Call an HVAC technician

Why this happens

Trane says that if the furnace is switched on and has power, the thermostat is set to Heat and the filter is clean, and it still won't turn on, it's time to call an HVAC company. Trane notes that newer furnaces have sensors and a limit switch that stop the furnace when they detect a safety issue, and that a breaker that keeps tripping after a reset means a more serious problem. Carrier lists a faulty ignition system and a disruption in the gas supply among the more complex causes, and says ignition problems usually need a qualified HVAC technician.

What to do right now

1. If the breaker tripped again, leave it off. Trane says not to keep resetting it.
2. If the breaker is holding, check the filter before you call. Carrier and Family Handyman both say a clogged filter can make a furnace overheat and shut itself off. Carrier says the filter usually sits in a filter cabinet with its own door on the side or bottom of the furnace. Turn the furnace switch off first, as Carrier advises, then open the filter cabinet door and slide the filter out to look. If you can't find the furnace switch to turn it off, skip the filter check and leave it for the technician. If it looks dirty or clogged, slide in a new filter of the same size with the arrow pointing toward the furnace. Close the cabinet door and switch the furnace back on. If the only way to reach the filter is to take off the furnace's front door panel, leave it for the technician.
3. Set the thermostat to Off for a moment, as Honeywell advises before this check. Then look at the furnace's front door panel without removing it. Honeywell says a loose panel can keep a safety switch from letting the furnace run. If it looks loose or crooked, press it firmly into place so it sits flush, without taking it off, then set the thermostat back to Heat and give the furnace a few minutes. If it's off the furnace or won't sit flush, tell the technician. Never tape or hold that switch down.
4. If you already know your other gas appliances, like the stove or water heater, aren't working either, Carrier says the gas supply may be disrupted. Contact your gas utility, and don't try to light anything to test it.
5. Don't open the furnace, press any reset button inside it, or try to relight a pilot.
6. While you wait, never heat the house with a gas oven. CDC and USFA both warn against it. CDC also says never to burn charcoal or use a portable gas camp stove indoors, and not to use portable flameless chemical heaters indoors. If you use a space heater, USFA says to keep anything that can burn at least 3 feet away and turn it off when you leave the room or go to bed.

When to stop. If you smell gas or rotten eggs, leave the house without touching any switch and call 911 or your gas utility from outside. If a carbon monoxide alarm sounds, or anyone feels dizzy, sick or has a headache, get everyone outside to fresh air and call 911. If you see smoke or flames, get out and call 911. If a breaker trips again, leave it off. If you notice a burning plastic or electrical smell once the furnace runs, Carrier says to turn the furnace off and have a qualified HVAC professional inspect it before you use it again.

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