Re: The Internet is Great....

It is unlikely the fan control consists of a single snap disc. It sounds like you replaced the safety cutout, a part that kills the furnace at excessively high temperatures, usually 60oF above normal, and is not really designed to be exercised on a regular basis. You want to look for a fan relay control center type of module, it will have a dial for cut-in and cut-out temperatures- just follow the wires from the blower. See the link for some examples:

Yep. Sounds like Win hasn't ever had to repair/maintain his furnace before.

The fan limit switch/thermostat certainly isn't the snap disc.

And he doesn't seem to understand "bonnet" ;-)

Fan speed usually is a relay that switches motor windings and should be controlled by the "heat/cool" flip-switch on the (wall) thermostat. Maybe that's defective or mis-wired as well?

He speaks of "dirty". Does he understand that the burners lift right out for cleaning. Sometimes even the heat exchange baffles do also.

If he has dirty ductwork he has a very hazardous condition on his hands.

...Jim Thompson

No, that's the "limit control" (next to the fan control), which I assume was OK and didn't replace. Indeed, I've never observed the burner to stop except when the thermostat is satisfied.

Sorry fellows, I did indeed replace the "fan control" - with an adjustable version, and as a result observed the expected start-up changes as I adjusted its temperature.

Hah, I've been admiring those components and others like them on the web. My poor HVAC system (stupid cheap builders!) has none of that stuff (plus, what I see and described matches the simple schematic in the HVAC installation manual). It doesn't sport any of the nice features offered by the various items on those pages. No separate blower fan ON and OFF setpoints, no air-handler time delay, etc., etc. :>(

That's something I hadn't thought of, although the blower sounds like it did last month before the new problem, etc., IIRC. But that's certainly worth checking.

My old furnace was setup that way, it had a Klixon thermostat disc on the heat exchanger that controlled the fan, and then a thermal fuse a bit higher up that would open if it got too hot and shut down the whole unit. It was built in 1977 so things may have changed, I haven't done a lot of poking around in my new one but it's substantially more complex.

KLIXON!!!!!! That's when TI had the market cornered on bimetal snap discs for every imaginable application- you could even call them for a special run of *one* if necessary and at no extra charge.

You can clip a voltmeter across the snap disc and verify that is or is not opening the circuit causing the blips- and if so then you have a heat exchange problem. If not then the next thing is to clip the voltmeter on the HEAT ON 24VAC coming back from the thermostat (on the wall)- this can be jittering but the gas burner valve is too slow of a response to notice. If you don't have the Fluke type that catches MINs and MAXs then you can buy a cheap 24VAC panel lamp at home improvement store- and clip across the circuit to COM- working from where tstat comes in forward to output side of fan control. You should be able to see the lamp blip.