Random Main Blower Cycling

[interalian]

I have a 2002/3 vintage Tempstar NTG3100GFG1 for the main furnace and a smaller Tempstar in my house. Aside from having noisy blowers, they work just fine. There's an AC unit on the smaller furnace which only serves the top floor.

Strange problem: when it's really windy/blustery outside, the main furnace blower will cycle randomly for anywhere from a half second to a few seconds every few minutes. There is no heat call during these cycles, and typically the furnace hasn't run for some time prior.

The only thing I can think is there's enough draw on the main flue that it's somehow tripping the draft inducer proving switch. However, I've tried drawing the valve closed and it doesn't provoke the problem. The other furnace never does this.

Do I have some odd control board gremlin? Or?

 

[interalian]

Circling back to answer my own question after a couple of years:

Turns out the problem was broken slats on the kitchen range exhaust where it runs through the wall. The original builder fitted a pressure switch in the duct between the range hood and the wall so the furnace fan would come on when the range hood is on. Air blowing in from outside was enough to trip the switch briefly, causing the main furnace blower to run for a second or two. Fixed with a new exhuast cap.