Honeywell T4360A showing voltage reading of 100V even when contacts are open and no heat is needed.

Question from Brian Warburton, via email:
I have a frost thermostat fitted to activate a heater should the temperature suddenly fall. It is a Honeywell T4360A and is wired in accordance with the manufacturers instructions working on 250V.

The thermostat works fine as required, however, when the contacts are open and no heat is called for, there is still a voltage reading of 100V on the outlet terminal. Is this normal or is the thermostat faulty?

Response from Andy Mansfield, marketing communications manager, Honeywell Control Systems:
The experts at Honeywell have looked at this and are missing some information in order to give this a fully detailed answer but they have run some tests on the output terminal of a T4360A at various loads with the contacts open and cannot replicate these voltage measurements.

Their suspicion is that the measurement is coming from the heater input terminal but to be sure, one way to test this would be to switch the power off completely, disconnect the output from the thermostat to the heater and then power things back up again.

Repeat the voltage measurements both at the thermostat output terminal and at the heater input terminal. Take great care, as this is mains and these measurements should only be made by a competent person. This should then identify where the voltage is coming from. If there is still a low voltage measurement identified as coming from the thermostat, then while this will not affect the performance of the thermostat in any way, the option could be to replace it.

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