Thornton Home Pros

Furnace blowing cold air? Check these six things first.

Most cold-air problems have a simple cause. Run through this list, and if it's still cold, one trusted local pro can be on the phone with you today.

  • Takes about 10 minutes.
  • No tools needed for the first four checks.
  • If you smell gas, skip this and leave the house.
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Takes about a minute. A trusted local heating and cooling company will call you, usually the same day.

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Smart thermostat on a living room wall
Photo: Unsplash

Step by step

What to check

  1. Thermostat fan setting. If the fan is set to ON, it blows air even when the burners are off. Set it to AUTO and HEAT, and set the temperature a few degrees above the room.
  2. Thermostat batteries. A weak battery can keep the thermostat from calling for heat. Replace them if the screen is dim or blank.
  3. The air filter. A clogged filter makes the furnace overheat, and a safety switch shuts the burners off while the fan keeps running. Replace it if you can't see light through it.
  4. The furnace switch and breaker. There's usually a light-switch-style power switch on or near the furnace. Make sure it's on, and check the breaker.
  5. The condensate line (high-efficiency furnaces). If your furnace vents through white PVC pipe, a clogged or frozen condensate drain will shut it down. Look for water around the furnace or a full condensate pump.
  6. The gas supply. Make sure the gas valve to the furnace is open (handle in line with the pipe) and that your other gas appliances work.

When to stop and call a pro

  • The furnace clicks or tries to light but keeps shutting off.
  • You hear banging at startup.
  • Your carbon monoxide alarm goes off. Leave the house and call 911.
  • You've checked everything above and it's still blowing cold.

Common pro fixes include cleaning the flame sensor, replacing the igniter, or clearing the drain and pressure switch tubing. Get a licensed Thornton furnace pro.

Smell gas? Leave first.

Don't use this form for a gas leak.

  • Get everyone out of the house, then call Xcel Energy's gas emergency line or 911 from outside.
  • Don't flip switches or use anything that could spark.

Questions

Good to know

Why is my furnace blowing cold air but the thermostat says heat?

Check that the fan is set to AUTO, not ON. If it's on AUTO, the burners likely aren't lighting or are shutting off early: often a dirty flame sensor, a clogged filter or a blocked condensate line.

Can I clean a flame sensor myself?

Many homeowners do, with the power and gas off, using fine steel wool. If you're not comfortable working inside the furnace, call a pro; it's a quick, inexpensive visit.

Who are you, and is this free?

Thornton Home Pros is a local referral service, not a contractor. It's free for you: the company we connect you with pays us a referral fee. All quotes, work and warranties come directly from that company.