My new house has a 6 month old Goodman 92% HE furnace. A couple of days ago,
it began cycling in an odd way. It would begin with the normal warmup phase
(whatever that is), go to the next stage where it's actually blowing heat
through the ducts, but afterward, the smaller warmup motor would continue
running for between 5 and 20 minutes. Other times, the warmup stage would go
on for 15 minutes BEFORE heat began blowing, and sometimes it would never
reach the true heating stage.

By the way, when I say "warmup phase", I don't mean there's a flame yet. I'm
referring (perhaps incorrectly) to the stage where the thing is checking for
proper ventilation or whatever it's doing.

ANYWAY.....my neighbor's car alarm went off at 3:30 AM (the tenth time since
this morning), and since nobody in my house is up watching TV, it's very
quiet. The furnace began doing it's odd thing again, so I went down in the
basement to curse at it. Because the house is quiet, I heard something
interesting. Standing right under the PVC vent pipe, I heard what sounded
like water - about as much as you'd hear if you were in the basement as the
last 3 seconds of water drained out of the bath tub. "WTF?", I said to
myself. I looked up and noticed that the installer had nailed a pipe hanger
in such a way that the thing had come loose, leaving the 15 ft span of PVC
with a downward bow in the middle. I went upstairs, had a few pieces of
fresh pineapple, went down to the cellar again, pushed up on the pipe to
straighten it, heard water head toward the outside, and voila! The furnace
has cycled normally 5 times so far.

Was collected moisture in the pipe preventing the furnace from being
satisfied with the venting?

Or was the pineapple a factor? On the next episode of Nova.....fruit and
appliances....what science doesn't know yet.