How-to
What to Do the Morning the Furnace Will Not Start
The house is cold and the furnace is quiet. Four checks take about five minutes. If you smell gas, skip them, leave the house, and call the gas company from outside.
The thermostat
Set it to heat, a few degrees above whatever the room says. If the display is blank, the batteries are dead. A blank thermostat looks exactly like a furnace that quit. Put fresh batteries in before you go downstairs.
The switches
A lot of furnaces have a switch on the cabinet, or on a box nearby, that looks like a light switch. It gets bumped when someone moves a shelf. Flip it on. Then the breaker. One reset is worth doing. If the breaker trips again as soon as the furnace tries to start, leave it off.
The filter
Hold it up to a light. If you cannot see through it, the furnace may be shutting down because it cannot move air. That shows up after a week of hard running, not on a mild day. Match the new one to the size on the frame.
When to stop
A scorched smell, or a furnace that clicks and then locks out after two resets, is the end of the project. Do not keep pressing the button. What a shop should look at next is the same list as a heating tune-up: the flame sensor, the vent, and whether the burner actually stays lit. If this is the second morning of a cold snap, the same checks are in keeping heat through the snap.