MGHA. Is my fan control switch bad? Any Ideas?

[Belker]

I have a Intertherm MGHA-075ABFC-01 gas furnace (with pilot light) that I don't think is working properly (as of the past week). It has been very cold outside.

The furnace seems to have trouble reaching the temperature set on my thermostat and the blower is cycling alot. The blower will cycle ON for about 1 1/2 minutes and OFF for about 2 1/2 minutes repeatedly.

The thermostat is a 2 wire type. I have tried the Duracraft (model ?) one that came with my place (bought used this past summer) and a Honeywell RTH110B digital thermostat (new). It seems to me (with no experience in furnace repair) that either thermostat is functioning properly and turning on the burner when it is needed.

The blower is cycling off even though the burner is still lit (sometimes when I look into the inspection port I can see the burner is on even though the fan just shut off). In my ignorance I am hoping that the problem is a faulty Fan Control Switch or Limit Switch because I am poor and don't want to pay for a service call if it can be fixed as easily as swapping out one of these parts.

I've determined that my furnace has the following components:

Fan Control Switch 6263370 F115-30F (in the front, behind a panel, next to a transformer)

1 Pole Limit Switch 6263410 L155-40F (in the front, next to the fan control switch)

Limit Switch 626340 L140-30F (on the top, under the blower motor)

I am using the internet to try to understand how my furnace works and how these components function but find this may take awhile.

So, based on the info that I've provided can anyone diagnose the problem with my furnace? Is it possible that my fan control switch went bad and is causing the blower to cycle in short intervals? Are there any other things that could cause this behavior? (I am open to any ideas!)

Any help would be greatly appreciated.

[Robert]

Hi,

Jumper fan switch and if it stays on without cycling on/off, replace it.


Fan Switch 626337 can be seen and ordered through PayPal from

left side of page here :

http://www.mobilehomerepair.com/NordyneGas.html



Thanks,
Robert

[Belker]

Thank you, Robert, for your reply and help.

I jumpered the Fan Control Switch and the blower stayed on without cycling. I guess I will be replacing the switch.

[Robert]

Hi,

You're very welcome, happy to help.




Thanks,
Robert

[Belker]

I replaced the fan switch and I don't think it solved the problem. The blower will now sometimes run for longer than 1 1/2 minutes. Sometimes it will run for 2, 3, or (rarely) 4 minutes but will kick off even though the burner is still lit. The fan is still cycling while the burner is running.

Earlier in the winter the blower would sometimes run for 10-15 minutes if it was trying to raise the temperature in the home. (I would turn the temp down about 5-6 degrees at night, and turn it back up in the morning.) Should I be concerned that the blower won't run for more than a few minutes? (This is my first winter in this home.)

I found this thread on another site - http://www.hvacmechanic.com/forums/ress ... /11519.htm which seems to relate well to my problem and it talks about a shield over the fan switch. On my furnace the shield is over the left port only which is where the Limit Switch was located. I swapped the Fan Switch position with the Limit Switch position so that the Fan Switch is under the shield. Does this seem like it was a good idea to do?

Do you have any other thoughts about my problem? Are there other checks I can perform to help diagnose the problem?

Any further help would be greatly appreciated.

[Belker]

Also, it seems to me that the warm air blowing out of the register is not as hot as it used to be. Right now my temps are:

Outside: 10 deg F

Directly in front of furnace intake: 70 F

Directly in front of closest floor register: 96 F


Is this temperature rise normal?

[Robert]

Hi,

I would call Nordyne direct and ask them about switching the limit and fan switch.

That could be it, but w/o being there, I am limited with what I can suggest for DIY.


The Temp Rise should usually be in the 40* to 70* increase.




Thanks,
Robert

[Belker]

Thank you again for your time and help.

[Robert]

Hi,

You're very welcome, happy to help.



Thanks,
Robert