Sunday afternoon as I was unplugging my computer I noticed that the air blowing on my feet was not actually warm. This explained why it was so chilly in my house–53°F–and why turning the thermostat up had not made it any warmer. In fact, as I processed the fact that my furnace was blowing non-warm air, it occurred to me that the blower had been on pretty much all day, as the heating system tried valiantly to live up to the expectations being put on it.
It turns out that the furnace has a switch in it that turns off its ability to heat if it gets too warm somehow, and that sometimes the switch gets stuck. The furnace guy showed up with admirable promptness to unstick it, but recommended that I get a new one as soon as possible, as one failure often leads to another.
It seems that the initial problem was caused by not having changed the furnace filter recently enough. I did put in a new one when the cold weather started, but my house is old and dusty so I’m going to have to keep a better eye on things in the future.
I’m glad it was an easy fix, but I could have wished for a less expensive one.