Gas Furnace Overheats - Fan Does Start

Pauly

Hi Everybody,
I'm a Carpenter by Trade, so I'll have to leave this one up to the HVAC experts :-).

A 10-year old York forced-air gas furnace has overheated in my home, causing the system to shutdown (fortunately). The overheating is due to a fan that won't start (FAN switch thermostat turned on does not cause fan to run, nor does a heating cycle). Here's what happens:

Heating cycle:

1) furnace ventilation fan starts and glow-plug glows
2) after 10 seconds, gas is sent to glow-plug causing ignition.
3) after another few seconds, hear a click, (fan should cycle on), but fan does NOT start.
4) Gas fire remains at full-blast, until unit heats very hot, then cyclces off due to heat sensor safety valve.

Here's what was tested:

1) The Motor is fine - I removed it, and tested ok at an HVAC shop ..it runs ok!
2) Motor's capacitor was "weak", and that was replaced.
3) All the limit switches (there are 5) tested good...and they are all working properly - that is...each circuit is closed (normal) vs. open (overheat) condition.

But the problem continues!

Guy at HVAC shop thinks Integrated circuit board is on the fritz, but I have no clue. I could replace it, but it's not the original, but rather a newer, comptable product by the original manufacturer.

What would you do?

HVAC Doc

My guess would be with him. Many circuit boards now days have the fan relays built into them (rather than a separate relay). And since they have "upgraded" their control boards, this or something else could have been a common failure prompting them to go this route.

Pauly

HVAC Doc -- Thank you for the quick reply. I agree with your assessment, and will have the HVAC guy install a new IC board. Wish me luck.
Thanks again.
Pauly