The fan is set to ON
With the thermostat fan on ON instead of AUTO, air keeps blowing between heating cycles, and that air feels cool.
Heating & cooling problem
Furnace blowing cold air? Start here.
Cold air from the vents usually means the fan is running but the burner isn't heating, or isn't staying lit. A few safe checks at the thermostat and around the furnace often point to why.
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What's likely going on
These are typical causes, not a diagnosis. A pro looking at yours can tell you which it is.
A clogged filter
A dirty filter restricts airflow. The furnace can overheat and shut the burner off as a safety measure while the fan keeps running.
The furnace is in lockout
After failing to light a few times, many furnaces stop trying and flash an error code. A dirty flame sensor is a common reason, and cleaning it is a job for a tech.
Blocked vent pipes outside
High-efficiency furnaces vent through plastic pipes on an outside wall. Snow, ice or debris over them can stop the furnace from heating.
It's still warming up
Some furnaces run the fan briefly before the heat comes up. Give it a few minutes from a cold start.
Safe things to check yourself
- Check the thermostatSet it to HEAT, above the current room temperature, with the fan on AUTO. Replace the batteries if it uses them.
- Check the filterSlide out the filter at the furnace. If it's grey and clogged, put in a clean one of the same size, with the airflow arrow pointing toward the furnace.
- Read the blinking lightMost furnaces have a small window with an LED. Count the blinks and photograph the code chart on the furnace door.
- Check the outside pipesIf your furnace vents through plastic pipes on an outside wall, make sure snow, ice or leaves aren't blocking them. Clear them gently by hand.
- Check the power switchThere's usually a switch like a light switch on or near the furnace. Make sure it's on, and check the panel door for a tripped breaker.
Stop and leave it to a licensed tech
- You smell gas. Leave the house and call your gas utility or 911 from outside.
- A carbon monoxide alarm goes off. Get everyone outside and call 911.
- You smell burning, or see scorch marks or soot around the furnace.
- The furnace goes back into lockout after you've reset it once.
Don't open the burner compartment or touch the gas valve or gas supply. Those are jobs for a licensed tech on site.
Show a licensed heating and cooling tech on video
Point your phone at it and a licensed heating and cooling tech tells you what's likely going on, what's safe to try next and whether it needs someone on site. $34 for 15 minutes or $59 for 30, and you only pay when a pro connects. Hollr is launching across Canada, so join the waitlist for your invite.
Questions
Advice only: Hollr pros diagnose and guide over video, and refer anything that needs hands on site. Live electrical panels and gas lines are never walked through on a call. Hollr checks every heating and cooling tech's licence with the provincial body that issues it. How we check.
Why is my furnace running but blowing cold air?
Common causes are the thermostat fan set to ON, a clogged filter making the furnace overheat and shut off its burner, or a furnace in lockout after failing to light. The blinking light on the furnace usually tells you which.
How do I reset my furnace?
On many furnaces, you switch the power off at the furnace switch, wait a minute, and switch it back on. Do it once. If it goes back into lockout, call a licensed HVAC tech rather than resetting it again and again.
Can a dirty filter really stop the heat?
It can. With too little airflow, the furnace can get too hot inside, and a safety switch shuts the burner off while the fan keeps blowing. A clean filter is the first thing to try.
Can an HVAC tech help over video?
Yes, for diagnosis. On a Hollr call, a licensed tech can read the error code with you, check your thermostat and filter, and tell you whether it needs a visit. They never walk you through the burner or gas supply.