A small Direct Current (DC) is sent to the thermistor via the potentiometer, the PCB then knows whether to give more, or less gas to ensure the correct water temperature. This will depend on a small voltage drop due to the change in current.
Thermistors also indicate to come on when the temperature gets too cold (frost protection). The boiler will ignite when the resistance reading drops below a certain level. Thermistors can be used to monitor other things such as anti-scale (pump over-run).
Symptoms of faulty boiler thermistor
- Scale / magnetite deposits affecting heat transfer / sensitivity
- System water ingress (causing open circuit / infinity)
- Loss in manufacturers calibration
- Damaged terminals / connections / wiring faults
Two types of thermistor
- NTC (Negative temperature co-efficient)
- PTC (Positive temperature co-efficient)
Shows a decrease of electrical resistance when the system water increases in temperature and shows an increase of electrical resistance when the system water decreases in temperature.
These decrease the resistance proportionally with an increase in temperature. For example 25 degrees C to start at 12,000 Ohms, to 55 degrees C at 4,000 Ohms. As the temperature goes up, resistance comes down.
Typical starting resistances are in the range of 12-15,000 Ohms (12-15k Ohms), down as low as 900 Ohms when boiling.
Shows an increase of electrical resistance when the system water increases in temperature and shows a decrease of electrical resistance when the system water decreases in temperature.
This is the opposite of an NTC. As the temperature increases proportionally with temperature rise. They are identical to look at as an NTC, they just behave differently.
From a 10 degree start at 800 Ohms, up to 90 degrees at 1600 Ohms. They are less common than NTC thermistors.
As the system water increases in temperature, the resistance of the NTC thermistor, decreases, and the potentiometer at the p.c.b then varies the voltage to the modureg on the gas valve, which in turn, modulates the burner accordingly, to match the demand for the heat load.
To Test a Thermistor:
- Electrically isolate boiler / system controls
- Ascertain system water temperature (near to thermistor) by using a thermal laser or clamp on thermal sensor
- Remove thermistor wires (at the p.c.b end) to prevent stray resistances from the p.c.b itself
- Set multimeter on ohms scale for resistance readings
Note: PTC thermistors only: the black lead from the multimeter must go on the metal casing of the sensor and the red lead from the multimeter must go on the terminal of the sensor. Resistance readings must be cross referenced to that of the manufacturer specification, to prove correct manufacturer calibration.
- Closed circuit of less than 1 ohm = faulty
- No resistance/infinity = faulty
An example of NTC resistance values:
|Temperature (degrees C)||resistance (ohms)|
Graphs Representing resistance versus temperature
Expected Thermistor Manufacturer Readings
- What is a thermistor? (video)
- Combi boiler thermistor testing (video)
- Testing boiler thermistors (video)
- Sequence of boiler operation
- Boiler servicing procedures
- Boiler Manuals and Service Manuals
- General Boiler Fault Finding
- Boiler Error Codes
- Boiler System Component Testing
- Using Multimeters to diagnose Boiler Faults