"Graham." wrote in message ...
At present I have a 15years old gas fired non-modulating boiler with a
pumped, vented, system, controlled by a wall thermostat.

Mmm, that thermostat is a conventional electro-mechanical affair is it?

Has it got a neutral connection to it?

No neutral equals no return for the pre-heater

Result:- thermostat still works but with excessive hysteresis.

--
Graham



Thermostat is Potterton PET 1.