What to check before you call anyone

The four checks

Why each one is on the list

The thermostat. Batteries die, screens go blank, and somebody in the house sets it to a mode nobody expected. Set it to heat, above the room temperature, and then leave it alone for a few minutes: many furnaces wait through a short delay before they start.

The filter. Restricted airflow overheats a furnace, trips it out on its limit switch, and weakens the air from every vent. It is one part and it costs very little. Hold it up to a light. If you cannot see through it, that is your first suspect.

The switch and the breaker. The switch on the side of the furnace looks exactly like a light switch, usually because it is one. People turn them off while working in a garage or crawl space and forget. Breakers trip. Reset one once; if it trips again, stop, because it is reporting a fault rather than being temperamental.

Any smell of burning or gas. This is the one that overrides everything else on the list.

What to have ready when you do call

Having these to hand genuinely shortens the visit, and sometimes the phone call resolves it.

• What it is doing, in ordinary words. "It lights, runs about ten seconds, then shuts off and tries again" is worth more than any technical term.
• Whether the air conditioner works, if you have one. If neither heats nor cools, that points at the blower they share.
• What kind of furnace. Gas or electric, and whether it vents through a metal flue or two plastic pipes. If you do not know, describe what you see.
• The brand and model number, from the rating plate inside the furnace door or on its side. Photographing it is easier than copying it.
• Any blinking light. Many furnaces have a small window with a blinking LED that reports a fault code. Count the blinks.
• When it started and whether anything changed: a power outage, work in the house, a filter change.
• Whole house or particular rooms.
• Your town, because the answer to some questions genuinely changes with it.

What to ask on the phone

Worth asking before you book, not after the visit:

• What the visit costs, and whether that comes off the repair if you go ahead.
• Whether that figure changes outside normal hours.
• What happens if the diagnosis finds something they do not handle.
• Whether a permit is likely, which office issues it, and what it costs. The permits guide has the state fee schedule in full.
• For heating work, the Idaho HVAC contractor certificate number. The about page explains what that is and why it is the right number to ask for.

What not to do

• Do not take the blower door off. On most furnaces that kills the power via a safety switch, and then nothing works and you have added a variable.
• Do not reset a tripping breaker repeatedly.
• Do not tape over, bypass or jumper a safety switch, whatever you read online.
• Do not light a pilot or restart a furnace you can smell gas around.