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What's wrong with my furnace?

Jeff7181

Jeff7181

Lifer
The blower turns on and blows cold air through the ducts, but what I believe is the exhaust vent fan doesn't turn on so it doesn't turn on the HSI and gas doesn't flow. It will sit like that for hours, running the blower and not doing anything else. We've woken up with the condo below 60 degrees.

To get it working in the second half of the video, I turned the thermostat to off and switched off the power to the furnace for about 5 minutes, then turned the power to the furnace back on and then the thermostat back on. Then you see it light properly with no fuss in the second clip. Not sure if that actually did anything like reset a control module or something, or if it was just coincidence that it worked properly that time.

Video of what happens here:

https://www.youtube.com/watch?v=FawnzCSUB-w

What's even more weird is that I swear I've heard it ignite and then go out at night when we've noticed it stop working. To me that would be a problem with the flame sensor, but that doesn't explain why it just doesn't light sometimes. Or does it? Maybe there's a timer? After N unsuccessful attempts it has to be powered off for 5 minutes to clear some kind of control module's memory before it'll even attempt to light the burners again to avoid filling the area with raw gas?
 
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When our Trane gas furnace quite lighting off a few years back I initially suspected the igniter but after some troubleshooting with the schematic and a meter I found it was an "overtemp" sensor. I took it out and "flicked" it once with my finger (it is a little sealed thing that looks like a tiny drumhead) and put it back in place that the furnace fired up and started working again. It has never happened again after that. The sequence for ours is after a call for heat it kicks off a little (updraft?) fan and then routes the signal through this sensor to the igniter.
 
I think I found the issue. I finally observed it lighting the burner, then shutting the burner off, then lighting again, then shutting off. I believe the flame sensor is bad/going bad and once it fails to see that it's lit enough times, it gives up and has to be powered down long enough to reset before it'll try again.

I took the flame sensor out last night and cleaned it with steel wool and it's been working since then. However, this is an intermittent problem to begin with, so time will tell if that actually helped.
 
My Trane had a similar issue. I checked and cleaned sensors, but eventually bought a control board for the unit off ebay. After the board swap everything was fine. Upon removing the old board, I could see signs of heat buildup.

I came across the board I bought by a cross reference Googling around. Same exact circuit boards but not Trane branded = Cheaper
 
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