Central heating pump not working or noisy: how to tell, common causes and what it costs to replace

How does a central heating pump work?

A central heating pump — sometimes called a circulating pump or boiler pump — has one job: to keep hot water moving around your heating system. It pushes heated water out of the boiler, through your radiators, and back again in a continuous loop. As the water circulates, it transfers heat into each room. The same pump also helps move hot water to your taps and shower if you have a system or regular boiler with a hot water cylinder.

Inside the pump is a spinning component called an impeller. Water enters the centre of the impeller, is flung outward by the spinning blades, and is channelled at pressure through the outlet pipe and around the system. Modern pumps are designed to do this efficiently — many automatically adjust their speed to match what the system actually needs, saving electricity in the process.

On a system or regular (conventional) boiler, the pump sits outside the boiler — usually inside or next to the boiler casing, or in the airing cupboard near the hot water cylinder. On a combi boiler, the pump is built into the unit itself, which means access is more involved.

Some pumps have a speed dial with settings 1, 2 and 3. The setting controls how hard the pump works and is typically adjusted during installation to suit the size of the property and the number of radiators. It is not something that needs regular adjustment, but it is useful to know it exists — more on that below.

Leading brands in the UK include Grundfos and Wilo. The Grundfos Alpha range is frequently recommended by heating engineers as a reliable replacement option.

Signs that your central heating pump is not working

Pump problems usually fall into two broad categories: the pump has stopped working altogether (electrical or mechanical failure), or it is still running but poorly (worn components, blockage, air lock). The symptoms differ slightly, but here is what to look for:

• Cold radiators with a warm boiler — the boiler fires and you can feel heat around it, but the radiators stay cold or barely warm. This is the classic sign of a circulation failure.
• Boiler locking out or overheating — without circulation, heat builds up rapidly in the heat exchanger. Modern boilers detect this and shut down to protect themselves, often displaying an error code. If your boiler keeps locking out with an overheat or flow fault code, a failing pump is a very common cause.
• Short cycling — the boiler fires, the water heats almost immediately (because it is not moving), hits the safety cut-off and switches off, then fires again a few minutes later. This rapid on-off pattern is a direct sign of poor or zero circulation.
• Strange noises from the pump area — humming, grinding, gurgling or knocking where there was previously quiet operation is a warning sign worth taking seriously.
• Increasing energy bills — a struggling pump has to work harder, using more electricity, and a system that cannot circulate properly may cause the boiler to run longer than it should.

As a rough rule: if you can hear the pump running but have no heat, the problem is likely mechanical. If the pump is completely silent when it should be running, suspect an electrical fault — either in the pump motor itself or in the wiring and controls.

Common causes of a central heating pump failing — most common first

Understanding why pumps fail helps you explain the situation clearly to an engineer and gives you a sense of whether a repair or a full replacement is more appropriate.

• Seized pump shaft — the most frequent cause, particularly at the start of the heating season in autumn. If the pump has sat idle over summer, the internal shaft can seize, preventing the motor from turning and stopping all circulation. A gentle tap on the casing sometimes frees it temporarily, but if seizing is a recurring problem, the pump is on its way out.
• Sludge and debris build-up — over time, corrosion inside radiators and pipework produces black iron-oxide sludge. This circulates through the system and can clog or partly block the pump impeller, causing knocking, sluggish circulation or complete failure. Older systems with no magnetic filter fitted are most at risk.
• Motor failure — the pump motor can burn out, especially in older units or those running at high speed continuously. When the motor fails, the pump will be completely silent and there will be no circulation at all.
• End of lifespan — central heating pumps typically last between 10 and 15 years, depending on usage and maintenance. If yours is approaching or beyond that age and starting to give trouble, replacement is usually more cost-effective than repeated repairs.
• Blown seals or leaks — worn seals around the pump body or connections can allow water to escape. Sometimes tightening a joint or replacing a seal resolves this; other times the whole pump needs replacing.
• Incorrect installation — rare, but if a system has never worked properly since installation, it is worth checking the pump has been fitted in the correct flow direction. A pump installed backwards will not circulate water effectively.

Central heating pump noise: what different sounds mean

A healthy pump runs quietly — a very faint hum at most. Any new or worsening noise deserves attention. Here is what the most common sounds usually indicate:

• Humming or buzzing — a low, constant hum can mean the pump speed is set too high for the system, that internal components are worn, or that the shaft is beginning to seize. Dropping the speed setting one notch (for example from 3 to 2) is a safe check the homeowner can do.
• Gurgling or knocking — almost always caused by air trapped inside the pump or pipework. Air bubbles passing through the impeller create turbulence that sounds like knocking or trickling. Bleeding the pump and the radiators usually resolves this.
• Grinding or scraping — this is a more serious sign, indicating that internal mechanical components are worn or failing. Do not ignore grinding; left alone it typically leads to complete pump failure.
• Whining or squealing — often heard when the pump is working too hard, either because the speed setting is too high or because a partial blockage is forcing the motor to strain. Check the speed setting first; if that does not help, call an engineer.
• Metal clanking — central heating pumps have a small fan to prevent overheating. A loose or damaged fan blade can cause a rhythmic clanking noise as the pump runs. This requires a professional to investigate.
• Banging related to sludge — black sludge and debris circulating in the water can pass through or partially block the pump, creating an uneven, labouring sound or intermittent bangs. A system flush is usually required alongside any pump work.

Safe checks the homeowner can do — try this first

Before calling an engineer, there are a few checks that are safe and straightforward for any homeowner. None of these involve opening the boiler casing or touching gas components.

