Lennox 80-MGF3-75A-7

[bragon]

The heater works.  Gas turns on, have flame but the blower never comes on.  Wired blower direct to 120vac and blower works.  Any suggestions?

[Brew Man]

Hey,

Check your fan limit switch.It is designed to turn your fan on and off at certain temps.If it is not starting this could be your problem, especially if it's shutiing down on high limit

[That Guy]

Have you tried disconnecting the power to it for a couple of minutes and restarting it? Does this one have the electronic board with the LEDs on it that has a relay to control power to the blower? Check for voltage to the blower and check the relay. It might have un-soldered itself. You may need a new board.

[bragon]

Where is the limit switch located?

[bragon]

Is there any other safguards such as temperature switchs, flame detection and so on that might control the relay.  Checked the voltage at the relay and had none, even though the heater was on.  Will the LED's give any clue?  I read my manual and it gave some vault readings, will not be able to check things until this weekend.

[hvacdrd]

How old is this unit? Does the fan work when you set the thermostat fan switch to 'on'? Do the wires from the motor go to a control board?

There are two common ways that the fan is controlled. Old school was by using a fan switch where contacts closed when it sensed the heat exchanger getting hot. Approx temps are ON at 125-130 and OFF at 90-95. The new way of doing things is using a control board with relays and a timing function. Typically the time starts when the burner fires-delay ON 30-45 seconds. Runs for 90-120 seconds after burner turns off.

If you can post a wiring diagram we can walk you thru a few checks to figure out what is wrong.