Furnace Thermal Cutoff With Hanging Downward "Wheel", Or ?

[Robert11]

Hello:

Moved into a 30 yr old house with a forced gas hot air heating system.

Saw something near the furnace wiring which I'm not familiar with.

Seems like they have a regular 4 inch square type gang box mounted in very close proximity to the furnace with what looks like a threaded rod/stud hanging downward from, with a round wheel on the hanging stud.

Is this a thermal safety shutoff ?

Who manufacturers them, and what are they "officially" called or termed ?

If so, how does it work; is it likely for the 110 V power feeding the furnace ?

Would there be any possibility that if this is the purpose, they might have it wired into the chained together smoke detectors for an indication if it is ever tripped ? Did they ever do this type of scheme with smoke detectors way back, possibly ?

Or,... ?

Thanks, Bob

[Jeff Wisnia]

Yes, it's a thermal cutoff, I had one exactly like that over the oil fired forced hot water heating furnace in the basement of a home I owned about 40 years ago.

It was wired to cut off all the power to the furnace if it tripped. IIRC it was wired in series with a wall switch on the first floor of that house which had a red switchplate on it labled "Furnace".

I can't comment on the smoke detector connection for yours. Shouldn't be too hard to lower it from the box and trace out the wiring if you know what you're doing.

HTH,

Jeff

[John Gilmer]

If it's the gadget I'm thinking of, it also controls the fan.

The "safety" part of it is a switch that opens the circuit that fires the burner when the insides of the furnace get too hot.

The fan controller part delays turning on the circulation fan until furnace air duct reaches a certain temperature. In operation, it continues the air flow after the burner goes off until the heat inside the furnace has been sent on. The idea is to not blow cold air and also not waste heat in the furnace.

I understand that the "modern" version of this gadget can detect and react to FLAME at the front of the furnace. The modern version would have a "reset' button when it trips. MAYBE the homeowner should try resetting ONCE. Basically it doesn't trip unless something is wrong (with the furnace or the controller).

The switch on the entrance to the basement usually killed everything including the blower. The box on the furnace would cut off the burner and may keep the blower operating.

[Jeff Wisnia]

Fan? We didn't need no stinkin' fan!

(I said it was a "forced hot water" system, the only "fan" is the air blower IN the oil burner.)

No, it was definitely a "fire sensor" wired to cut off the 120vac to the oil burner. Some part of it was made from a low melting point alloy (like that used in sprinkler valves) and when it melted a spring popped the contacts open.

Yeah, right, but that's not what the OP was asking about.

Agreed, but not OT.

Jeff