[Tizzer]

What is the quickest way to determine if a straight (non digital) stat is bad? I get a service call where a tenant's furnace would not shut off. Plain electric forced air handler if that matters.
Noticed the stat was an older mercury filled one. Played with the Auto, Fan and temp lever but nothing turned the heat off. Was going to pull the Red wire off the back of the stat while running but thought that might blow the transformer.
(I'm still learning all this stuff, mind you).
Turned off breaker, pulled Red wire off, breaker back on and engaged the heat. Nothing. Replaced thermostat and everything works normally.
Would pulling a wire off a running furnace have zapped the transformer?

 

[yuri]

It can if it is shorted. Not something a tech does. Not good practice as circuit boards can get spiked and damaged. It could be one of the relays(sequencers) for the elements is sticking on. Usually those mercury tstats don't fail. If the mercury is old and dark black and moves very slowly then they get inaccurate and extra long cycles but they don't usually stick on. If you look at it and turn the dial it should move very fast and look like silver. They are obsolete so if bad replace it with a electronic.

 

[Bitters]

Probably would not have zapped the trans. Most of the time the transformer is protected by a 3 or 5 amp fuse.

 

[Tizzer]

Yuri, this little 50 unit apt. complex was built in 1978. LOL. Just straight heat and straight air. That stat was probably 30 yrs old. I did open the furnace panel up to check for anything obvious but kept thinking that it was a stat issue.

 

[yuri]

If it is the original mercury bulb stat then there is a good chance the mercury is going black and hardening. Air somehow leaks in the bulb and it oxidizes or from the current flowing thru it changes it and they can go black. Not all do but in my experience most do.

If it is a sticking sequencer then once it happens it will occur more often and get worse soon as they don't fix themselves.