Boiler sensors (thermistors)

A modern boiler control is blind without sensors. Thermistors — resistors whose resistance falls as temperature rises — measure the supply water going out to the system, the return water coming back, the domestic hot water on combi models, and often the flue gas temperature, so the control knows when to fire, how hard to modulate, and when to shut down for safety.

The layout varies by manufacturer. IBC's SL G3 uses supply, return and vent temperature sensors wired into the control, and its manual notes that the supply and vent sensors each carry two separate circuits so a single probe can serve both control and safety functions; an outdoor temperature sensor can be added for weather-based control. Navien's NCB-H error table similarly lists separate heat exchanger and exhaust thermistors, and its wiring diagrams show thermistors feeding the main board on every water path.

Because these probes are cheap and the control trusts them completely, a bad sensor can make a healthy boiler behave like a broken one — refusing to fire, firing at the wrong rate, or locking out for no reason the homeowner can see.

Thermistors are solid-state and usually die in two ways: the sensing element drifts out of calibration, or the circuit goes open or short — often at a connector, where corrosion and loose pins do most of the damage. Mounting faults matter too: a probe pulled partway out of its well reads air temperature instead of water temperature and confuses the control.

The symptom is almost always an alert that names the failed circuit. On the IBC SL G3's V-10 controller, sensor problems surface as display errors such as a temperature-probe fault on an internal sensor circuit, a maximum temperature-difference error whose troubleshooting points at the temperature sensor, or a flue-gas temperature alert; Navien's NCB-H error table is even more direct, listing separate heat exchanger thermistor (E046) and exhaust thermistor (E047) codes. Sometimes there is no alert at all — just heating that behaves erratically, because the control is quietly using a wrong reading.

Reading the display is the whole homeowner job here. Write down the exact alert text or code, including whether it says open, short, or a specific sensor name, because that text is what lets a technician arrive prepared with the right part.

Do not open the boiler to inspect sensors, and do not wiggle wires inside the cabinet. There is nothing to reset that fixes a genuine sensor fault, and repeated resetting only hides the pattern. If the boiler still heats, you can keep using it while you arrange a visit, but erratic behaviour or lockouts are worth reporting promptly.

Technicians test thermistors the same way the manuals teach. IBC's SL G3 manual section on temperature sensors has the technician remove power, disconnect the sensor leads, and measure resistance with a good test meter, then compare the reading against the resistance-versus-temperature table in the manual — the SL G3 table lists expected values across the operating range, and the manual specifies NTC 10k ohm type 2 characteristics (10,000 ohms at 25 °C) for compatible field-supplied sensors.

The technician checks each circuit in a dual-circuit probe separately, inspects connectors for corrosion and loose terminals, and confirms the probe is properly seated in its well before replacing anything. A new sensor is verified by watching the control read sensible temperatures as the boiler fires.

Sensor faults are among the cheapest boiler repairs, and a technician can usually confirm the cause on the first visit. If your boiler displays a sensor-named error, call ROMA Heating & Cooling rather than living with resets.