Check the thermostat and timer — make sure the heating is actually being called for. It sounds obvious, but a thermostat that has been accidentally turned down, or a timer that has not been updated for daylight saving, accounts for a surprising number of call-outs.

Check system pressure — look at the pressure gauge on your boiler. Most systems should sit between 1 and 1.5 bar when cold. If it reads below 0.5 bar, the system may have lost pressure and the pump may not be able to circulate water properly. Top up via the filling loop following your boiler manufacturer's instructions, then reset the boiler and see if heating returns.

Try a boiler reset — if the boiler has locked out, reset it up to two or three times following the manufacturer's instructions. If it locks out again immediately, stop resetting and call an engineer.

Adjust the pump speed setting — if your pump has a manual speed dial (usually marked 1, 2, 3), try reducing it by one setting. A pump running at full speed on a small system can cause noise and even short cycling. Equally, if it has accidentally been set to the lowest setting, a small system may not be getting enough circulation.

Bleed the radiators — if you have gurgling noises and cold patches at the tops of radiators, trapped air is likely. Bleed each radiator using a radiator key, starting at the ground floor and working upward. After bleeding, check the boiler pressure and top up if it has dropped below 1 bar.

Bleed the pump itself — many modern pumps (including most Grundfos models) have a bleed screw. With the pump running, carefully open the screw a little until you hear hissing. Hold it there until the hissing stops and a small dribble of water appears, then close the screw. This removes air from inside the pump body.

If none of these steps restore heating, or if you hear grinding, detect leaks, or the pump remains completely silent, it is time to call a Gas Safe registered engineer.

What is pump overrun and should you be worried?

Pump overrun is a built-in safety feature on most modern boilers that often confuses homeowners. When the boiler burner switches off — either because the room has reached temperature or because you have turned the heating off — the pump continues to run for a short period, typically between one and ten minutes.

This is entirely normal. The pump is circulating residual heat away from the heat exchanger to prevent the water inside it from boiling or causing damage. Think of it like a car engine cooling fan continuing to run after you switch off the ignition.

Many boilers display a pump overrun indicator or code on their screen during this phase. On its own, this is not a fault — it means the boiler has done its job and is protecting itself as it should.

You should start to be concerned if pump overrun is happening alongside other symptoms: cold radiators, regular lockouts, noises, leaks, or if the pump continues running for noticeably longer than ten minutes after the heating shuts down. In those situations, the overrun period may be extending because heat is struggling to dissipate — which can point back to a circulation problem or a failing pump.

What a Gas Safe registered engineer will do

When you call in a professional, they will carry out a systematic diagnosis before recommending a repair. Here is what to expect:

The engineer will check whether the pump is receiving power and whether it is actually running. They will listen for noise, feel for vibration, and check whether heat is reaching the radiators. They will also look at the system pressure, check for visible leaks around the pump body, and may use a thermal camera or flow meter to assess circulation.

If the pump has seized, they may attempt to free it manually — but if it seizes again, replacement is the right call. If sludge is suspected, they will advise on whether a hot-flush with chemical cleaner or a full power flush is appropriate. Power flushing uses specialist equipment to push clean water at high velocity through the system to dislodge debris; it should always be done before fitting a new pump on a contaminated system, otherwise the new pump will degrade quickly.

When replacing the pump, the engineer will isolate the water supply and either use isolation valves (if fitted) or drain the relevant part of the system before unbolting the old unit. The new pump is fitted, the system is refilled and vented, and the engineer will test that circulation is restored to all radiators before leaving.

If you have a combi boiler, the internal pump is accessed by opening the boiler casing — this must be done by a Gas Safe registered engineer. Do not attempt to access the inside of a gas boiler yourself.

If sludge has caused the failure, ask the engineer to fit a magnetic system filter (such as a Magnaclean or similar) at the same time. This captures future debris before it reaches the pump, significantly extending the life of the new unit.

Step by step

1. Check the thermostat and timer

   Make sure your thermostat is set above the current room temperature and that your timer or programmer is calling for heat. Update the schedule if daylight saving has recently changed.

2. Check boiler pressure

   Look at the pressure gauge on the front of your boiler. It should read between 1 and 1.5 bar when cold. If it is below 0.5 bar, top up using the filling loop following the instructions in your boiler manual, then reset the boiler.

3. Reset the boiler

   If the boiler has locked out and is showing an error code, press the reset button as directed in your boiler manual. Reset a maximum of two or three times. If it locks out again each time, stop and call a Gas Safe registered engineer.

4. Adjust the pump speed setting

   If your pump has a manual speed dial marked 1, 2, 3, try reducing the setting by one notch. A pump set to maximum speed on a smaller system can cause unnecessary noise and poor performance. Give the system 20–30 minutes to respond.

5. Bleed the radiators and pump

   If you hear gurgling or have cold patches at the top of radiators, bleed each radiator starting from the ground floor. Then bleed the pump itself using the pump bleed screw (usually a slotted screw on the front of the pump). Open it slowly until hissing stops and a small drop of water appears, then close it. Check and top up boiler pressure afterwards if needed.

6. Call a Gas Safe registered engineer

   If the pump is completely silent, making grinding noises, leaking, or if none of the above steps restore heating, call a Gas Safe registered engineer. Do not attempt to open the boiler casing or work on gas components yourself.

Typical costs

Typical UK ranges as a guide only — prices vary by region (expect the top end, or 20–30% more, in London and the South East) and by how accessible your system is. Always get a written quote.

Frequently asked questions