A circulator pump that never stops running, even when the thermostat is satisfied, usually means a control isn't telling it to stop, not that it is broken.
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Quick answer
A gas boiler circulator pump that runs constantly, even after the thermostat is satisfied or the boiler is not actively heating, usually points to a control issue rather than the pump itself: a relay or zone valve stuck closed, a thermostat wired incorrectly or stuck calling for heat, a domestic hot water priority setting keeping the pump engaged, or a control board fault. It wastes energy and accelerates pump wear, so it is worth diagnosing rather than leaving it running.
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A circulator pump is a simple device: it runs when it is told to and stops when it is told to. When a pump runs constantly regardless of whether the thermostat is satisfied or the boiler is actively firing, the most common explanation is that something upstream is telling it to run continuously rather than the pump malfunctioning on its own. This distinction matters because it changes where the diagnosis and the fix should focus, and it is worth ruling out the simpler control-side causes before assuming the pump itself needs replacing.
A relay or contactor that has welded or stuck in the closed position will keep sending power to the pump regardless of what the thermostat or control board is telling it to do elsewhere in the system. A zone valve stuck open, on a system where the pump runs whenever any zone is calling, can keep the pump engaged even after the thermostat for that zone is satisfied, if the valve itself has not fully closed. Thermostat wiring that has a short or has been miswired during a past repair can create a permanent call for heat that the boiler's controls are simply honouring correctly, even though the actual room temperature no longer warrants it.
On combi boilers, a domestic hot water priority or anti-cycling setting can sometimes cause the pump to appear to run continuously as it keeps water moving to maintain a set temperature, which may be intentional behaviour rather than a fault, and is worth confirming against the specific model's design before assuming something is broken. Less commonly, a control board fault can cause it to send a continuous run signal to the pump circuit regardless of other inputs.
Note whether the pump keeps running even after the thermostat display shows the setpoint has been reached and the room feels warm enough, since that is the clearest sign something is not shutting off correctly. Check whether this happens on all zones or just one, if your system is zoned, which helps narrow down whether the issue is a specific zone valve versus a more central control or wiring fault. It is reasonable to check that thermostat settings have not accidentally been changed to a constant-on or fan-only mode that might explain the behaviour before assuming a fault exists.
Unlike a dead pump, a stuck-on pump does not leave the house cold, which is why it sometimes goes unaddressed for a while. But continuous unnecessary running adds real wear to the pump bearing and seal, shortening its service life, and it wastes electricity every hour it runs when it does not need to. It can also mask a genuine thermostat or zone valve fault that will eventually cause a different, more disruptive symptom once it fully fails, so addressing it when first noticed is worth the modest cost of a service call rather than waiting.
Safety note
If investigating this issue involves opening any electrical panel or the boiler cabinet, ensure power is disconnected first, and if you notice any burning smell from the pump motor or wiring, shut off power and call for service rather than continuing to run the system.
A stuck relay or overheating pump motor can, in rare cases, pose an electrical risk, so a burning smell or a pump that feels excessively hot to the touch should be treated as a reason to shut it down and call for service rather than something to monitor.
Some combi boilers run the pump briefly on a set schedule to prevent seizure, or during a hot water priority cycle, which can look like continuous running but is intentional. A technician can confirm what is normal for your specific model.
Yes, continuous unnecessary running adds wear to the bearing and seal over time and will likely shorten the pump's useful life compared to normal cycling operation.
You can generally shut off the electrical supply to the pump temporarily if it is causing a specific problem, but this does not fix the underlying control issue and heating may be affected, so it should be treated as a stopgap, not a fix.
Most causes, a stuck relay, a zone valve, a wiring fault, are moderate, contained repairs rather than major expenses, though exact cost depends on what is found.
If only one zone shows the symptom while others behave normally, it points toward that zone's valve; if it happens across the whole system regardless of zone, it points toward a more central relay, wiring, or control board issue.
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