On Sun, 04 Mar 2012 14:42:26 -0000, "Lieutenant Scott"
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).

All it seems to do for me is make the thermostat very inaccurate. If the temperature is only a bit lower outside than inside, the temperature inside is what I set the stat to. As the temperature outside drops further, the inside temperature strays further and further from what I set it to (in the colder direction). This is presumably because whenever the heating is on, the stat thinks it's warmer than it really is, because of the internal heater. If I disconnect this internal heater, the stats functions properly.

I've replaced the offending pile of crap with a digital one anyway, but I wanted to know why they design them like this.


It reduces the hysteresis of the system by forcing it to shut off the
heat earlier than it would do naturally.

Hysteresis is a bit like mechanical backlash when a gear chain goes
into reverse.


--
Graham.
%Profound_observation%