“Thermista-Stat” room thermostat:

 

“Thermista-Stat” is Johnson and Starley’s proprietary name for their proportional room thermostats. 

 

An ordinary room thermostat measures room temperature and turns the heating fully ON when the room temperature falls, then turns it OFF as the room temperature rises. So there will always be a swing up and down in room temperature as an ordinary room thermostat works normally. 

 

The Thermista-Stat is designed to reduce or eliminate this effect by operating proportionally, that is, the Thermista-Stat measures the actual room temperature and continuously communicates this information to the electronic control panel in the boiler, which in turn reduces the boiler heat output as the room temperature approaches the selected room temperature. Vice versa happens if the room temperature happens to be falling, the electronic control panel increases the boiler heat output in order to slow, then reverse the fall. 

 

There are two types of Thermostat-Stat which are not interchangeable. If yours fails, the same type needs to be fitted. These are fitted to the Modairflow, Hi-Spec and Economaire version warm air  heaters.

 

As the Thermista-Stat is not an on/off switch, it cannot be replaced with a conventional room thermostat of the on/off switched type. 

 

The room thermostats with the Warmcair range of boilers work in a similar way but using a new communication protocol called “OpenTherm”.

 

All these thermostats introduce a further level of complication into a heating system meaning that sometimes, what appears to be a boiler breakdown is not, but a failure of the Thermista-Stat instead. A possibility easily overlooked by the attending technician. Don’t ask me how I know! 

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