We have heat! It turned out to be a relatively minor thing - a filter that had apparently not been cleaned since sometime during the Carter administration. It was so badly clogged that light couldn't get through, much less air, and the motor was shutting itself off. A little embarrassing, but now at least we know it's there, how to remove it, etc.
overgrown garden
what happens when clutter escapes to the outdoors
alisonf at gmail dot com