There’s not much worse than being cold in your home, turning on the heat and then you get a blast of cold air for your trouble. If your furnace starts to blow cold air, there could be a number of different causes to investigate. There are four main possibilities, and in this article, we will examine each one in depth and offer a solution.

  1. The Thermostat Needs Adjusting

This will be easy to identify; you may notice that your thermostat position is set to “cool” or your fan is set to “on” and not “auto.” This seems like an obvious thing to mention, but HVAC professionals are often called out for these issues, and they are very simple to correct. Many people have their thermostat set to “cool” during the summer, and they simply forget to adjust their settings when the colder weather arrives. Of course, the simple solution here is to set the fan position to “heat,” and this should get your home warm again.

An even more common occurrence for a furnace blowing cold air is that the homeowner is using the wrong fan settings. The furnace uses an indoor fan to push the heated air throughout the air ducts and into the home. This fan has two settings “on” and “auto” when the fan is set to “on” it will run continuously regardless of whether the furnace is producing heat or not. So, if the heating cycle finishes and the furnace burner is cut off, the fan will carry on blowing, and if there is no heat, it will be blowing cold air. This cold air will be distributed through the registers, and the home will feel cold.

During the colder months, simply set your thermostat to “heat” and ensure that your fan is set to “auto.” If you still get cold air or the fan runs constantly you may have a more complex issue that’s are covered below.

  1. The Air Ducts are Leaking

This issue can be identified in five main ways:

  • Your utility bills are higher than normal.
  • Your cooling and heating systems seem to struggle to reach the desired temperature.
  • Some rooms don’t cool or heat properly or feel stuffy.
  • The home seems dustier than usual.
  • You notice holes, kinks or loose connections in the ductwork.

When you have leaks in your ductwork, the cold air that is located in your attic is sucked into the system. This cold air then mixes with the warm treated air from your furnace, and the temperature is lowered. This will cause the air from your vents to be colder than the desired temperature that has been set on the thermostat.

This a problem that needs to be fixed by a trained HVAC professional. The ductwork will be inspected; any leaks can be identified and repaired to restore heat to the home.

  1. The Furnace is Overheating

This issue can be identified in the following two ways:

The furnace begins to blow hot air, but then the air turns cold quickly, and the furnace shuts off.

This sequence of hot and then cold air repeats and your home will not warm up.

These are sure signs that the furnace is overheating, the heat exchanger is getting too hot, and the fan limit switch is shutting the furnace down for safety. As soon as the heat exchanger has cooled down, the furnace tries to heat the home again, and this process is repeated. Of course, the fan will still work, so cold air will be forced into the home because the furnace cannot produce any heat.

There are two simple fixes that can correct this issue and restore normal operation:

Examine the air filter, if it looks dirty, replace it and the airflow will be improved. This will allow the cooler air to run through the furnace to avoid overheating.

Check the supply vents and make sure that they are open, these vents are needed to push the treated air into the home. If you close too many vents pressure can build up in the ducts, and this will put too much strain on the furnace blower. If the blower slows down, it cannot push enough cool air over the heat exchanger, and the furnace will overheat.

If neither of these fixes work and you still have overheating and cold air issues, it’s time to call for some expert help. The system may need to be inspected for more complex problems that need to be fixed by an HVAC professional.

  1. The Fan Limit Switch is Faulty

We discussed the fan limit switch above, this device is usually located under the furnace hood, and it tells the furnace when to turn on.

If you have a problem with the fan limit switch you will notice that the air coming from your vents is very cold initially, then it will turn warmer, but then cold again before the fan cuts off entirely. This process will repeat, and you will notice that the fan always turns off between these failed heating cycles.

During normal operation, the fan limit switch will allow the furnace burners to fully warm up before the fan is turned on. Then the fan will be kept running until the burners cool slightly and heating will continue as normal. This is a safety measure that’s designed to ensure that your furnace isn’t damaged by overheating issues. If the fan limit switch is working properly, you should never feel colder air coming out of your vents.

Sadly, this isn’t a fix that you will be able to carry out yourself and an HVAC professional should be contacted. They will be able to test the unit and replace it if necessary. In many cases, the settings can be adjusted to correct the problem and ensure that cold air is not being blown into your home.

In Conclusion

As you can see, there are some steps that you can take to identify and correct this issue yourself. If the steps covered above don’t work, it’s time to call out an HVAC professional to fix the problem.