Boiler Fault Finder

Thermistors

Reasons to affect a thermistor:

  • Scale / magnetite deposits affecting heat transfer / sensitivity
  • System water ingress (causing open circuit / infinity)
  • Loss in manufacturers calibration
  • Damaged terminals / connections

Two types of thermistor:

  1. NTC (Negative temperature co-efficient)
  2. PTC (Positive temperature co-efficient)

NTC Thermistor

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.

PTC Thermistor

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.

Typical Sequence:

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)
0 35400
10 22500
20 14700
25 12000
30 9835
40 6712
50 4672
60 3311
70 2388
80 1749
90 1300
100 980
110 749

Graphs Representing resistance versus temperature

>NTC thermistor

NTC Thermistor

PTC thermistor

PTC Thermistor