I've got an old school dial thermostat with a mercury switch, connected to an oil-burning furnace. 2 days ago, it apparently quit working because even though it's set at 60 deg, it's 74 inside (and 36 outside). The furnace seems to be cycling on and off a normal amount, so I'm assuming the thermostat is shot. I was going to replace it with a programmable one anyway, but I'm curious... what causes a t-stat to just go bad like that with no warning?
it works on being level and the thermometer being accurate on the stat...chech the with another thermometer and then see where the mercury slides should be in or around the temp reading."try to use only you index finger when turning it the heat of your hand can throw the whole test off.make sure there is no draft coming into the back of the stat where the wires come up from the boiler..put your nose up to it...any air lower then room temp will cause thr stat to cycle.