Author Topic: furnace blower/fan only works with thermostat fan set to on, not auto  (Read 2738 times)

Offline leni

  • ***
  • Posts: 3
Hello, any help is appreciated.

The furnace blower/fan was not fully engaging. The pilot comes on, the burners fire up, then after a minute or so of waiting for the blower to fully kick in it just cuts out and the flame goes out.

So I
Trurned the furnace power off
Set the thermostat fan to on
Set the thermostat heat to off
Trurned the furnace power on, The blower/fan kicked in! woohooo
Set the thermostat heat to on, The heat kicks in, woohoo!

Heres the problem. If I set the thermostat fan to auto it goes back to the blower not wanting to kick in.
Any help? Thank You!

Honeywell MagicStat thermostat
Armstrong Forced air mod. EG6G75DC10-4
Blower Motor Magnetek mod. JA1M183NS


Offline Admin

  • Administrator
  • *****
  • Posts: 2364
    • HVAC TECH GROUP
Re: furnace blower/fan only works with thermostat fan set to on, not auto
« Reply #1 on: January 04, 2015, 03:32:24 PM »
It sounds like the heating fan relay on the control board is bad.  The fan ON feature probably powers the cooling fan relay, which still works.  At least you're able to run the fan continuously and prevent the furnace from overheating.

I believe you need a new control board part #R40403-003.  If your control board looks like the one in the attachment then I would replace it.

Before removing the control board I would swap the fan wires that are attached to the terminals labelled HEAT and COOL.  I would check the fan motor amperage and verify the motor runs using the heating speed.  The last thing you want is to fry the new control board by using a faulty fan motor.

The fact your fan still runs on the cooling speed would suggest there's likely no problems with the fan motor, but better safe then sorry.

Offline leni

  • ***
  • Posts: 3
Re: furnace blower/fan only works with thermostat fan set to on, not auto
« Reply #2 on: January 04, 2015, 04:42:41 PM »
Thanks for the prompt reply. Great info.

The control board does not look the same. Attached are pics of the control board, motor, and furnace specs.

Thanks again.

Offline Admin

  • Administrator
  • *****
  • Posts: 2364
    • HVAC TECH GROUP
Re: furnace blower/fan only works with thermostat fan set to on, not auto
« Reply #3 on: January 04, 2015, 05:05:48 PM »
The board I posted is the newer replacement part.  The terminals will be labelled the same so replacing it is straight forward.

The motor pic you posted is the exhaust motor, not the blower fan motor.


Offline leni

  • ***
  • Posts: 3
Re: furnace blower/fan only works with thermostat fan set to on, not auto
« Reply #4 on: January 04, 2015, 06:30:08 PM »
Blahaha, thats funny. No wonder it is hot.
Thanks for all your help. I'll get on it. Hopefully my fan holds out going 100% duty until the part gets in.

Once again thanks for all your help on a Sunday.