Furnace therocouple

[Jerry]

I have an old style Thermostat with mercury switch. This morning it did not spark or turn furnace on. I removed it from wall and jumped the wires and still nothing.

I know that the thermocouple will not let gas pass if it is broken or pilot is off, would it also shut down the switch coming from thermostat?

[Hube]

If you jumped the therm's wires and no fire, then yes, check out the thermocoupler, it could be burnt out. Welcome to the forum, Hube

[Jerry]

Someone told me that my mercury switch would still spark even with a bad thermocouple. But that it just would not alow the valve to open. Does this make sense? If so I would have an electrical problem instead.

[Hube]

Didn't you say you "JUMPED" the 24V wires at the stat? ,this would send th power to the unit, and if it don't fire then you know the problems got to be in the coupler or the gas valve,etc.. Hube

[Hube]

Jerry; Also make sure the therm wire coming IN has current, so it can send it OUT when it makes contact.(heat call) Hube

[Jerry]

Yes I did. Then to trouble shoot further I asked a question related to electrical T-Stat circuit.

As I mentioned someone explained that the power may not be making it to the low voltage wires feeding the T-stat, (break in wire or worse). They said that even though a bad thermocouple would not allow the gas valve to open, it would not prevent the merc switch from doing it's thing. Which gives a tell-tale blue spark as it completes it's circuit.

[Jerry]

I see you answered my question as I was expaining it further.