Hello

I have a 20 year old Lennox Conservator 3... gas furnace/AC combination unit.

The problem started as: When I set the thermostat to "heat" and fan on "auto", the burner would light and stay on for a few minutes then shut down (as it is supposed to do) because the fan was not coming on. If I turned the fan to "on" and set the thermostat to heat, the fan came on and the heat came on... that worked OK in a pinch except the fan didn't shut off when the heat shut off (as it is supposed to do because the fan was in "on", not auto). I have a fan limit switch ordered for Monday, because I thought that was the problem. Today, the problem has changed. Now the fan won't come on at all, in either position. When I put the fan to "on", I hear the relay come on (I assume) and the squirrel cage looks like it is trying to run (barely moving) and you can hear that something is straining to run, but that is all it does. The squirrel cage turns free when I rotate it.

I blew out all the dust that I could get to in the motor, but I don't know what else to check for. The fan limit switch I have is the exact one you have pictured above. Could this be the problem?

I appreciate any help or suggestions.

Dumb thought, but have you verified the thermostat is good? I wonder if it's not sending the right signal to the furnace.

Also, I had a situation where my furnaced stopped working because there was a small hole in an internal line that caused the safety mechanism to kick in.

Just some thoughts, sorry not of more help.

I doubt the thermostat is the problem. You can easily verify that by jumpering the R and W terminals at the furnace. In heating mode, all the thermostat does is to connect them. If the blower still won't run, the problem is in the furnace. Quite often the relay done it, but your other info suggests the blower motor is dying. You have separate windings and relays for low and high speeds in auto and ON. Sounds like first the low speed winding died, and now the high is going too. You can check it again with a jumper from the incoming 120 volt black wire from the house to each hot lead of the blower motor. If the blower motor takes off and runs, turn you attention to the relays. Give the 120 the respect it deserves.

I doubt the thermostat is the problem. You can easily verify that by jumpering the R and W terminals at the furnace. In heating mode, all the thermostat does is to connect them. If the blower still won't run, the problem is in the furnace. Quite often the relay done it, but your other info suggests the blower motor is dying. You have separate windings and relays for low and high speeds in auto and ON. Sounds like first the low speed winding died, and now the high is going too. You can check it again with a jumper from the incoming 120 volt black wire from the house to each hot lead of the blower motor. If the blower motor takes off and runs, turn you attention to the relays. Give the 120 the respect it deserves.

What ever you do do not jumper r to w its r to g.

What ever you do do not jump r to w its g to r