It's one thing after another. We woke up this morning to an especially cold house and found the heat not working. Checked the heater for anything obvious that someone who knows nothing about heaters might be able to correct. Wiggled a loose switch and heard the pilot come on and decided that the problem must be a simple but potentially dangerous electrical problem. Went upstairs and the heat was off again. MonsterMustDie grumbled and emailed landlord before going to work. I fiddled with thermostat a bit later and the heat finally came on. Then the theory changed to "must be a bad thermostat... and potentially dangerous electrical problem with switch as well.
Fortunately, our landlord reacted very quickly and got their Heating/AC man out to the house ASAP. Because the heater was now working, I was imagining them seeing the heat on and leaving without taking a look. Another TV-repairman type moment.
John Austin, the technician got to the house and let me tell you, it was a pleasure to meet him. Very patient and knowledgeable about his work - and he looks like David Tennant.
He went over the entire system. Found the missing pocket for the never-used air filter, which I immediately bought and installed today, and gave me a lesson on heating units and their maintenance, most of which I'll probably forget by tomorrow morning.
He also took apart the thermostat and pointed out the presence of batteries that needed replacing. We've always had the little hard-wired round thermostats so we never heard of thermostats that needed batteries.