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A homeowner adjusts the thermostat

What Causes a Furnace to Blow Cold Air?

Sean S.Verified

HVAC Owner/Technician

Residential HVAC

292 positive ratings
First Steps

First, check the setting on the thermostat is switched to heat. Next, check the air filters for grime and the pilot light for flame. Finally, inspect the thermocouple and flame sensor.

There are four main components to most furnaces. These components are the:

  1. power system (gas or electric)

  2. burners

  3. fan(s)

  4. duct work

If something is obstructing the function of any of these systems, it can compromise the way your furnace blows air.

Fixing an electric furnace or gas furnace blower on your own can be a daunting task. It is always a good idea to troubleshoot the simplest things first.

Before you start

When did the furnace start blowing cold air?

Before you tear into your furnace, ask yourself, “When did you first notice the furnace blowing cold air?” Sometimes people are concerned when they first turn on the furnace because it did not instantly blow hot air. Warm air production happens gradually and it may take a few moments to begin blowing warm air, because of how long it takes to travel from the heat pump, through the ductwork, and out of the radiator vents.

Switch the furnace thermostat to heat

When summer is over, it is easy to forget to switch the thermostat from air conditioner to heat. If your furnace is blowing cold air, check to make sure you have switched the thermostat over so the heat exchanger can ignite.

Check thermostat batteries and wiring

In some furnace models, the thermostat solely runs on batteries. If this is the case for your thermostat, your heater will still produce air if the batteries need to be replaced. However, if the batteries are weak or drained, the thermostat may not be able to send the messages to the heating element so it won’t know to turn on.

Many heater/furnace thermostats are hard-wired into the the furnace, air handler, or boiler. Check the wiring to make sure all wires are properly connected and tightened.

Check the thermostat settings

Ensure that your thermostat is set higher than the temperature in your home. This will trigger your electric heater or gas furnace to turn on.

A typical thermostat

 Easier fixes

Rule out condensation

Many of the newer furnaces have a condensation drain line to remove water during the heating process. Sometimes a line or drain can become clogged and signal the safety switch that prevents the burners from lighting.

Rule out inadequate gas supply

Your gas supply could be turned off. Or the gas line is may not be supplying enough gas to your furnace. This can cause your gas furnace to lock down for safety.

Check for a dirty air filter

A dirty air filter can block air travel and potentially cause the furnace to overheat. Many furnaces have a safety switch that will shut the heater coils down when they overheat. This safety mechanism, however, often does not shut the blowers down at the same time. Instead, they will continue to blow, producing only cool air. Change your air filter at the beginning of each season or approximately every 2-3 months.

A dirty air filter

Inspect the oil tank

If your furnace is oil-fired, make sure you have adequate oil. If you are low or out of oil, your blower may keep circulating air even after the burners fail to light, resulting in the cold air blowing from the furnace.

Fix a furnace blowing cold air intermittently

If this is the case, check your thermostat first. If the fan is set to ON, it can cause the blower to run constantly even though the furnace is not heating the air. Set your thermostat to AUTO to enable the furnace to regulate the heat based on the air temperature in the home.

Examine pilot light issues

If you have an older furnace with a continuous pilot light, the pilot may have gone out. This could happen from a draft coming from an open window or door or inadequate gas supply.

Turn off the heat first then try relighting the pilot light. If the pilot does not light at all, check if there is gas supply, a faulty gas valve or a draft. Or, if your system has not been cleaned in a while, a valve could be stuck due to dirt and debris.

Pilot lights firing

Troubleshoot ductwork

Time, age and changes in temperature may cause your ductwork to crack or tear. If this happens, it can allow the warm air to escape and force cold air into your home. While this can be a big problem, it is an easy fix; generally covering the hole or rip with tape or sealant will do the trick.

Also, dirt and debris can build up in the ductwork, which may cause the furnace to blow cold air. Rarely is it severe enough to block airflow fully; though it is still possible. If it happens, it can make your furnace overheat and cause the safety switch to shut the heater coils down. If the heater coils are off, cool air can still travel through the ductwork.

Investigate furnace control panel problems

Some models of the newer furnaces come with an electronic control panel. If a surge of electricity or a glitch of any kind causes the control panel to stall or fail, air can still circulate. However, it will not be warm air because the safety lock is signaled to shut down the heating element. Just like any computer, a glitch can cause the need for a reboot. Usually, if you turn the furnace off at the power switch, wait a few minutes and turn it back on the computer control panel will reset.



Cost of Parts

Cost of Labor

Estimated Time to Fix

Dirty air filter




Faulty gas valve




Broken pilot light




Porous ductwork




Broken control panel




Harder fixes (may require professional help)

Maintain proper furnace size

Many people do not realize that furnace size is important; it is not a one-size-fits-all appliance. If your furnace is too big for your home, it will turn on more frequently. This could cause the heating elements to become overworked and shutdown, sending unheated air throughout the air ducts. If the furnace is too small, it may not have enough power to push the air needed to warm your home. This can also cause the furnace to shut down due to overheating.

Fix faulty thermocouple

If you are having issues with the pilot light staying lit, or if it keeps going out, it could be a faulty thermocouple, which is a sensor that controls the gas valve and ignition. If your furnace is blowing cold air (or not working at all), contact a professional for help.

Correct faulty electronic ignition

If you have a newer furnace with an electronic ignition, such as an intermittent pilot or a hot surface ignition, your system may need adjustments or replacement of failed parts. To be safe, a professional would be best suited for repair.

Flame sensor

If your furnace starts blowing warm but then quickly turns cold, the flame sensor may need attention. A flame sensor that’s covered with grime or worn out can cause the burner to keep shutting off. Try cleaning the sensor. If, however, you are not experienced, contact a HVAC Expert.

Dirty oil or gas burner

If the burner itself is neglected, the grime can cause it to have trouble igniting. Have your system professionally cleaned to take care of this problem.

Yearly maintenance and inspections

Sometimes just having your furnace inspected once a year can help prevent problems that cause a furnace to blow cold air. One of the most common problems, dirty flame sensors, is entirely preventable with regular maintenace, for example. Several components can become cracked over the years or even eaten by vermin like mice and squirrels. This can become a dangerous situation, especially if your heater is powered by gas or propane, because carbon monoxide can leak into your house and lead to serious health problems for you and your family. A thorough inspection can prevent many problems before they occur.

A technician performs maintenance on a furnace

Is your furnace still blowing cold air after troubleshooting?

Many of us know how to put water in our cars, but few of us have the knowledge to change our own radiator. The same applies when it comes to heating and air units. There are several things we can try on our own, however, sometimes after you have exhausted your efforts, it is just faster and cheaper to ask an Expert. A licensed HVAC professional will be geared with the knowledge to troubleshoot a furnace that continues blowing cold air.

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