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  • Writer's pictureRob Mitchell

Why Is My Furnace Blowing Cold Air?

Updated: Jan 9

There are several reasons that could be causing your furnace to blow cold air. Sometimes, the issue is simple to resolve. In other situations, you need to call a professional for repairs.

Furnace Problems

(1) Your furnace isn’t warmed up yet.

Your heating cycle needs time to get rolling before it starts blowing hot air. Particularly if it’s the first time your furnace has been running this season.


After approximately 10-15 minutes, if your furnace is still blowing cold air you might investigate the following possible problems.


(2) You might have a thermostat issue.


The thermostat sends instructions to your heating and cooling system, telling it when to heat, when to cool, and when to run its fan. A simple reason for a furnace to be blowing cold air is that the thermostat settings are incorrect.

Make sure the thermostat is set to HEAT, not COOL. One reason you might feel cold air from your vents is that your air conditioner is running – not your furnace.


Check the thermostat’s fan settings and make sure AUTO is selected, not ON. The ON setting allows your HVAC system’s blower fan and motor to operate continuously, even when the furnace is not heating. Between heating cycles, the blower will continue to circulate air through your ducts; it just won’t be hot because the furnace isn’t running to create heat. The AUTO setting enables the blower to run only when the furnace is also on.


There is also a possibility your thermostat isn’t working properly. A wire may have become disconnected. Or, if it has batteries, they might be dead.


(3) You could have a blockage in your ductwork.


A compromised airflow can cause your furnace to overheat. This will trip the high limit switch and the furnace will shut down. This is a safety measure to prevent a fire or further complications. This might be why your furnace is not blowing hot air.


A dirty furnace air filter is a common culprit of an airflow restriction. Check your air filter on a regular basis, and change it at the recommended interval or sooner if needed. Try changing the air filter and restarting the furnace.


Investigate for other airflow issues. Make sure all your vents and registers are open and the airflow is not blocked by furniture, rugs or other items. A dirty air conditioning evaporator coil could also restrict the airflow.


(4) There might be leaks in your ductwork.


If you have holes, disconnections, or leaks in your ductwork, then all the hot air could be escaping before making it through your vents. Sometimes this can be due to age. Other times it is due to small animals biting or clawing at them.


(5) There could be a pilot light or ignitor malfunction.


Older furnaces were equipped with pilot lights, while newer models use an electric ignitor to spark gas for combustion. If the pilot light goes out, or the ignitor fails, the gas doesn’t ignite. This can lead to cold air blowing from the furnace. Follow the instructions in your furnace owner’s manual to safely relight the pilot if you have one– if it continues to go out, call for furnace repair. If your unit uses an ignitor, call for service to have the broken component replaced. Don’t worry, ignitors typically have to be replaced every seven years or so.


People tend to use the terms “flame sensor” and “thermocouple” interchangeably. In truth, while they basically perform the same job, they're not the same thing. If your furnace uses an electronic controller, it's got a flame sensor. Thermocouples are used on standing pilot furnaces that have pilot lights.


(6) You may have a faulty thermocouple.


If you do manage to relight the pilot but it keeps going out, a faulty thermocouple is likely causing your furnace to blow cold air. A thermocouple is a safety device that’s used to detect whether or not your furnace’s pilot light is lit. It contains a metal rod that is placed very close to your pilot light’s flame. This rod is connected to your furnace’s gas valve. When your pilot light is lit, the heat from the flame sends a voltage through the thermocouple that keeps the gas line open. If the pilot light goes out, the thermocouple will lose its voltage and the gas line to your furnace will be closed. This is usually a simple fix, but you’ll need to call a technician to adjust or replace it.


(7) A dirty flame sensor might be the problem.


A flame sensor detects the presence of a flame created by burning fuel. This safety control shuts down the heating system if it cannot detect a flame in the burner assembly while gas is going to the furnace. Soot from combustion can cover the flame sensor, making it unable to detect a flame. The flame sensor can be cleaned by a technician in some cases, or it may need to be replaced.


(8) The electronic ignition could be the problem.


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.


(9) There may be a gas valve issue.


If the pilot won’t light at all, you may have a faulty gas valve. Or, if your system has not been cleaned in a while, a valve could be stuck due to dirt and debris.


(10) You might have a dirty gas burner.


Over time, accumulated dust and grime on the furnace burner itself can make igniting difficult. Having your system professionally cleaned will normally take care of this problem.


(11) There could be a problem with the computerized controls.


If you have a newer furnace with an electronic control panel, you may need to reset the system. Try turning the furnace off with the power switch. Wait a few minutes, then restart. This is similar to rebooting your computer to fix a software glitch. It may be that the control board has gone bad and will need to be replaced.


(12) The condensate line might be clogged.


High-efficiency furnaces have condensate lines that move water away from the furnace. A clog in the condensate line will prevent your furnace from igniting. No ignition means only cold air coming through your vents.


(13) There could be a gas line leak.


One sign of a natural gas leak is the smell of sulfur or rotten eggs in the air. Gas supply companies add this odorant to gas to aid in gas leak detection.


If you suspect a gas leak, get everyone out of your home right away. Move a safe distance away from your home and call 911. Then contact the natural gas provider for your area.


(14) The gas line might have a blockage.


A gas line problem might be inside or outside of your home. A gas line can become blocked with dirt or other debris; insects; tree roots; or even water. Alternatively, a gas line can develop a bad dent or kink in the line that blocks the flow of gas through it.


Are your other gas appliances working properly? When you have a blockage in your gas line your gas range may only produce a low flame when you have the burner set on high, or your gas water heater may take much longer to heat water than it used to.


If you suspect you have a gas line blockage, shut off the gas supply to your home and contact a professional with experience in gas line repair.


If troubleshooting on your own doesn’t solve the issue, call Elbert Heating & Air for a diagnosis and an estimate for repair.



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