A Worcester Bosch boiler fan may continue briefly after the flame goes out. That can be a normal purge or overrun used to clear combustion gases and manage residual heat. The exact time varies by model: Worcester service literature for some Greenstar appliances shows a fan overrun of about 30 seconds, while other sequences allow the pump and fan to run for up to roughly three minutes.
A fan that never seems to stop, runs for long periods with no apparent demand, becomes unusually loud or is accompanied by a fault code needs model-specific diagnosis. Do not block the flue, remove the case or disconnect power repeatedly to silence it.
First make sure it is the fan
The fan, circulation pump and water flow through pipes can all make a low hum. After the burner stops, the pump may continue to move water even if the fan has already stopped. Listen from outside the casing and watch the user display; never remove panels to identify a component.
- Airflow or smooth whir near the flue area: may be the combustion fan.
- Low hum with water movement in radiators: may be pump overrun.
- Boiler fires briefly with no tap or heating demand: combi preheat may be active.
- Grinding, scraping or surging noise: is not a normal quiet overrun and needs attention.
Why the fan runs after the flame stops
During a normal demand, the fan helps provide the air path required for combustion. When the gas valve closes, some Worcester operating sequences keep the fan running for a short purge. This is a controlled part of the cycle, not proof that gas is still burning.
Published Worcester Greenstar service diagrams show different timings across products. Some state “fan run 30 seconds” after the end of demand. A Greenstar Highflow diagram says the pump and fan may run for up to three minutes. Use the exact boiler manual rather than expecting one universal duration.
Time it rather than guessing
With the boiler operating normally, note when the flame symbol disappears and time how long the suspected fan sound continues. Repeat once for central heating and, on a combi, once after a normal hot-water draw.
Also note whether the display shows a temperature, standby symbol, fault code or continued demand. Do not keep creating demands to test a boiler that is noisy, hot, leaking or locked out.
Could combi preheat make the boiler run?
Worcester explains that, in PreHeat mode, a compatible combi may fire during the day without an active heating or hot-water demand. It keeps the boiler warm so hot water reaches the tap sooner. In Eco mode, it does not fire throughout the day solely for that purpose.
A brief preheat burn and its following fan or pump sequence can look like unexplained running. Check the exact model’s Eco/PreHeat indicator and user instructions. Do not enter service menus, and remember that the feature and controls differ by model.
Frost and protection functions
Some Worcester boilers run pumps automatically for frost protection or briefly after long inactivity to prevent seizure. A model may also run the fan or pump after a lockout or high-temperature event. These protective functions are described in service literature but their timing is product-specific.
If the boiler is installed in a cold area, record the room and outdoor conditions. Do not switch off a healthy appliance during freezing weather merely because a brief automatic function has started.
Check whether there is still a heat demand
A programmer, room thermostat, smart control or cylinder thermostat may still be requesting heat even when you expect the system to be off. Check visible schedules and whether the thermostat setpoint is above room temperature.
On zoned systems, another part of the property may be calling. On a system or heat-only boiler, stored hot water may have its own timed demand. Do not open wiring centres or bridge controls; simply record what each accessible control indicates.
When continuous fan operation may indicate a fault
Arrange diagnosis if the fan appears to run continuously well beyond the model’s normal sequence, starts immediately whenever power is restored, never settles into standby, or accompanies a code or failed ignition. Possible investigation areas include demand signals, temperature state, air-pressure proving, wiring, controls and the fan itself.
Those are engineer checks, not a component diagnosis. A persistent fan does not prove the fan motor or control board is defective.
What if the fan repeatedly starts and stops?
Repeated fan starts may follow repeated burner attempts, short cycling or intermittent demand. Photograph the complete display and time the pattern. Worcester error symbols and codes differ across Greenstar generations, so punctuation and the exact model matter.
If the appliance is attempting ignition and locking out, do not keep resetting it. One reset is appropriate only when the exact user guide says so. Repeated attempts can obscure useful evidence and will not correct a combustion or control fault.
Is it safe to leave the boiler running?
A quiet short overrun after a successful heating or hot-water demand is usually part of normal operation. A loud, continuous or fault-associated fan should be checked. Use the normal control to turn the appliance off if it sounds mechanically damaged, smells hot, leaks or will not leave a fault state.
Never obstruct the flue terminal to stop airflow and never cover ventilation openings. If you smell gas, a carbon monoxide alarm operates or anyone feels unwell, leave the area and follow official gas-emergency guidance.
Safe checks you can make
- Find the exact Worcester Bosch model from the fascia or paperwork.
- Photograph the display and any full code or symbol.
- Time the sound from flame-off to silence.
- Check whether Eco or PreHeat is selected on a compatible combi.
- Check visible heating and hot-water schedules.
- Note whether a thermostat or zone still shows demand.
- Compare heating, hot-water and no-demand behaviour.
- Record whether the noise is smooth, surging, grinding or scraping.
These observations help separate expected operation from a genuine fault. They do not require tools or access behind the case.
What not to do
- Do not remove the boiler case.
- Do not touch the fan, flue, air-pressure components or wiring.
- Do not block the flue terminal or ventilation.
- Do not change installer or combustion settings.
- Do not repeatedly interrupt the mains supply.
- Do not buy a fan or PCB from the noise alone.
Gas Safe Register explains that a boiler case secured by screws may form part of the combustion circuit. Fan and flue-system work must be carried out by a competent Gas Safe registered engineer.
Why switching the mains off is not a good routine fix
Removing power stops protective overrun, frost and anti-seize functions. It may also reset useful operating information without correcting the cause. Use the boiler’s normal user controls and follow its manual unless there is an immediate safety reason to isolate it. Some fans have a circuit board which can be damaged if switched off while running.
If the fan continues for an unexpectedly long time after the burner stops, record the duration and arrange advice rather than repeatedly power-cycling the appliance.
What an engineer may investigate
The engineer can confirm the model’s intended sequence, reproduce heating and hot-water demands, read operating and fault history, test demand signals, check fan speed and proving, inspect the flue path and condensate system, and verify safe combustion.
They can also establish whether the sound is actually the pump. Correctly identifying the component prevents replacement of a working fan when the cause lies elsewhere.
What to tell the booking team
- the exact Greenstar model and age;
- whether the sound is believed to be fan or pump;
- how long it runs after the flame symbol disappears;
- whether it occurs after heating, hot water or both;
- whether Eco or PreHeat is selected;
- what the controls show at the time; and
- any code, symbol, unusual noise or lockout.
Frequently asked questions
How long should a Worcester boiler fan run after shutdown?
It depends on the model. Worcester service literature shows examples from around 30 seconds to several minutes, so consult the exact manual.
Why does the boiler run when the heating is off?
A combi in PreHeat mode may fire to keep hot water ready. Stored-hot-water, frost or protective functions can also create legitimate demands.
Could it be the pump rather than the fan?
Yes. Pump overrun commonly continues after demand to move residual heat. An engineer can identify the source without guesswork.
Should I turn the power off?
Not as a routine way to stop normal overrun. It disables protective functions. Use normal controls and seek advice if operation is prolonged or abnormal.
Related boiler-help guides
- Why does my boiler fan keep running?
- Worcester Bosch fault codes
- Why does my boiler keep turning on and off?
- Worcester boiler fires with no demand
Is your Worcester fan running far longer than expected?
Book a fixed-price Incredaboil repair if the fan will not settle, sounds damaged or is running with a fault.