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Reasons Why Your Furnace Is Blowing Cold Air

When your furnace is blowing cold air instead of warm, it might be due to a small, fixable issue, or it could be a sign of something more serious.

Reasons Why Your Furnace Is Blowing Cold Air

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Roger Bakies


February 1st, 2023

It’s no coincidence that we at Fire & Ice Heating and Air Conditioning receive a lot of service calls when homeowners turn on their furnaces in autumn for the first time since they used them in the spring.

If you’ve skipped your yearly maintenance - and even the most conscientious people do - your furnace has been lying dormant for five, six months. And it may have been trouble-free all of last year, but that doesn’t mean that it’s good to go now.

So during that first cold snap, you realize that there’s a problem. Your furnace is blowing cold air. You wait, but the temperature in the house drops.

Or maybe it’s been working fine for months, then one day: no heat. What could have gone wrong?

We’ve been troubleshooting these problems for years and years, and have learned quite a few things. Some solutions are easy fixes. Some require an HVAC contractor with a good tool kit.

So why is your furnace blowing cold air? Let’s go through some possible scenarios and fixes.

Related: Furnace Troubleshooting: Common Heating Problems & Repairs

Cool Air Is Normal When Your Furnace First Starts

When a gas furnace first starts, it goes through a warm-up cycle. The flames come on first and there’s a delay until the blower comes on. And of course, the ductwork is full of cold air because the furnace hasn’t run for a while. So it has to purge that cold air out of the ductwork before it’s going to start to feel warm.

So cold air at first is normal, as long as it turns warm soon.

If it doesn’t, several things could be wrong.

The Problem Could Be Your Thermostat

If your thermostat has been set to “cool” for months, it may still be set there. A quick adjustment to “heat” might do the trick.

Another setting may be the cause. If it’s set to “fan,” that doesn’t give the furnace a call to heat. It only sets the blower in action. “Auto” might turn the full system on. 

One more simple mistake can be remedied easily. If the temperature is still set for an air conditioner - say, 68 degrees or colder - raise the temperature to 72 or so, and your problem might be over.

And one problem will necessitate a call to your local HVAC system professional. The thermostat wires could be shorted, meaning it’s lost communication with the furnace. It might cause it to run continuously without a call for heat.

The Problem Could Be a Dirty Air Filter

Air filters trap dirt, dust, and other contaminants, cleaning the air and helping to keep the ductwork and furnace clean. If they are left in place too long, they become choked, which restricts airflow to the furnace.

The furnace then has to work harder to compensate for the blockage of airflow. This can cause the heat exchanger to overheat and shut off too quickly. Furnaces have limit switches that open when the temperature exceeds a set point. At the high limit, the burners turn off and the heat stops.

If you have a dirty air filter, the cool air from inside your home will struggle to travel through it and into the furnace. This means you have insufficient air to heat and redistribute. The result is your heater blowing cold air.

The switches will usually reset when the temperature of the heat exchanger cools. But if the filter remains clogged, the problem will recur and the furnace will not provide the proper amount of heat. If it does that enough times, the limit switch will not reset.

Be sure to check your air filter monthly and replace it if it’s dirty. Some filters should be changed every three months or so. Others can last a year. Others can be washed and reused.

RELATED: How Often Should Furnace Filters Be Changed?

Sometimes the limit switch will get stuck in the open position, which will require a service call from a contractor to replace the part.

A Lack of Airflow Might Be an Issue

Just as a clogged filter can restrict airflow, so can other issues. For instance, if you’ve moved furniture around, and are now blocking return air registers, you’re starving the furnace of needed air. It’s the same effect as having a clogged air filter.

The less air your furnace gets from the return vents, the less it can generate air to send through the supply vents. It might be warm, but the airflow will be weak.

Cold Air Might Be Due to Leaky Ducts

According to Energy Star, in a typical house, about 20 to 30 percent of the air that moves through the duct system is lost due to leaks, holes, and poorly connected ducts.

The holes in your ductwork allow cold air from unconditioned rooms to enter the vents. And all the hot air might be getting deposited in the basement before it has the chance to reach the far rooms of your house.

