Why Is My Furnace Blowing Cold Air? 9 Causes and Easy Fixes

Your furnace is probably blowing cold air because the thermostat fan is set to ON, the air filter is clogged, or the burners are not lighting. The first two are quick fixes you can do in a few minutes. Burner and ignition problems sometimes need a pro, but there are a few safe things you can check first.

Before you call for service, work through the list below from top to bottom. The easiest fixes come first, and many people solve the problem before they reach the end.

Safety first: If you smell gas (a rotten egg smell), do not touch any switches. Leave the house right away and call your gas company or 911 from outside.

9 reasons your furnace is blowing cold air

1. The thermostat fan is set to ON

This is the number one cause, and the easiest fix. Most thermostats have a fan setting with two choices:

• AUTO: The blower only runs when the furnace is heating. Air comes out warm.
• ON: The blower runs all the time, even between heating cycles. Some of the air coming out is not heated, so it feels cool.

Fix: Switch the fan setting to AUTO. Then check that the thermostat is set to HEAT, not COOL or OFF.

2. The thermostat settings or batteries are wrong

A thermostat set too low will not call for heat. And a thermostat with dead batteries can act strangely or stop sending signals.

Fix:

• Make sure it is set to HEAT.
• Set the temperature at least 3 to 5 degrees above the current room temperature.
• Replace the batteries if the screen is blank or dim.
• If you have a programmable thermostat, check that a schedule is not overriding your setting.

3. The furnace is still warming up

Many furnaces light the burners first and wait a minute or so before the blower starts. The first bit of air may also be cool because it was sitting in the ducts.

Fix: Wait 3 to 5 minutes. If the air turns warm, your furnace is working normally.

4. The air filter is dirty

A clogged filter is a very common cause of heating problems. When air cannot flow through the furnace, the inside can overheat. A safety part called the limit switch then turns off the burners to protect the furnace. The blower keeps running, so you get cool air.

The Department of Energy and ENERGY STAR both say to check your filter every month in heating season and change it at least every 3 months.

Fix: Turn off the furnace. Slide out the filter and hold it up to a light. If you cannot see light through it, replace it. Match the size printed on the edge of the old filter, and point the arrow on the new filter toward the furnace.

A dirty filter also means more dust in your home. Our guide on cutting down on household dust explains how filters help.

5. The pilot light is out or the igniter failed

Older gas furnaces have a small pilot light that stays lit and lights the burners. If it goes out, the furnace cannot make heat. Most newer furnaces use an electronic igniter instead. Igniters can crack and stop working over time.

Fix for a pilot light: Follow the relighting steps printed on the furnace label. If it will not stay lit, call a pro.

Fix for an igniter: Look through the small window on the furnace while it tries to start. If you never see a glow or a flame, the igniter may be bad. This part is best replaced by a technician.

6. The flame sensor is dirty

The flame sensor is a thin metal rod that checks for a flame. If it gets dirty, it cannot "see" the flame, so the furnace shuts the gas off after a few seconds for safety. A common sign is burners that light and then go out quickly, again and again.

Fix: A technician can clean or replace it in a quick visit. Some handy homeowners clean it themselves with fine steel wool after turning off the power and gas. If you are not comfortable working inside the furnace, leave this one to a pro.

7. The gas supply is off

If the gas valve near the furnace is turned off, or your gas service has been shut off, the furnace cannot heat.

Fix: Check that the handle on the gas valve runs in the same direction as the pipe, which usually means it is open. Check that your other gas appliances, like the stove, are working. If none of them work, call your gas company.

8. The condensate drain line is clogged

High-efficiency furnaces make water as they run. The water drains out through a plastic tube. If that line clogs, a safety switch stops the burners to prevent a leak or water damage.

Fix: Look for water around the furnace or a full drain pan. Check the drain tube for clogs, and clear it with a wet/dry vacuum from the end where it drains. If you are not sure where the line is, a technician can clean it.

9. The ducts are leaking

Your warm air travels through ducts to reach each room. If ducts in the attic, garage, or crawl space have gaps, warm air escapes and cold air gets pulled in. Rooms far from the furnace may get only cool air.

Fix: Look at the ducts you can reach. Seal small gaps with metal foil tape or duct sealant, not regular cloth duct tape. For bigger problems, an HVAC company can test and seal the whole duct system.

What about a heat pump?

If you have a heat pump instead of a furnace, cooler air can be normal. A heat pump often blows air that feels warm but not hot, since it is usually cooler than air from a gas furnace.

Heat pumps also run a defrost cycle in cold weather. For a few minutes, the system may blow cool air while it melts ice off the outdoor unit. This is normal. If cool air goes on for a long time, or the outdoor unit is covered in thick ice, call a technician.

Check for a blinking error code

Most modern furnaces have a small light on the control board that blinks a code when something goes wrong. You can often see it through a small window in the furnace door.

1. Count the blinks, and note if they are long or short.
2. Look for a chart printed inside the furnace door.
3. Match the blinks to the chart to see the problem.

Write down the code before you call a technician. It can save time and money on the service call.

Quick troubleshooting checklist

Keep your furnace running well

• Check the filter monthly and change it at least every 3 months.
• Get a tune-up every year before the cold season starts. ENERGY STAR suggests a yearly check by a pro.
• Keep vents open and clear of furniture and rugs.
• Clear space around the furnace. Do not store boxes or paint cans near it.
• Test your carbon monoxide alarms. Any gas furnace should have a working CO alarm nearby. If one is chirping, see our guide on why smoke and CO alarms chirp.

A warm home in winter can also mean foggy windows. If you see water on the glass, learn why condensation forms on windows and how to stop it.

When to call a pro

Call a licensed HVAC technician if:

• You smell gas or suspect a gas leak
• The burners will not light after you check the basics
• The furnace shows an error code you cannot clear
• You hear loud banging, screeching, or grinding
• The problem keeps coming back after you fix it

The bottom line

A furnace blowing cold air is usually an easy fix. Start by setting the thermostat fan to AUTO and checking that it is on HEAT. Then change a dirty filter and make sure the power and gas are on. If the burners still will not light or stay lit, call a technician. A yearly tune-up and monthly filter checks will help your furnace keep you warm all winter.