A noteworthy geek-out

I must share in my shame as well as my glory. And since nothing glorious has happened yet I'll kick things off with a little shame.

I have spent 6 hrs troubleshooting a 90% afue downflow furnace. And in order to defend my stupidity, I shall bore you (or just myself once again) with all the details. The initial call was "running constantly but not hitting thermostat set temperature". So I discovered the collection box off of the secondary heat exchanger had filled with condensate water. So i cleared the drain, poured some alkaline cleaner in it and flushed it. I ran the furnace through three cycles and checked to be sure it was draining correctly. That was fine for five days. Then a call "it's not working at all". This time the Induced draft assembly (more specifically the motor) was cooked. Stands to reason, it was slinging water for a while a week before. So I replaced it(the assy). Well now it would not pull in the pressure switch. Great, so Being as I no longer, no wait, never have owned a magnehelic guage, I assumed it probably was not the switch, a low incedent of failure part it is. I took some 18 guage copper wire and reemed all of the nozzles where the silicone tubing attaches, then ran some insulated 18 gauge wire through the length of the tubing.
Still no make on the pressure switch. I went and got an after market adjustable switch. Notta, I had to adjust it way down to nothing, and thjis was still not reliable and not the type of practice to allow for a good nights sleep. So back to the factory pressure switch (it was most likely fine). I removed the collection box, the burner box, new draft assy, gas valve, manifold and burners. Plugging all of the heat exchanger entries with rags, I one at a time blew high pressure c02 thru the primary and out the secondary, fully expecting a rewarding amount of something to come out.Nothing. I re-sealed the collection box with silicone, put everything back together, cut the flue pvc off the furnace as well as the combustion in take, in spected them for obstruction, nothing. Now I franticly went over all the collection attachments, the drain lines, plugged the silicone tubing and blew, expecting to find a leak, nothing. Re-attach everything jumper the furnace pressure switch after the board check for normally open position. Let it run ten minutes and checked for co in the house near the supply vents, almost hoping for a cracked primary, nothing 2ppm. OK smoke bomb the burner compart ment with the fan on , nope no cracks, dammit. Maybe the dang induced draft assy isnt sealing well, remove again, nope good gasket all looks well. Boredom, spin the draft blower wheel with my finger, what IS that sound??
Oh a piece of packing plastic covering a couple of the blower vanes, nice. What did we learn from this? Always inspect a new part very well before installing? I like to think I learned how to disassemble and reassemble a 90 % downflow gas furnace, you know , just for fun.