A central-heating pump that can be heard or felt running does not prove that hot water is circulating through the radiators. The pump may be receiving power while flow is restricted by trapped air, closed valves, sludge, a zone-control fault or a mechanical problem. The boiler may also be running a short pump-overrun cycle rather than answering a heating demand.
Start by confirming that the boiler is actually heating water for the radiators, then check pressure, controls and the pattern across the house. Do not remove the boiler casing, open an electrical pump cover or change pump settings. If all radiators stay cold after the safe checks below, arrange a qualified diagnosis.
First confirm what “the pump is running” means
A faint hum or vibration can mean that the pump motor has power. It does not confirm the impeller is moving the required amount of water. Equally, some boilers run the pump briefly after the burner stops to carry residual heat away. Worcester Bosch also notes that certain controls can keep a pump running to monitor flow temperature.
Record when the sound begins, whether a flame or burner symbol appears and whether it stops after the heating demand ends. This separates normal overrun or control behaviour from a pump that seems to run continuously while the home remains cold.
Check that there is a genuine heating demand
Set the programmer to heating on and raise the room thermostat above the current room temperature. If you use smart controls, confirm they are online and showing a demand. Make sure the boiler is not set to hot-water-only or summer mode.
Energy Saving Trust explains that the programmer decides when heating is allowed to operate and the room thermostat decides whether heat is needed. A pump sound without a valid demand may be overrun, frost protection or another control mode rather than normal central heating.
Look at the boiler display
Check for a heating symbol, burner indication or fault code. Record the code exactly and use the manual for the full model. A display showing standby or no demand suggests the controls are not asking the boiler to heat, even if a pump or valve elsewhere can be heard.
If the boiler fires for hot water but not for heating, the fault route is different from a boiler that fires normally but sends no heat to any radiator. Tell the engineer which pattern you have.
Safe checks in the right order
Check the cold system pressure
Read the boiler gauge or digital pressure when the system is fully cool and compare it with the exact user manual. Low pressure can stop or restrict heating. Repressurise only when the manual provides a clear homeowner procedure and no active leak is visible.
If pressure falls again, do not keep topping up. Record the cold reading and arrange an inspection.
Open the normal radiator controls
Make sure the thermostatic radiator valves are turned up in the rooms you are testing and that any ordinary manual valves have not been closed. Do not remove valve heads, pull valve pins or use tools. If one radiator is cold while others heat, the problem is likely more local than a whole-system pump fault.
Compare every radiator
Check which radiators warm first and which remain cold. A radiator cold at the top may contain trapped air. A radiator warm at the top but cold at the bottom can indicate sludge or restricted flow. If every radiator stays cold, focus on the boiler demand, zone controls and circulation route.
Listen without opening anything
Note whether the sound is a smooth hum, a buzz, gurgling, rattling or repeated starting and stopping. Do not put your hand inside a boiler or pump enclosure. A sound recording from a safe distance can help an engineer when the symptom is intermittent.
Check the visible pipe-temperature pattern carefully
Without touching very hot surfaces, notice whether the heating flow pipe becomes warm while the return and radiators stay cool. Baxi lists a hot flow with a cool return among the signs of sludge or poor circulation. Pipe temperature alone cannot identify the blocked component, so do not dismantle a valve or strainer.
Why a running pump may not heat the radiators
The pump is powered but not producing enough flow
A worn, obstructed or mechanically stalled pump can hum without circulating water correctly. Internal boiler pumps and their controls require professional electrical and hydraulic testing. Do not remove a centre screw, free a shaft manually or probe wiring.
Air is trapped in the circuit
Air can interrupt circulation, produce gurgling and leave radiator tops cold. Bleed only affected radiators with the heating off and cool, then recheck pressure. If air returns repeatedly or all radiators stay cold, the system needs more than routine bleeding.
Sludge or debris is restricting flow
Baxi lists cold radiator bottoms, hot pipes feeding cool radiators, a cool return and pump problems among sludge clues. Worcester Bosch likewise associates cold-bottom radiators with poor circulation or debris. An engineer can test system water before recommending cleaning.
