Last updated: October 4, 2026
The heat is on, the fan is running, and the air coming out of the vents is cold. It is one of the most common furnace complaints, and a good share of the time the cause is something you can check yourself in a few minutes.
Short answer: Check that the thermostat fan is set to AUTO, not ON, and check the air filter. Those two cause many cold-air complaints. If the burners light and then shut off, or never light at all, the furnace needs a technician.
Safety first
If you smell gas (a rotten-egg or sulfur odor), don't try to relight anything or flip any switches. Leave the house right away, then call 911 and NW Natural's 24-hour emergency line at 800-882-3377. If your carbon monoxide alarm sounds, get everyone outside and call 911.
Check these first
- Thermostat fan setting. With the fan set to ON, the blower runs all the time, including between heating cycles when the air isn't being heated. Set it to AUTO.
- Thermostat mode and batteries. Make sure it is set to HEAT and set above the room temperature. Replace the batteries if the display is dim or blank.
- It just started. A furnace can blow cool air for a minute or so at the start of a cycle while it warms up. That is normal.
- Air filter. A clogged filter restricts airflow, the furnace overheats, and a safety switch shuts the burners off while the fan keeps running. Replace the filter and give the furnace time to cool down and restart.
- Vents and returns. Blocked or closed vents restrict airflow the same way a dirty filter does. Open them and move furniture away from returns.
- Gas supply. Check that the gas valve at the furnace is open. If your other gas appliances aren't working either, call your gas utility.
Are you in need of professional services for your home’s heating systems?
Contact Tri-Tech Heating, Inc. for expert solutions today!
Causes that need a technician
- Dirty flame sensor. The flame sensor confirms the burners are lit. If it is dirty, the furnace lights and shuts off within seconds, one of the most common no-heat calls.
- Failed igniter or pilot. If the burners never light, the fan may still run and push unheated air through the house.
- Limit switch tripping. The limit switch shuts the burners off if the furnace overheats. If it keeps tripping with a clean filter and open vents, something else is restricting airflow or the switch itself is failing.
- Clogged condensate drain. High-efficiency furnaces produce condensation. A blocked drain can stop the burners from firing.
- Leaking ductwork. Ducts that leak in a crawlspace or attic lose heat before it reaches the room, so the air arrives lukewarm. See ductwork.
If you have a heat pump
In cold, damp weather a heat pump briefly reverses to melt frost from the outdoor coil. Air from the vents can feel cool for a few minutes during that defrost cycle, and that is normal. Cool air that doesn't go away is not. See heat pump repair.
Repair or replace?
Most of these are repairs. If the furnace is older and the breakdowns are getting more frequent, it is worth comparing the repair against a replacement. See furnace installation and replacement.
Preventing it
A yearly tune-up catches a dirty flame sensor, a weak igniter and a restricted drain before they leave you without heat. See HVAC maintenance and our seasonal maintenance checklist. For other symptoms, see common HVAC problems and when to call a professional.
Need a technician?
Tri-Tech Heating, Inc. repairs gas and electric furnaces, all makes and models, across Vancouver, Camas, Salmon Creek and Clark, Cowlitz and Skamania Counties and the Portland metro area. Learn more about furnace repair in Vancouver WA, or contact us.
Our office is open Monday through Thursday, 8:00am to 5:00pm, and Friday, 8:00am to 4:00pm, and a call to (360) 891-2002 during those hours reaches a person. The office is closed on Memorial Day, July 4, Labor Day, Thanksgiving and the day after, Christmas Eve, Christmas Day and New Year’s Day. After hours and on holidays, call the same number and a live person from our answering service will take your information. They'll text our on-call technician right away, or, if you'd rather wait, pass your request to our office and we'll respond by 9:00am the next business day. Licensed HVAC #TRITEHI952LL, CCB #161426.
Are you in need of professional services for your home’s heating systems?
Contact Tri-Tech Heating, Inc. for expert solutions today!
Frequently Asked Questions
Why is my furnace blowing cold air when the heat is on?
The most common causes are a thermostat fan set to ON instead of AUTO, a clogged air filter that makes the furnace overheat and shut its burners off, or an ignition problem such as a dirty flame sensor.
Is it normal for a furnace to blow cool air at first?
Yes. A furnace can blow cool air for a minute or so at the start of a cycle while it warms up. Cold air that continues is not normal.
Can a dirty filter make a furnace blow cold air?
Yes. A clogged filter restricts airflow, the furnace overheats, and a safety switch shuts the burners off while the fan keeps running.
Should I turn my furnace off if it is blowing cold air?
If you smell gas, leave the house and call 911 and your gas utility. Otherwise, if the furnace keeps trying to light and failing, turn it off at the thermostat and call a technician.
Company Information
Tri-Tech Heating, Inc.
6406 NE 116th Ave Unit C
Vancouver, WA 98662
Phone: (360) 891-2002