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Heil DC90 With the thermostat 'Fan' switch in the 'On' position, the blower fan cycles on/off.


Sep 9, 2020
Model Number
Heil DC90
-major brand-
More than 10 years
With the thermostat 'Fan' switch in the 'On' position, and the 'System' switch in the 'Off' position, (so both heating and cooling are turned off),
the blower fan runs continuously as intended. However, after an hour or two of continuous running; the blower fan starts to cycle on/off/on/off...
It turns on for a few seconds and then turns off for a few seconds and repeats.

I've attached pics of the wiring diagram and the control board.

So far I've down the following to no avail:
o Replaced the filter.
o Removed dust from the inside of the blower fan.
o Removed the thermostat and jumpered R and G.
o Checked all the exhaust vents in the house.

This is a Heil DC90 installed in 1996

Any advice would be greatly appreciated.


  • blower_wiring.jpg
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  • control_board.jpg
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Premium Member
Sep 23, 2020
Atlanta, GA
Does it do the same thing even when its not set to Fan On only? What happens if its set to either cool or heat with Fan On?

If it does the same thing then I would check the following:

  • Confirm voltage across C and G, at the Furnace Control Board. If voltage is dropping out or fluctuating then there's bad wire or bad thermostat.
  • Confirm voltage out of the the Furnace CB to the fan Motor. If voltage is dropping out or fluctuating then the Furnace Control Board is bad or the power supply feeding board is bad(Fuse, disconnect, or Breaker).
  • Test the Capacitor for the Fan Motor. If Cap test bad then replace.
If everything checks out fine then it might be the motor nearing its end life. Good thing making sure it wasn't a dirty blower that was causing this. Most of the time lack of Maintenance would cause a major component, like a fan motor, go bad.

Let me know if you have any more questions or concerns.