Went to our local Chamber of Commerce. No A/C. Found the furnace inside a closet and the receptionist nearly bumped her head on the frozen suction line. She didn't even realize it was frozen. The filter was relatively clean - sat in a housing on the outer wall. Gee, this must be why!

That's my arm from the outside of the furnace, left, to underneath in the blower compartment.

So I crawl down and what do I see? Here's the rear of the furnace as viewed from the top of the AC box.

The indoor coil wasn't really bad. The condenser on the roof was plugged and low on charge. The vapor line schrader was bad. Oh, and the heat exchanger had 3 holes. (sorry no pics)

Here's what I found after the ice thawed - sorry for the crappy pic.

Corrosion from flux and bird-sheet from plumbing solder. Guess they need both a furnace and AC now.