Cold air from a furnace usually means the blower is running but the burners aren’t. Common causes are a thermostat fan setting, a dirty filter, an ignition problem, or a safety shutoff. Some of those checks are safe for you, and some are for a licensed pro, which Climate Pros Tucson can match you with.

A Tucson furnace sits idle most of the year. The airport averages fewer than 12 nights a year at or below 32F (1991-2020 normals), so a problem that built up over summer often shows itself on the first cold night of the season.

What are the usual causes?

  • The fan is set to ON. The blower runs constantly and pushes unheated air between cycles. Set it to AUTO.
  • The thermostat is set wrong or has dead batteries. Check the mode and the target temperature.
  • A dirty filter. Poor airflow can overheat the furnace, and a safety switch shuts the burners off while the fan keeps running.
  • An ignition or pilot problem. The burners never light, or they light and drop out.
  • A dirty flame sensor. The furnace lights, senses no flame and shuts the gas.
  • Gas supply or a tripped limit switch. Both can stop the burners.
  • A heat pump, not a furnace. Heat pump air feels cooler than furnace air even when it is working. EIA’s 2020 survey counts 25% of Arizona homes with a central heat pump as main heat (a statewide figure), so check which one you have before you troubleshoot a furnace. See heat pump not heating.

What can I check safely?

  1. Set the thermostat to heat, target above room temperature, fan on AUTO.
  2. Replace the thermostat batteries.
  3. Check the filter and swap a dirty one.
  4. Make sure the supply registers and return grilles are open.
  5. Check that the furnace switch near the unit is on.

If you smell gas, leave the home and call 911 or your gas utility from outside. Don’t flip switches. Because carbon monoxide is a heating-season risk, read carbon monoxide in a Tucson home.

When does a pro need to look?

Call a pro if the furnace still blows cold after the basic checks, if it clicks and shuts down, if you hear a repeated start and stop, or if it smells of gas or burning. A pro can clean the flame sensor, test the igniter, check the limit switch and gas valve, and check the heat exchanger. Our guides on why a furnace won’t ignite, furnace blower motor repair and signs a furnace is going out cover each symptom.

When to call us

Climate Pros Tucson matches you with a licensed local pro. See furnace repair and heat pump service, or the Tucson HVAC hub. Call (520) 353-2520 and we’ll match you with a licensed local pro.