Cold Air Coming From Vents When Heat Is On

Cold Air Coming From Vents When Heat Is On

About a century ago, residential cooling and heating systems were unheard of. However, with the advent of technology, most of us cannot live without it. While your home’s cooling and heating system keeps the climate and temperatures within your home comfortable, any homeowner knows that their home’s HVAC is no easy feat to address when something goes wrong.

If you know what you’re up against coupled with some fundamental tips, you might not always need to rope in an HVAC technician. One common mishap is cold air coming from vents when heat is on. What should you do when you find yourself in this predicament? It’s an indicator that there’s a problem with your home’s heat. The thermostat may need to be readjusted, you could have a leaky duct, or a problem with the fan limit switch.

Read on to learn why cold air coming from vents when heat is on could be a problem.

1. The Thermostat Settings Require Readjustment

You’ll know this is your issue if:

  • Your fan runs continuously and doesn’t stop
  • Your thermostat fan button is set to ‘Cool’ or ‘On’


The two thermostat problems that may cause your furnace to blow cold air are:

  • It’s set to ‘Cool’ rather than ‘Heat’
  • The thermostat fan setting is set to ‘Auto’ rather than ‘ON’

Granted, we know these causes seem trivial. However, you’d be surprised by the number of residents who leave their thermostat set to ‘Cool’ year-round and forget about the ‘Heat’ button. The result is a confused homeowner who believes the cold air from their vents is the marker of a malfunctioning furnace when the truth is their HVAC unit is responding as it should.

Another common reason behind cold air coming from vents when heat is on is that you might be using the wrong fan settings. Furnaces use an indoor fan to circulate hot air through the ductwork and into your home.  

However, the fan typically has two modes: Auto and On. If your furnace is set to ‘On,’ it operates continuously regardless of whether it’s generating heat. Therefore, after each heating cycle and the furnace burners cut off, the fan keeps blowing heat. Given that the furnace is no longer heating, the air being circulated in your home is cold.

The Solution:

  • Check whether your thermostat is set to ‘Heat’ and your fan is set to ‘Auto’
  • That should stop the fan from its continuous operation
  • If that proves futile or your vents are still blowing cold air in heating mode, then have a professional examine the unit for more adverse problems

2. Leaky Ducts

It’s an issue when:

  • You notice a spike in your utility bills
  • Your home is excessively dusty
  • You notice the air ducts have loose connections, kinks, or holes
  • Particular areas or rooms in your home don’t heat or cool properly
  • Your cooling and heating system don’t get to your desired temperature


The leaks in the air ducts indicate that they are sucking in unconditioned, cold air coming from your attic. When the cold air mixes with the warm air from your furnace, the result is ventilation that feels cooler than normal.


Unfortunately, you’ll need to rope in an expert to sort out this issue because it entails inspecting the ductwork for leaks. If the leaks are minimal, the professional will repair them and if not, sealing the ductwork is the best course of action.

3. A Problem with the Fan Limit Switch

It’s an issue that’s caused when your vents initially blow cold air that becomes warm before getting chilly again. That’s because the fan cuts off entirely between heating cycles.


Your fan limit is found right below the furnace hood and signals the furnace blower to turn on. Typically, the switch allows the furnace burners to warm up entirely before activating the fan and should keep it operating until the burners cool off slightly to curb overheating or damage. In a nutshell, when your fan limit switch is set accurately and functioning as it should, the vents won’t blow chilly air.


Once an expert examines the fan limit switch, they might be able to adjust the settings in a way that prevents the fan from blowing cold air into your home. However, if the switch is bad, it will need a replacement.

4. An Overheating Furnace

It’s marked by:

  • Your furnace is blowing hot air that quickly becomes cold, after which the furnace stops running
  • A sequence of cold and hot air that occurs repeatedly but isn’t warming up your home


  • The overheating of your furnace means the heat exchanger is getting excessively hot
  • When that happens, the fan limit switch will automatically turn off the burners and keep the fan operating until the heat exchanger cools off
  • Once the exchanger cools off, the furnace will attempt to warm your home, during which you’ll notice it continuously going on and off
  • Meanwhile, you’ll get cold air as the fan continues running for safety purposes, long after the furnace fails to generate heat


  • Check to see if your filter is clogged. If so, it restricts the amount of cool air running through the furnace and results in the unit overheating and abruptly shutting off
  • Confirm that all supply vents are open. After all, they blow warm air into your space. However, closing too many elevates the pressure inside the ductwork, putting a strain on the furnace blower and slowing down its operation. When the blower functions slower than it should, it cannot push as much cool air over the heat exchanger, causing it to overheat, eventually
  • If your furnace continues overheating and blowing cold air after you open all supply vents and change the filter, that’s your cue to rope in an expert to inspect the unit for other underlying problems

The Maintenance Checklist For Your Furnace Heating System

Furnace Heating System
Source: Peter Gudella

Before performing any maintenance on your heating system, ensure you turn off the unit to curb costly repairs and injuries. Once you do so, follow the steps below.

1. Clear the area surrounding the furnace

Although you shouldn’t use the area around your furnace as a storage space, your annual or bi-annual maintenance is a great time to ensure that there are no blankets, toys, or other items lurking.

2. Examine the blower motor

If you notice the accumulation of dust and dirt around it, use a vacuum to clean it. However, if you frequently change the filters, you likely won’t have as much accumulated dust around the unit.

3. Use a damp cloth

Wipe the extra dirt or dust that your vacuum was unable to pick up.

4. Examine the oil ports

These are located right above the blower motor or to the sides. Remove the caps and pour a few drops of motor lubricant in them.

5. Take off the flame shield

Inspect the burner, keeping your eyes peeled for signs of corrosion.

6. Inspect the blower motor belt

By inspecting the blower motor belt, you can ensure there are no signs of fraying or cracks. If your belt is seemingly damaged, it requires replacement.

7. Inspect your flue for any gaps or dents

You can do this by visually inspecting the flue for any apparent damage, such as gaps or dents. If there is damage, you want to get it fixed as soon as possible.

8. Check your air vents for any signs of mold growth

To do this, open the air vent and wipe the inside of the vent with a white cloth. Is the cloth damp? You also want to look for black and white spots inside the vent cover and the duct. Another way to check for mold growth is by smell. Take a whiff of the inside of the vent when there is air flow; you may be able to smell mold.


When the cold season rolls around, the last thing you need is chilly air blowing from your vents when you turn on the heater. Fortunately, resolving this issue is a breeze with these easy at-home troubleshooting techniques. Furthermore, when all else fails, an HVAC expert is available to sort out the problem efficiently.