Originally Posted by #CARRIERMAN
Hi again SteveAnnArbor
I just read your second response. When the inducer is running and all the wires are in their proper location. You should show 0 volts across the pressure switch. If you are showing 26 volts across the pressure switch with the inducer running, the switch is not closing. If this is the case, start the furnace up with the thermostat. You will want the common wire off of the pressure switch to do this test. When the inducer starts, press the terminal of the common wire against the wire going to the n/o terminal and hold it there. If the furnace fires up you know the switch is not closing. Could still point to same problem as I listed before.
Well, thanks for the tip! I did this and--voila`!--HEAT. The furnace ran normally. I was getting set to go buy the pressure switch and I decided to recheck the outside intake (my wife had felt around and not found anything); we'd had a lot of wasp/bee activity this summer, and I was a little concerned.
We have one of those combined intake/exhaust ports which cover the pipe-inside-a-pipe PVC configuration (exhaust on the inside, intake on the outside; I presume that part of this idea--besides only having ONE hole through the house's "skin"--is that the heat from the exhaust air could be partially recaptured to heat the incoming air...so that you aren't wasting heat from the burning gas to heat the air, as well as cooling the exiting air; this is actually exactly what is done with liquid O2, say, for rockets, which you can see at most rocket park-style displays and at various museums). There was nothing except some condensate inside the exhaust pipe, and just a little dust and a cobweb in the intake space; I cleaned it up using a refrigerator coil brush and a vacuum (crevice tool) and...well, wouldn't you know it...HEAT with no problems.
I had no idea just HOW sensitive the overall flow was to what appeared to be relatively minor restriction of just a portion of the intake area...what an eye-opener! What a wild goose chase from an incomplete investigation of a fundamental; if I'd been thorough at the beginning, I'd have not had a few weeks of wondering if I could get this, myself. [ What was more misleading is that the exhaust flow was strong; it wasn't weak and so didn't seem as though anything was blocked... ]
[ FYI: I was also misapprehending the action of the pressure switch. I thought that the switch opened due to a signal from the board; clearly, it is a mechanical switch which operates directly in response to an (adequate) suction due to the airflow created by the induction motor, and this then is used to create a signal read by the board to say, "Hey, it's OK to proceed w/gas and ignition, because we've got enough air flow"... Your comments were part of what helped to get my head corrected in this respect; thanks!]