Why Is My Furnace Blowing Cold Air? 7 Common Causes and Fixes

If your furnace is blowing cold air, the most common causes are a thermostat fan set to "On" instead of "Auto," a clogged air filter, a pilot light or ignition problem, or leaky ductwork. Start by checking your thermostat settings and your filter. If the air is still cold after that, the problem is probably inside the furnace and needs a technician.

That first cold morning of fall is when many homeowners in DC, Maryland and Virginia find out something's wrong with the heat. Here's how to figure out what's happening.

First: Is It Actually Cold, or Just Cool?

For the first minute or so after a furnace starts, the air from your vents may feel cool. Many furnaces wait for the heat exchanger to warm up before the blower runs at full speed. So if it warms up after a minute or two, that's normal. If the air stays cold, keep reading.

7 Reasons Your Furnace Is Blowing Cold Air

1. The Thermostat Fan Is Set to "On"

This is the most common and easiest fix. When the fan is set to "On," the blower runs nonstop, even when the furnace isn't heating. Between heating cycles, it just pushes room-temperature air through the vents. Switch the fan setting to "Auto" so it only runs while the furnace is heating.

2. The Thermostat Isn't Set to Heat

It sounds obvious, but after a summer of cooling, it's easy to forget. Make sure the thermostat is on "Heat" and set a few degrees above the current room temperature. If you have a smart or programmable thermostat, check that the schedule hasn't overridden your setting. Thermostat acting up? See our thermostat services.

3. A Clogged Air Filter

A dirty filter chokes airflow. The furnace can overheat, and its safety switch shuts off the burners while the blower keeps running to cool things down. That's cold air coming out of your vents. Check the filter and replace it if it's dirty.

4. Pilot Light or Ignition Problems

Older gas furnaces use a standing pilot light, and newer ones use an electronic igniter. If the pilot goes out or the igniter fails, the burners never light, but the blower may still run. Don't keep trying to relight or reset a furnace that won't stay lit. That's a job for a technician.

5. A Dirty Flame Sensor

The flame sensor confirms the burners are lit. When it's dirty, the furnace may shut the burners off right after they light, and you'll get a short burst of warm air followed by cool air. A technician can clean or replace it.

6. Leaky or Disconnected Ductwork

Sometimes the furnace is making heat, but the heat isn't reaching you. According to ENERGY STAR, about 20 to 30 percent of the air moving through a typical home's duct system is lost to leaks, holes and poor connections. Ducts running through attics, crawlspaces or garages can also pull in cold air. If some rooms are warm and others stay cold, ducts may be the culprit. Learn about our ducts and vents services.

7. Gas Supply or Condensate Drain Issues

If the gas valve is closed or there's a supply problem, the furnace can't heat. High-efficiency furnaces also have a condensate drain line, and if it clogs, a safety switch can stop the burners. Both are best handled by a pro.

Quick Checks You Can Do Safely

1. Set the thermostat to Heat and the fan to Auto.
2. Replace thermostat batteries if the screen is blank or dim.
3. Check and replace the air filter.
4. Make sure the furnace power switch (it often looks like a light switch near the unit) is on, and that the breaker hasn't tripped.
5. Open supply and return vents and clear anything blocking them.

Safety First: Gas Furnaces and Carbon Monoxide

If you smell gas, don't try to troubleshoot. Leave the home and call your gas utility from outside. Also make sure you have working carbon monoxide (CO) alarms. The U.S. Environmental Protection Agency lists leaking furnaces and poorly maintained combustion equipment among the sources of carbon monoxide in homes. That's one more reason to schedule a fall heating check-up.

When to Call a Professional

Call a heating technician if:

• The air stays cold after you've checked the thermostat and filter
• The furnace won't light, or lights and then shuts off
• You hear unusual noises or see error lights flashing on the furnace
• Some rooms are warm while others stay cold

D&D HVAC Solutions offers heating and cooling repair, maintenance and installation for homes in DC, Maryland and Virginia.

Frequently Asked Questions

Why does my furnace blow cold air at first, then warm air?

Many furnaces wait for the heat exchanger to warm up before running the blower at full speed. A short burst of cool air at start-up is normal. Cold air that never warms up is not.

Should my thermostat fan be on "On" or "Auto"?

For most homes in heating season, "Auto" is the better choice. The fan only runs while the furnace is heating, so you won't feel cool air between cycles.

Can I fix a furnace that's blowing cold air myself?

You can safely check the thermostat settings, batteries, filter, power switch and vents. Anything involving the burners, igniter, gas valve or internal parts should be handled by a qualified technician.