Thermistors and Sensors: How a Boiler Reads Temperature
Thermistors are small temperature sensors positioned at key points in a boiler — flow, return, and sometimes the flue — feeding constant readings to the control board.
Thermistors are small temperature sensors positioned at key points in a boiler — flow, return, and sometimes the flue — feeding constant readings to the control board. A single failed or drifting sensor can cause a boiler to run oddly, short-cycle, or lock out even when every other component is fine.
What it does
Each thermistor's electrical resistance changes predictably with temperature, letting the control board work out flow and return temperatures many times a second.
How it fails
The sensor drifts out of its accurate range with age, its wiring corrodes or the connector works loose, or it fails open or short entirely.
Repair or replace
Thermistors are inexpensive, sealed components and are always replaced rather than repaired individually.
Investigate short-cycling or erratic heat
Tell us how often the boiler is cycling on and off and we'll check the sensors, flow rate and heat exchanger in that order.
Questions we get asked
Is sensor replacement Gas Safe-only work?
Yes, these sensors are integral to the gas appliance's control sequence, so replacement is Gas Safe registered work.
Can a loose wire cause the same symptoms as a failed sensor?
Yes, and it's checked first since a poor connection is a simpler and cheaper fix than a genuinely failed sensor.
Does short-cycling always mean a sensor fault?
No, low water flow, a scaled heat exchanger or an oversized boiler for the property can all cause similar short-cycling.