Why is my furnace not turning on?

A gas furnace starts up like a careful person lighting a gas grill. It runs through a safety checklist, and it won't go on to the next step until the last one checks out.

Here's the checklist, step by step:

1. The thermostat asks for heat. It's the boss on your wall. When the room gets cooler than the setting, it sends a small signal to the furnace.
2. A small exhaust fan clears the air. It pushes old air out the vent pipe, like opening the grill lid before you light it. Pros call it the draft inducer. This is the hum you hear first.
3. A safety switch checks the air is moving. Pros call it the pressure switch. If the vent pipe is blocked, it says no.
4. The igniter glows. It's a small part that glows bright orange, like the wire in a toaster. Some furnaces click and spark instead.
5. The gas valve opens. Gas flows to the burners and catches fire from the igniter. You'll see blue flames.
6. The flame sensor proves there's a flame. It's a thin metal rod that sits in the fire. If it can't feel a flame within a few seconds, it shuts the gas off.
7. The blower sends warm air to your rooms. About a minute later, the big fan pulls room air in through the filter, past the heat, and out your vents.

A furnace that "won't turn on" is stuck at one of these steps. The questions at the top check them in the same order: thermostat, power, airflow, then watching the start-up through the little window.

This guide covers gas furnaces, which most homes have. If yours is electric, the thermostat, power and filter steps work the same way.

Water around your furnace? See our guide to an AC or furnace leaking water.

This tells you whether your furnace is really heating. You need a few minutes and, if you have one, a kitchen thermometer.

1. Set the thermostat to Heat, 5 degrees above the room.
2. Stand by the furnace for the first 2 minutes. A small fan hums first, then a glow or clicks, then blue flames. That order is normal.
3. About a minute after the flames light, the big fan starts. You'll hear a rush of air.
4. Go to the nearest vent. Within about 5 minutes, the air should feel clearly warm, like a hair dryer on low.
5. Hold a kitchen thermometer in the airflow for a minute. It should read well above the room temperature.

• Warm air, and it keeps running: it's working. Give the house an hour or two to warm up.
• Cool air from the vents: check that the thermostat says Heat, not Cool or Fan only.
• Flames, then it shuts off and starts over: take the questions at the top.

• You smell gas or rotten eggs. Don't touch the furnace. Leave the house and call the gas company from outside.
• Your carbon monoxide alarm goes off. Get everyone outside and call 911.
• You smell burning or hot plastic that doesn't fade. A dusty smell for the first few minutes of the season is normal.
• The breaker trips again after one reset.
• You hear a loud boom or rumble when it lights.
• The flames are mostly yellow, floppy or reaching out of the front. Normal flames are mostly blue.

To turn it off, set the thermostat to Off. For anything on this list except a gas smell, also flip the furnace switch off.

Most furnace repairs cost a few hundred dollars. HomeAdvisor puts the typical repair at $133–$505. Ask yourself three things before a big repair:

• How old is it? Most gas furnaces last about 15–20 years, and some last longer. The date is usually on a sticker inside the front panel.
• Is it still under warranty? Many furnaces have parts covered for years. Check before you pay for a part.
• The half rule. If the repair costs more than half the price of a new furnace, get a replacement quote too.

Example: a $200 flame sensor repair on a 16-year-old furnace is worth doing. A $1,000 exhaust fan motor on an 18-year-old furnace is worth comparing with a new one.