How to Troubleshoot a Furnace Blowing Cold Air

So you’ve turned on your furnace, but it’s giving you the cold shoulder by blowing cold air.

What’s wrong?

Well, it could be multiple issues; some of them you can solve yourself.

So before calling a professional for help, try these 4 furnace troubleshooting tactics.

Check the thermostat’s FAN setting

Does your furnace blow hot air sometimes, but cold air other times? Your thermostat’s fan setting may be set to ON.

The fan setting controls the blower, the part that circulates air throughout your home, Setting it to ON means the blower will run 24/7—regardless of whether the furnace is heating the air or not, thus why you get cold air sometimes.

Do this: Ensure your thermostat’s fan setting is set to AUTO, not ON. AUTO ensures that the blower will run only when the furnace heats the air.

Check the furnace’s air filter

Your furnace may be blowing cold air because the filter is too dirty.

A dirty air filter blocks airflow over the furnace’s heat exchanger, causing it to overheat. When overheating, your furnace can trip a high limit switch, causing the furnace burners to shut off so that the heat exchanger does not crack.

Do this: Turn off your furnace at the thermostat, and check the furnace filter. If it’s dirty, change it. You may need to call a technician to help you reset the furnace.

Your furnace filter may be located here, next to the blower.

Check the pilot light

Do you have an older furnace with a standing pilot light? If the pilot light isn’t lit, then the furnace’s burners won’t light, meaning no heat.

Do this: Try relighting the pilot light following these steps:

Step 1: Turn off your furnace

Turn your thermostat from HEAT to OFF.

Step 2: Find the furnace’s pilot light assembly and reset switch

The assembly and switch should be near the bottom of your furnace. Can’t find them? Check your furnace’s manual.

Also, this image may help:

You’ll have to open your furnace’s cover to access the reset switch. You should see a knob with these 3 settings:

"Pilot"
"Off"
"On"

Step 3: Turn the knob to "off" and wait 3-5 minutes

This will shut down gas coming through the pilot.

Step 4: Turn the knob to "pilot" and press down the knob

Now you’re re-starting the flow of gas to the pilot.

Step 5: As you press the knob, hold a lighter to the pilot opening until the flame lights

When the flame lights, it should be a steady blue cone that hits the middle of the thermocouple (a small copper bar).

Step 6: Turn the knob to “on”

Now, your furnace should ignite.

Step 7: Turn the furnace on at the thermostat

Turn your thermostat back to HEAT and ensure that the temperature is set 5 degrees below room temperature. You should be getting hot air now.

What to do if the pilot won’t light or stay lit

At this point you should call a furnace technician for help. You may have a malfunctioning thermocouple (an inexpensive fix) or other issues that cause the pilot light to burn improperly.

Check your condensate line

Do you have a high-efficiency furnace? Do you see water pooling around the furnace? Then the furnace’s condensate line (usually a PVC pipe) may be blocked, which causes the furnace to shut down.

Let us explain.

When high-efficiency furnaces run, they create water (condensate), which is emptied out a drain line. However, if that line gets blocked, water backs up into the furnace, causing an overflow kill switch to shut down the furnace to prevent water damage.

Common causes of condensate blockage are:

Dust
Dirt
Mold
Ice (only during cold snaps and if the line runs through an unconditioned space)

Besides blockages, the condensate overflow may also be caused by a broken condensate pump. In that case, you’ll need a professional the fix the pump.

Do this: Assuming that ice isn’t the problem, unclog the condensate line by following this tutorial on Energy Vanguard.

If you suspect ice is the problem, then wrap the condensate line with heat tape and pipe insulation.

Need a furnace repair in metro Atlanta?

Why not give Coolray a call? We’ve been serving the metro Atlanta area since 1966. And if you need help ASAP, we can help since we offer emergency service 24/7.

Coolray is your Atlanta-area home comfort expert with specialists in heating, air conditioning, air quality and plumbing. Have more questions? We’d be happy to help – just contact us online.