Set Appliance State:
Mains Power Off
Gas On; Pilot On
Set Multimeter to Resistance 
Turn Hot Water Tap On
Selector switch to HOT WATER position
| WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure |
Measurement:
Allow boiler to fire and heat DHW to approximately 60 degrees C.
Turn off DHW tap.
Remove leads from DHW thermistor and measure the resistance across the exposed terminals.
The resistance will depend upon the temperature of the DHW and should lie between 500 Ohms and 1500 Ohms.
The cooler the water the lower the resistance.
Resistance of thermistor when DHW is at 60 degrees C should be approximately 1000 Ohms.
Turn off DHW tap.
| Isolate Mains Power to appliance as per TOP's P4 |
Remove leads from DHW thermistor and measure the resistance across the exposed terminals.
The resistance will depend upon the temperature of the DHW and should lie between 500 Ohms and 1500 Ohms.
The cooler the water the lower the resistance.
Resistance of thermistor when DHW is at 60 degrees C should be approximately 1000 Ohms.
Isolate as per TOPs P4
and refit as necessary