The Dampers May Need to Be Reset

In some ductwork, dampers help direct the air toward or away from certain areas of the house. They can be closed manually, or some electronic dampers can be opened and closed through a smart thermostat.

The dampers help with creating zones. In the winter, if one room remains stubbornly cold while another is too warm, a damper can redirect the air.

A closed damper means no airflow.

RELATED: What is an HVAC Zoning System? (And Why It Works)

Dampers are typically set differently for different times of the year. These settings can help compensate for seasonal changes. If you notice that part of your home seems chilly, your dampers may not be positioned for the heating season.

You can adjust your dampers yourself, but it may involve a lot of trial and error.

Pilot Lights and Flame Sensors Could Be the Issue

A modern furnace without a pilot light relies on flame detectors to ensure the successful completion of a heating cycle. If the flame detector is covered in dust, dirt, or grime, you will quickly lose heat and your gas furnace will start blowing cold air. The flame sensor will need to be cleaned.

With an igniter, it’s going to recognize that the flame isn’t there. Depending on the furnace’s makeup, some of them will go into a full cycle and run the blower constantly even with no flame.

We still see pilot lights on very old furnaces. The shaft that the pilot light comes out of might be covered in debris, which can interfere with the light’s operation. If there is noticeable flickering or one side struggles to light, or the base of the flame is yellow instead of blue, you are likely dealing with some sort of blockage.

That will diminish the amount of heat the system can generate.

RELATED: Furnace Pilot Light: How to Re-Light, Fix and Protect Your System

What Happens When an Electric Furnace Blows Cold Air

Electric furnaces and air handlers create heat through the use of heat strips. They work much like a toaster or hair dryer. It’s a wire that glows red hot, and the blower moves that heated air.

If the strips go bad, the blower will still run. It could be a failure of the board or the heat strips. The unit will go through its algorithm for making heat. That will get the blower in motion but the heat won’t be occurring.

What Happens When You Own a Heat Pump and It Blows Cold Air?

If your main source of heat is coming from your heat pump, it should work fine as long as the temperature stays above 40 degrees Fahrenheit. When it drops below that you’ll run into a heating issue. At lower temperatures, heat pump “de-rate,” they can’t take as much warmth out of the air to distribute.

The air coming out of it and being distributed will feel cooler and cooler as the temperature drops. Eventually, it will freeze and stop producing any heat until it can defrost.

This is where it’s vital you have back-up or emergency heat. That can come from an air handler equipped with heat strips, or else your gas furnace kicks on and carries the load.

In most systems, your thermostat will know when to make the switch from heat pump to back-up. If it doesn’t the problem most likely will be with the thermostat.

Other Problems that Can Cause Your Furnace to Malfunction

Yet more issues can crop up:

  • Poor airflow. If your system never supplies enough warm air, it could be that your ductwork is not sized properly. Unfortunately, this is too common. If the return air ducts are too small, the furnace will not be able to take in the required amount of air. Due to the lack of airflow, your heater will blow cold air.
  • Check the gas line. If something drastic has happened to it, your furnace won’t function, and neither will your water heater or gas stove. Your furnace is designed to shut off if it’s not receiving enough gas to perform its regular heating process. Call your gas company ASAP.
  • Some gas furnaces are engineered with condensate drain lines that aid with the removal of moisture. If there is a blockage in these lines, this may deactivate the burners and cause the furnace to blow cold air. This can be diagnosed and remedied quickly. A PVC pipe will lead to a floor drain. If it’s blocked, that can be cleared.

Working with Problematic Furnaces in Columbus, Ohio

The average homeowner who’s facing a furnace that blows cold air, and has no idea where to begin, should treat it like they do with their computer. Turn it off. Let it sit for a minute, then turn it back on. Usually, it will try to cycle again.

If that doesn’t work, and you can’t solve your furnace problem easily, it’s time to make a call to an expert. We would love to be the company you reach out to.

Simply fill in your zip code to see if you’re in our Central Ohio service area. We hope to hear from you soon.

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