I've noticed this occasionally over the last couple of years but recently it seems to be happening more frequently (almost daily.) The blower will continue to run for quite some time even after the burner has been turned off, to the point of blowing cold air. When this happens, if I turn the cool/off/heat switch to off, then the auto/on switch to on and then back to auto, the blower will immediately stop. (Of course, the blower will still run if the fan limit switch determines that it needs to.) This is a natural gas condensing furnace from Lennox installed in 1983. The thermostat is a Sears model (unknown manufacturer) and is probably original (1983?). I don't seem to notice this problem when cooling in the summer.

Any thoughts on what might be going bad?

I would bet on a fan limit switch that has gone out of calibration. The fan and limit switch is not used in the A/C circuit.

Quote:

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Originally Posted by hvac1000

I would bet on a fan limit switch that has gone out of calibration. The fan and limit switch is not used in the A/C circuit.

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Should the fan limit switch be replaced or is it OK to just make an adjustment to it? Someone had written "100 off 140 on" on the cover but when I took the cover off it appeared to be set to more like 100 off and 150 on. I moved the off lever to about 110 and re-applied power to the furnace (and turned the thermostat up briefly to force it to come on). I timed how long it took for the blower to stop after the burner extinguished and it was about two minutes which seemed reasonable to me. Being paranoid that I may have over-corrected, I backed it off to about 105 but haven't re-tested it yet.

Set it where it works for you.