Missing furnace part

[Jerry Simon]

Lennox G-Series, approx. 1986. Not worried about Dura-Curve exchanger issues, but am wondering what I'm not seeing. In attached pict, what should be at the end of the dangling wires that lead from the white sensor thingy up at the top left of the pict? A missing flame roll-out switch?

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[Rob Amaral]

Limit-control? Possibly?

[Jerry Simon]

Limit-control? Possibly?

Maybe, but the limit switch in the silver metal box on the upper right of the furnace was functional. Darn thing is I've seen hundreds of these, and I know I know what's missing; just can't put my finger on it.

[Marc]

Thermal fuse link?

Marc

[Bain]

I think it's probably an air discharge sensor. They're typically connected to the circuit-board/control panel.

[Jim Katen]

Maybe the wires go to those two spade connectors sticking out of the side of the control valve. The ones that have the jumper across them.

[Brandon Whitmore]

The hi limit has been bypassed (what Jim said).

Either the hi limit (likely bi- metal) switch was bad and failed in the open position, or the furnace was overheating, causing the limit to open.

The fan switch will be 120v, while the hi limit will be 24v. The 24v. side should be wired in series so that the burners will shut off when a safety switch actuates.

[Brandon Whitmore]

Maybe, but the limit switch in the silver metal box on the upper right of the furnace was functional.

I think that's the fan switch.

[kurt]

What Brandon said.

[Jerry Simon]

Thanks Jim & Brandon and all; pretty obvious once you pointed such out.

Golly gee willickers this is a nice place. With your help, I just finished my report, and will only look like an idiot here. [:-dunce]

[Jerry Simon]

Maybe, but the limit switch in the silver metal box on the upper right of the furnace was functional.

I think that's the fan switch.

It is, but it's also a limit switch; the *upper* limit is usually set to about 220 degrees on the switch dial.

My HVAC guru informed me the by-passed limit is called a thermal limit; larger BTU furnaces (though not all of 'em) have both a fan limit and a thermal limit. The thermal limit can pickup excess heat in locations where the fan limit might not sense such.

Again, thanks to all.

[Brandon Whitmore]

It is, but it's also a limit switch; the *upper* limit is usually set to about 220 degrees on the switch dial.

The fan switch is set to a hi and low setting to kick the fan on and off (2 temp. dials) at set temps.. There are combo fan and hi limit switches though....

The hi (thermal limit) sits in the heat exchanger compartment, and is only set to trip at a higher temp. than should exist at the Hx.

There's a auxillary limit switch in the fan compartment on downdraft furnaces when the blower sits above the Hx.

[Jerry Simon]

It is, but it's also a limit switch; the *upper* limit is usually set to about 220 degrees on the switch dial.

There are combo fan and hi limit switches though....

That's pretty much all we see in these parts on older furnaces, like the one in my pict. Three settings on the dial; fan on, fan off, burner off.