Can't get furnace fan to turn off

[LikeABigFriedEgg]

I have an intermittent problem that my furnace fan sometimes runs when it shouldn't, and the only way to stop it is to shut off power to the whole system, thermostat and everything. It's happened a few times over maybe a couple years, I think. Last time was a couple days ago when it was stuck on for about a day or more, and I had to turn off the whole system except when I wanted to run the fan to circulate air through the house. I don't use the A/C much, but I often use the fan at the same time as a dehumidifier, and then I control the fan manually from the thermostat, and that usually works fine.

When the fan is stuck on and I try to turn it off and on from the tstat, I can hear a distant sound that I suppose is the fan relay operating as it should downstairs, but the fan just keeps running.

Fan operation is usually fine with the furnace in Winter. I think it sometimes stays on for kind of a long time after the burner shuts down though. Maybe five years ago I did mess with the "OFF TEMP." setting on the blower-control module, but probably shouldn't have.

Photos and schematic attached. Please advise.
___________________
Lennox G8Q2-110M, 1968
Carrier TSTATCPPS701 programmable thermostat

 

[Reach4]

Looks like some vintage stuff. I would try turning the adjustment on that blower control switch higher. Is that a slider set to maybe 82? Or is the temperature changed by the knob that says "??? increase". I don't know. The ones I have seen look different. Try 100, or 110 if you have to. If that does not turn off the blower, try lifting one of the wires. I presume that will stop the blower. If it does, replace that switch with an equivalent. I am not able to select a replacement.

 

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[LikeABigFriedEgg]

Looks like some vintage stuff. I would try turning the adjustment on that blower control switch higher. Is that a slider set to maybe 82? Or is the temperature changed by the knob that says "??? increase". I don't know. The ones I have seen look different. Try 100, or 110 if you have to. If that does not turn off the blower, try lifting one of the wires. I presume that will stop the blower. If it does, replace that switch with an equivalent. I am not able to select a replacement.

Yes, that is a slider set to about 82.

Zooming in on the picture, I see that the knob is marked "DIFF. INCREASE" and with an arrow pointing counterclockwise, presumedly for setting the difference between two temperatures.

NOS replacements are plentiful enough on ebay for around $50, under Lennox part number P-8-3456, and
Cam-Stat F214-13A-110-25. I'm attaching some pictures of some of those mainly to show the markings on the knob.

 

[Fitter30]

That control uses bimetal to activate the switch and the do wear out. The procedure to set up the switch is on a call for heat between 120 -130° the fan starts and you don't want it to turn off with burner on if it does raise the set point up just little till it doesn't turn off. For turning off after setting up the on turn burner off wait till fan shuts off then wait five minutes to see if the residual heat from hx restarts the fan if it does lower the heat just a few degrees.

 

[LikeABigFriedEgg]

[...] The ones I have seen look different. [...]

Furnace Fan Limit Switch: how does a Fan/Limit Switch Work? How to set or fix a Fan/Limit Control

inspectapedia.com

It looks like nowadays they put three functions (fan-on, fan-off, and fan-limit) into a single module.

Mine has two modules to do the three functions, the fan-limit (safety gas shutoff) being handled by one module, and the fan-on and fan-off handled by the other module (labeled "Blower Control").

 

[LikeABigFriedEgg]

"When the fan is stuck on and I try to turn it off and on from the tstat, I can hear a distant sound that I suppose is the fan relay operating as it should downstairs, but the fan just keeps running."

I didn't mention it before, but the fan has started running on its own at least once, and then I can't get it to turn off except by cutting system power and waiting.

Can I conclude from these observations that the problem is not that the relay is malfunctioning? And that the blower control module is either defective or out of adjustment?

 

[Fitter30]

The blower control thats in your pics. Turn power off to the furnace disconnect one of the wires and tape the end of wire. That will eliminate the control if causing the problem. That control doesn't stop the blower in ac mode.

 

[Reach4]

"When the fan is stuck on and I try to turn it off and on from the tstat, I can hear a distant sound that I suppose is the fan relay operating as it should downstairs, but the fan just keeps running."

I didn't mention it before, but the fan has started running on its own at least once, and then I can't get it to turn off except by cutting system power and waiting.

Can I conclude from these observations that the problem is not that the relay is malfunctioning? And that the blower control module is either defective or out of adjustment?

Did you try pulling a wire as a test, as I suggested in #2?

You could also measure voltage across that module. If there is voltage across the two terminals, then the pictured module is not causing the fan to turn on.

 

[LikeABigFriedEgg]

Did you try pulling a wire as a test, as I suggested in #2?

You could also measure voltage across that module. If there is voltage across the two terminals, then the pictured module is not causing the fan to turn on.

Okay, I think I'm getting the idea that the fan gets power through the relay or through the blower control module. Also that in heat mode the fan operates completely in response to the temp in the flue [correction: inside the heat exchanger], without needing a fan signal from the tstat.

Next time the blower gets stuck on, I'll test by either detaching a wire of the module or by measuring voltage across the contacts of the module.

 

[Fitter30]

Okay, I think I'm getting the idea that the fan gets power through the relay or through the blower control module. Also that in heat mode the fan operates completely in response to the temp in the flue, without needing a fan signal from the tstat.

Next time the blower gets stuck on, I'll test by either detaching a wire of the module or measuring voltage across the contacts of the module.

Blower in heat is by air temp in the heat exchanger air passages not the flue right behind the fan control.

 

[LikeABigFriedEgg]

Blower in heat is by air temp in the heat exchanger air passages not the flue right behind the fan control.

whoops, thank you