Yep, it's forced air, gas-fired. Ducting in the basement, so no chance
of crushing that. The furnace filter was changed just before I took
posession of the house. FHA wouldn't approve the loan otherwise. It's
only been 2 months so I can't believe the filter is that clogged yet.
Besides, the change was SUDDEN, as in I went up in the crawl space with
12 rolls, and came down 90 minutes later to the furnace on this new
cycle. It was actually colder outside, and windier, before I installed
the insulation and the furnace didn't run this often.
More observation, it appears that the burner runs about 6-7 minutes,
and the fan 4-5 minutes afterwards. Time between furnace runs has
actually increased to 25-30 minutes.