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Is the exhaust fan kicking on at start up?
Ok that is the issue...see if the exhaust fan is stuck..has a bird, nest...or maybe just needs a nudge....we nee that to run first in order to activate a switch
do you know where the exhaust pipe is?
It is the chimney pipe
or take door off and take pic of furnace and post here using the attach files link.
Then I can draw what we need on it
see a basic drawing HERE
Ok, see the picture HERE
The exhaust fan MUST start and run...which in turn closes the air switch
no..if the fan does not run to force the carbon monoxide outside it will cause great problems.
That safety switch is to make sure the excess heat and carbon monoxide go outside instead of burning the furnace.
Can you remove that fan or tap on side while it is humming?
we need to see if it just needs to be unstuck, or worse if motor is bad
And please note that without this fan running it is 100% dangerous to try and bypass
In that case you are pretty much stuck with replacing the assembly.
This is how it comes
hopefully you can coax it to run...
I certainly wish you the best and thank you for your patience.
Just rate service to move off active list.