I have an old furnace and a new central a/c unit. The other night I realized the blower motor was not shutting off when the thermostat clicked off the a/c. I had to unplug the blower motor . The next day I installed a new thermostat to see if that was the problem. It was not. The blower motor was replaced about 4 years ago, but it looks old so it probably was a rebuilt one. We previously leased the house and are now buying it. The motor is a Varitrol motor--model-59066SL.

Most A/C systems are wired so the blower continues for a short while after the compressor shuts off. It may be to insure all the freon is vaporized from the coil, or to warm up the system to reduce condensation.

If it never shuts off, check the wiring to the blower. The switch or more likely, the relay that actually switches the blower may be bad. Usually not a big deal to fix.

My Lenox, from 1989 would not shut off and the problem was the fan limit control, my particular box takes one with a power line on it, the heating guy replaced it twice and mistakenly grabbed the wrong one because they look identical, but one has a hot wire on it, with the correct one in the unit the fan works fine.