11 November 2026. ↩
- Ideal Heating. What is a combi boiler?. Checked 11 November 2026. ↩
- Health and Safety Executive. Domestic gas: frequently asked questions. Checked 11 November 2026. ↩