Furnace not turning on
A furnace that won't start often has a simple cause: a thermostat setting, a power switch that got flipped off, a tripped breaker, a clogged filter, or a front panel that isn't fully closed. You can check all of those in about 15 minutes. If you smell gas, skip the checks and get out of the house. Take a quick look at the small window on the furnace before calling. Many models blink a code that points straight at the problem, and the key is often on the panel.
Why furnaces refuse to start
A modern furnace runs through a safety checklist every time it starts. If any one thing looks wrong, it won't fire. That's good design, but it means one small, easy-to-miss problem can look like a dead furnace.
Most homes have a power switch on or near the furnace that looks just like a light switch, often in a spot where someone flips it off by accident.
What the furnace is doing, or not doing, narrows it down fast. Dead silence points to power. A blower that runs without heat points to gas or ignition. A burner that lights and quits after a few seconds points to the flame sensor.
What stops a furnace from starting
Set to COOL, the wrong schedule, or dead batteries. The cheapest thing to rule out.
The furnace service switch looks like a light switch and gets flipped by accident.
Restricted airflow trips the high-limit safety switch.
A dirty flame sensor makes the furnace light, then shut off after a few seconds.
High-efficiency furnaces make water as they run. If the drain line or pump backs up, a float switch shuts the furnace off to prevent a flood.
| What you notice | Most likely | Try this |
|---|---|---|
| Completely silent, no lights | No power | Check the furnace switch, the breaker, and that the front panel is seated. |
| Blower runs, but the air is cold | Gas or ignition | Make sure the gas valve handle is in line with the pipe. Look for a status light code. |
| Lights, then shuts off in a few seconds | Dirty flame sensor | A technician cleans it quickly and cheaply. |
| Starts and stops over and over | Overheating or airflow | Replace the filter and open closed vents and registers. |
| High-efficiency furnace (white plastic vent pipes) won't start | Blocked vent or full condensate trap | Clear snow or leaves from the outside pipes. Check the for a clog. |
| Status light blinking | Stored error code | Count the blinks and match them to the chart inside the door. |
Five checks, in order
- Check the thermostatSet it to HEAT, turn it at least 5 degrees above room temperature, and replace the batteries if the screen is dim or blank. Wait a few minutes; many furnaces have a short delay before anything happens.
- Find the furnace switchLook for a light switch on the furnace or on the wall near it, sometimes at the top of the basement stairs. Make sure it's ON. It's easy to mistake for a regular light switch.
- Check the breakerLook for a tripped breaker labeled furnace or HVAC. Push it fully OFF, then ON. If it trips again right away, leave it off and call for service.
- Replace a clogged filterA badly clogged filter can make the furnace overheat and shut itself off. Swap it if it's gray, with the arrow pointing toward the furnace.
- Close the door panelFurnaces have a safety switch that keeps them off if the front panel isn't seated. After a filter change, it's the first thing to check. Push it firmly into place.
Watch it done
- The furnace starts after its short delay when the thermostat calls for heat
- Warm air comes from the vents
- The breaker stays on
- The front panel is seated firmly
If there's still no heat
If it lights for a few seconds and then shuts off, the flame sensor may be dirty. It's a common, inexpensive fix, but it means opening the burner area. If you're not comfortable with that, call a technician.
If the blower runs but the air is cold, check that the gas valve to the furnace is open. On most, the handle sits in line with the pipe when it's open. If you're unsure, or the valve won't move easily, leave it and call for service.
On high-efficiency furnaces, look outside for the two white plastic pipes. Snow, ice or a bird's nest over them will stop the furnace cold. Clear them gently by hand.
What a pro typically charges (a rough estimate; prices vary by area): A furnace service call typically runs $100–$300. Flame sensor cleaning is often under $150; igniter replacement usually costs $150–$450.
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