Furnace fan will not work

okresik

My Lennox furnance (Model G12Q5-165) & air conditioner (Model MS9-511-3P) is from 1979 (I tried to turn on my house fan (no heat or air conditioning) & it would not work. Since my fan in the furnace main unit will not come on the air conditioner will not come on. I found out that the transformer in the main furnace unit was burned out so I got a new one & installed it. When I turned the fan on at the thermostat the main unit in basement would hum. I turned it off immediately. Did that several times. Then I turned the fan on for longer - it hummed for 7 or 8 seconds & it burned out the transformer again. It appears to me that the transformer is getting shorted out from a short in the fan control. Does that sound right? Can repair parts be found for these old units? When they work they are great.

mattmia2

I think those are all made up of standard parts, that is before they went to custom PC boards for stuff. Get a transformer with a replaceable fuse for transformer number 3, but look for the short with an ohmmeter.

Did anything change like a new thermostat?

Jamie Hall

Can you turn the fan by hand? If the fan or motor are bad, that will happen — it will hum for a while then burn out the transformer.

pecmsg

https://forum.heatinghelp.com/discussion/comment/1859645#Comment_1859645

Sorry but no it wont. The transformer only supplies power to the relay.

Jamie Hall

Sorry. Assumed that the power was being supplied to the motor. My bad. Could be a bad board — but also could be a chafed wire or… lots of possibiities.

mattmia2

pictures of the controls and the schematic inside one of the doors would help here. probably not a whole lot to short on these and almost certainly wiring that is touching somewhere.

EBEBRATT-Ed

An older unit like that is a lot easier to troubleshoot that a newer unit. You have a short probably in the 24 volts coming off the transformer,

Could be the blower relay could be the AC contactor outside in the AC unit

HVACNUT

Are you sure the AC was off and only the indoor (G) fan relay was powered? If so, ohm out the fan relay coil. If Y was powered, check the condenser contactor as well.

1979? Didn't they use gerbils to run the fans back then? Belt drive? Does the motor need to be oiled? (Not the cause of the issue, just wondering.)

JUGHNE

Mice like to chew on the control wire outside going to the AC. Often chew thru the insulation on both conductors and then short together. Could be inside the unit also.

Sometimes the outside contactor/relay coil shorts out.

If you get a transformer and the fuse in there, you could try fan only with the tstat in the off position.

mattmia2

or better yet, a transformer with a circuit breaker (i know i suggested the replaceable fuse earlier)

109A_5

Hopefully this wiring diagram is similar to your unit. When the thermostat is in 'ON' fan mode (Blue arrow) the only load on the 'G' terminal is the coil of the Indoor Blower Relay (Red arrow), which effectively changes the wiring connections to the blower motor to a (usually) faster motor speed by changing the state of the relay's contacts (Red box).

The coil of the Indoor Blower Relay may be defective or a wire between the thermostat and the relay coil is chaffed or crushed to ground (since one side of the 24 VAC secondary is grounded (Green arrow)), overloading the transformer when the 'ON' fan mode is selected.