You turned the heat on for the first time this fall and the furnace is blowing cold air. Before you book a repair, check a few things yourself. The most common cause takes ten seconds to fix and costs nothing.
This guide starts with the easy checks, then covers the problems that need a pro and ends with the signs that mean you should stop and get help right away. If you smell gas, skip to the last section now.
Check this first: the thermostat fan setting
Look at the fan setting on your thermostat. If it says ON instead of AUTO, the blower runs all the time, even when the furnace is not heating. Between heating cycles it pushes room temperature air through the vents, and that air feels cold on your hand. Switch it to AUTO and wait through one full heating cycle. If warm air comes out and then the blower stops, you found it.
Why Your Furnace Is Blowing Cold Air: The Easy Causes
Work through these in order. Each one takes a few minutes and needs no tools.
1. The thermostat is set wrong
Besides the fan setting, make sure the mode says HEAT and not COOL or OFF. Set the temperature a few degrees above what the room is now. If the screen is blank or dim, replace the batteries. A thermostat with dead batteries can stop calling for heat.
2. The furnace is still warming up
Most furnaces light the burners first and wait about a minute before the blower starts, so the metal inside can heat up. The first air out of the vents can also be cool air that sat in the ducts. At the end of a cycle the blower runs a little longer to push out leftover heat, and that air cools off too. Give it a few minutes before you decide something is wrong.
3. The filter is clogged
A dirty filter chokes the airflow. The furnace can get too hot inside, and a safety switch shuts the burners off. The blower keeps running to cool things down, so you get cold air. Pull the filter out and hold it up to a light. If you cannot see light through it, replace it. Write down the size printed on the frame so the next one is easy to buy.
4. The furnace has no power or no fuel
Check that the switch near the furnace is on. It often looks like a light switch on the wall or the side of the unit. Check the breaker panel for a tripped breaker. Make sure the front panel of the furnace is fully seated, because a safety switch keeps the furnace off when the door is loose. If you have gas, make sure other gas appliances work and the gas valve near the furnace is open.
5. The pipes outside are blocked
High efficiency furnaces vent through white plastic pipes that run out the side of the house or the roof. Snow, ice, leaves or a nest can block them, and the furnace will shut down to protect itself. If you can safely reach the pipes from the ground, clear away anything blocking them.
If you have a heat pump instead of a furnace, cooler air at the vent can be normal. Heat pump air feels warm but not hot, and the system blows cooler air for a short time while it defrosts on cold days. Not sure which one you have? Read the metal nameplate on the outdoor unit. It says air conditioner or heat pump.
If only some rooms get cold air, the furnace may be fine and the air is getting lost on the way. Our guide to why one room is always hotter explains how air moves through your ducts and what you can check.
Furnace Still Blowing Cold Air? Causes That Need a Pro
If you checked everything above and the air is still cold, the problem is likely inside the furnace. These jobs involve gas, high voltage or both. Book a pro rather than opening up the burner area yourself.
Flame sensor or ignition problems
Watch the furnace through its small window if it has one. If the burners light, run for a few seconds and shut off, a dirty or failing flame sensor is a common cause. If they never light at all, the igniter or the gas valve may have failed. Older furnaces with a standing pilot light may simply have a pilot that went out.
A clogged condensate drain
High efficiency furnaces make water as they run. It drains through a small tube to a floor drain or a pump. If that line clogs, a switch shuts off the burners to stop a leak. Water around the base of the furnace is a clue.
Error codes
Most newer furnaces have a small light on the control board that blinks a code when something is wrong. You can often see it through a small window in the lower panel. The code chart is usually printed on the inside of the panel door. Write down the blink pattern before you call. It helps the pro arrive with the right part.
Duct problems
If warm air comes out of some vents and cold air out of others, ducts in an attic, crawlspace or garage may be leaking or disconnected. The furnace is working. The heat is just not arriving.
Wondering what these fixes usually run? Our guide to HVAC repair and replacement costs in Ohio lists typical price ranges for parts like igniters and blower motors, and helps you decide whether an older furnace is worth fixing.
When to Stop and Book a Pro Right Away
If you smell gas
Do not touch light switches, the thermostat or anything that could make a spark. Get everyone out of the house. Once you are outside, call your gas company or 911. Do not go back in until they say it is safe.
Also stop checking and get help if:
· Your carbon monoxide alarm goes off. Get out and call 911 from outside.
· You smell burning that lasts more than the first few minutes of the season. A short dusty smell the first time you run the heat is common.
· A breaker trips again after you reset it.
· Water is pooling around the furnace.
· The furnace shows an error code or the burners keep shutting off.
If your heat is out on a cold night, our emergency HVAC guide covers what to do first.
What to Tell the Pro When You Book
A few details save time on the visit:
· What you already checked: fan setting, filter, power switch and outdoor pipes.
· What the burners do: never light, or light and shut off.
· Any blinking code, written down exactly.
· The brand and age of the furnace, from the label inside the panel door.
· Whether the cold air comes from every vent or only some.
And ask to see the pro's license and proof of insurance before work starts. Any good pro will show you.
Furnace Blowing Cold Air: Common Questions
Why is my furnace blowing cold air when the heat is on?
The most common cause is the thermostat fan set to ON instead of AUTO. The blower then runs all the time and pushes room temperature air between heating cycles. Other common causes are a clogged filter, a furnace that is still warming up and a burner that lights and then shuts off.
Is it normal for a furnace to blow cold air at first?
A short burst of cool air can be normal. Many furnaces wait about a minute after the burners light before the blower starts, and some air sitting in the ducts is cool. If the air is still cold after a few minutes of running, something else is going on.
Can a dirty filter make a furnace blow cold air?
Yes. A clogged filter chokes airflow, and the furnace can overheat inside. A safety switch then shuts the burners off while the blower keeps running, so you feel cool air. Replace the filter and see if the heat comes back. If it keeps happening with a clean filter, call a pro.
What does a flame sensor problem look like?
The burners light, run for a few seconds and then shut off. The furnace may try again several times and then stop trying. A dirty or failing flame sensor is a common cause. It is a small part, but it sits inside the burner area, so it is a job for a pro.
My furnace has white plastic pipes. Does that matter?
White plastic pipes usually mean a high efficiency furnace. These make water as they run, and it drains through a small tube. If that drain clogs, the furnace can shut its burners off. Snow, leaves or a nest blocking the pipes outside can do the same thing.
When should I stop checking and call someone?
Stop right away if you smell gas or your carbon monoxide alarm goes off. Leave the house and call your gas company or 911 from outside. Call a pro if the furnace shows an error code, the burners keep shutting off, a breaker keeps tripping or you see water pooling around the unit.
Find a Furnace Repair Pro Near You
Checked the easy things and still no heat? Start with our HVAC services page, or pick your city below.
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