It’s the first cold morning of October. You nudge the thermostat up, hear the furnace click on, and a minute later the air coming out of the vents smells… off. A little like a hot toaster, maybe. Or a little like a damp basement. Or, for some unlucky homeowners, like a gym bag that’s been in the trunk since July.
Most of the time, that first-heat smell is harmless a...