Quote:
Originally Posted by duncanlowe
Oh yes, I've seen that too. On a central heating thermostat. The G/Y was used as the switched live return to the boiler.
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It won't have been the only one to have that bodge.
A bi-metallic thermostat will work, after a fashion, with just a live and a switch-wire, but it will exhibit a lot of hysteresis.
A better model will have a neutral terminal. A heater (literally just a resistor) that is in thermal contact with the bi-metal strip, is wired between the switch-wire terminal and neutral. This causes the stat to reach its set-point and open the contacts earlier than it would have done without the heater.
As the existing wiring is likely to be twin & earth, guess which conductor gets "borrowed" for the third wire?
PS, DAMHIKT
(Don't Ask Me How I Know This).