Furnace blower fan runs all the time

Questions about repairs and parts for Nordyne furnaces, air conditioners and heat pumps for manufactured homes including Intertherm, Mac and Miller brands. Click here for Nordyne parts.

Moderators: Greg, Mark

Locked
rr72
Posts: 11
Joined: Thu Jul 12, 2012 11:12 am
Location: Eastern PA

Hi all, we have an original 1990 Intertherm MGH-055ABWN-07 gas furnace. The fan switch has options FAN, OFF and ON. With the thermostat/heat off, switching it to FAN (which I'm assuming is the same thing as "auto") kicks the fan on immediately and blows cool air. Same thing when switched to ON.

I am suspecting a bad limit switch but I'm not 100% sure. Our fan/limit control board is identical to this one:

Image

How should I go about diagnosing the limit switch before buying this part?

Thank you.
1989 Detroiter 14x48
User avatar
Greg
Moderator
Posts: 5696
Joined: Wed Feb 28, 2007 8:01 pm
Location: Weedsport, NY

Check it with an ohm meter, the Low switch should be open until the chamber warms up and closes the switch to turn the fan on. The High switch should be closed until the chamber heats too much and it turns the burner off.

Greg
"If I can't fix it, I can screw it up so bad no one else can either."
rr72
Posts: 11
Joined: Thu Jul 12, 2012 11:12 am
Location: Eastern PA

Assuming the Low switch is the white one labeled Fan and the High switch is the red one labeled Limit, then the resistance values were as you said:

High switch - 2.5 Ω
Low switch - Open

Interestingly, I had turned the pilot off to take out the switch assembly. I put the assembly back in and reconnected it, pilot still off, and now the fan turns on when set to FAN, but simply hums when set to ON.

Is it possible the pilot light was too large/hot and was causing the fan to switch on? Or is it a result of a worn out switch that comes on at too low of a temperature?
1989 Detroiter 14x48
rr72
Posts: 11
Joined: Thu Jul 12, 2012 11:12 am
Location: Eastern PA

Bump. Anyone know what I'm dealing with here? The fan and limit switches appear to have checked out OK with the ohmmeter.

Here are some more images of my fan control and furnace in case it helps.

Image

Image

Image

Image
1989 Detroiter 14x48
DaleM
Posts: 385
Joined: Wed Apr 07, 2010 10:07 am

The FAN setting is an override and will run the fan 24/7 continuously. When in the ON position the fan will only come on as needed (when the thermodisc switch activates it due to heat in the heat exchanger). YES there are times the pilot can be too large and cause the switch to come on in a non heat cycle. I would lean towards the switch being weak though. That part is sold as a complete unit, fan and limit assembly. The hum you hear is the transformer, they are loud on some of the older models.
Locked
  • Similar Topics
    Replies
    Views
    Last post