If you smell gas, leave the house first and call Southwest Gas or 911 from outside. Otherwise, a furnace that sat idle since spring often balks on the first cold night because of dust on the burner, a tired ignitor or a dirty flame sensor. A few checks are safe for you; the rest is a job for a licensed pro, and Climate Pros Tucson matches you with one. The general causes of a furnace that won’t light are in why won’t my furnace ignite. This page is about that first start of the season.

Why does a Tucson furnace sit idle so long?

Because the heating season is short. At Tucson International Airport, heating degree days stay tiny from May through October (1991-2020 normals): 3.9 in May, 0 in June through August, 18.3 in October. They jump to 151.4 in November and peak at 372.9 in December. So a furnace can sit unused for six months or more, then get asked to work on a November night when the normal low is 47.9F. Dust and spider webs settle on the burner and sensor in that time, and a part that hasn’t moved in months may fail on its first call.

What should you try first?

  1. Thermostat set to heat, a few degrees above room temperature, with fresh batteries.
  2. A clean filter. A clogged one can trip a safety switch.
  3. The furnace power switch on, which often looks like a light switch near the unit, and the breaker not tripped.
  4. The gas valve handle in line with the pipe, which means open.
  5. The furnace door fully closed. Many furnaces won’t run with the panel ajar.

Try a restart once or twice, not over and over. Repeated tries can lock the furnace out.

What is the furnace telling you?

  • A dusty burning smell for the first few minutes is common after months off and should fade. If it stays, or you see smoke, shut it off.
  • No glow and no flame: the ignitor may have failed.
  • Flame lights, then goes out in a few seconds: a dirty flame sensor is a common cause.
  • Clicking with no flame, or a blinking light and nothing else: the control has locked out. Note the blink pattern for the pro.

What about a gas smell or carbon monoxide?

Gas smell: leave, don’t flip switches or use the phone inside, and call Southwest Gas or 911 from outside. A CO alarm, or headache, dizziness or nausea that eases outside: get everyone out and call 911. Read carbon monoxide in a home and test your CO alarms before the first cold night.

What if it’s a heat pump, not a furnace?

There’s no burner to light, so the checks are different. See heat pump not heating. Next year, run the heat for a few minutes while the weather is still mild, as the fall HVAC checklist suggests, so you find the problem before you need the heat.

When the furnace still won’t light

If the basic checks don’t bring it back, or it lights and drops out, a licensed pro should look at the ignitor, sensor and controls. Check any pro’s license at roc.az.gov. See furnace repair or the Tucson HVAC hub. Call (520) 353-2520 and we’ll match you with a licensed local pro.