Do not assume that every cold system needs a powerflush. A closed valve, failed zone component or pump fault can produce a similar result.
A zone valve has not opened
Systems with a hot-water cylinder often use motorised valves to direct water to heating, hot water or both. The pump may run while the heating zone remains closed. Note whether the cylinder heats normally and whether only one zone of the house is affected.
Motorised valves and wiring centres involve mains electricity and hot pipework. Leave testing and replacement to a competent engineer.
The system is badly balanced or too many valves are closed
Water follows the easiest route. Poor balancing can make nearby radiators hot while distant ones remain cool, while closed radiator controls can leave too little useful flow. Vaillant explains that balancing adjusts the flow available to each radiator so heat is distributed evenly.
If every radiator is cold, balancing alone is unlikely to explain the complete loss of heat. Do not alter lockshield settings at random because that removes useful evidence and may make the system harder to commission.
The boiler is not producing heating flow
A pump can run during overrun, frost protection or a fault sequence even when the burner is off. The display, heating symbol and flow-pipe temperature help distinguish circulation failure from a boiler that never begins the heating cycle.
A diverter or internal hydraulic component is not changing position
On a combi boiler, hot water can work while radiators stay cold if heat is not being directed into the heating circuit. This is internal diagnosis. Do not remove the case or try to move a component by hand.
Useful patterns to report
- All radiators cold, boiler in standby: check demand and controls first.
- All radiators cold, burner runs briefly: record pressure, noises and flow/return pattern.
- Near radiators warm, distant ones cold: flow distribution, balancing or restriction may be involved.
- Radiator tops cold: trapped air is more likely.
- Radiator bottoms cold: sludge or restricted flow is more likely.
- Hot water works but no radiators: heating demand, zone or changeover components need checking.
- Pump continues after heating stops: it may be normal overrun, a control mode or a fault depending on duration and model.
When to switch the system off
Use the normal accessible control to switch the boiler off, if it is safe to reach, when the pump or boiler becomes unusually loud or hot, pressure rises rapidly, water is leaking near electrical parts, a burning smell appears or the appliance repeatedly locks out.
If you smell gas or a carbon-monoxide alarm sounds, leave the property and follow emergency guidance. Those are not circulation checks.
What to tell the engineer
- the boiler make and full model;
- whether the pump sound starts with a heating demand;
- whether the burner symbol appears;
- the fully cold pressure reading;
- whether hot water still works;
- which radiators warm first, partly or not at all;
- whether flow pipework becomes hot while the return stays cool;
- any gurgling, buzzing, rattling or fault code; and
- any recent radiator, control or system work.
This evidence helps the engineer decide whether to investigate demand controls, zone valves, air, system water, the pump itself or an internal boiler component.
If you rent the property
Report the lack of heating to the landlord or managing agent promptly. GOV.UK says landlords are normally responsible for heating and hot-water systems. Explain whether hot water works, whether all radiators are cold and whether anyone in the home is particularly vulnerable to cold.
Do not open electrical controls or the boiler casing. Keep screenshots, pressure readings and the date the fault began.
Frequently asked questions
Can a pump make noise but not circulate water?
Yes. Power or vibration does not prove adequate flow. Mechanical wear, obstruction, trapped air or a closed route can prevent useful circulation.
Why does the pump run after the boiler switches off?
Many systems use a short overrun to move residual heat. Some control strategies also keep a pump running for monitoring. Long or unexplained operation with cold radiators needs model-specific diagnosis.
Should I turn the pump speed up?
No. Pump settings affect flow, noise and system behaviour and should be matched to the installation by an engineer.
Will bleeding every radiator fix it?
Only if trapped air is the cause. Bleed radiators showing a cold-top pattern, then check pressure. All-cold radiators may have a different fault.
Does hot water working rule out the boiler?
No. A combi can produce hot water while the heating demand, diverter function or radiator circuit has a fault.
Related boiler-help guides
- Hot water works but radiators stay cold
- Why are all my radiators cold?
- How to recognise heating-system sludge
- How radiator balancing works
Pump running but the house still cold?
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