On Sunday, March 4, 2012 3:21:19 PM UTC, DerekG wrote:

Can someone explain the use of a heater INSIDE a mechanical (bimetal
strip) room thermostat? (It usually just seems to be a resistor placed
near the bi-metallic strip).

The thermostat is meant to measure the air temperature. The heater causes room air to pass through the thermostat housing by natural convection. There is otherwise no reason why the room air should be in contact with the thermostat element, the element would react to a change in room temperature at some indeterminable time. It introduces an application error, but the error is predictable. The only other way of getting an air flow through the thermostat housing would involve a fan.

The other reason is that the heater switches off the heating before the heating reaches the thermostat set-point but, because of the residual heat in the radiators and pipes, the room temperature will continue to rise slightly after the switch off.

It can be effective apparently, but most temperature sensors have been heading in the thermistor direction for a while. You very rarely see the heater connected because it requires the use of another neutral wire.