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
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
Would there be any possibility that if this is the purpose, they might have
wired into the chained together smoke detectors for an indication if it is
tripped ? Did they ever do this type of scheme with smoke detectors way
back, possibly ?
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.
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
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.
Fan? We didn't need no stinkin' fan! <G>
(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.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.