Reasons Why Your Furnace Isn’t Blowing Hot Air

The furnace and HVAC system are delicate pieces of equipment that operate to keep your home warm in the winter and cool in the summer, but because of their complexity, they may cause a variety of problems. The problem of the furnace spewing chilly air instead of hot air is a common thing.

It’s not simple to figure out what’s causing the problem and how to fix it, especially if you’re not familiar with how a furnace works. Take a look at these common reasons your furnace isn’t blowing hot air to assist you to get hot air flowing again, and if the problem persists, consult a professional.


The thermostat, which regulates the temperature and fan on the furnace, is one of the first things you should examine if your furnace is blowing cold air instead of hot air. The thermostat’s fan setting might be set to “on,” causing the fan to constantly blow air even if the temperature in the house is too high for the furnace to generate heat. 

Simply set the fan to “auto” mode, and it will only turn on when the furnace generates heat. Another person in the house may have modified the thermostat setting, so double-check the settings before calling an HVAC professional. 

A drained thermostat battery is another item to keep an eye on. While some thermostats are wired into the home’s electrical system, others feature a self-contained battery that must be replaced when it runs low. Because these batteries often last a long period, this isn’t usually an issue with new thermostats.

However, a common issue with new thermostats, particularly after a DIY thermostat installation, is that they are not properly connected or are incompatible with the home’s present HVAC system. 

Unfortunately, given you made the error in the first place, this can be a bit more difficult to figure out on your own. If you can’t find the source of the problem, hire an HVAC technician to repair or replace the current system.


The furnace has a filter that traps dust, dirt, and other particles before they are blown throughout the house, but the filter can get clogged over time. The furnace filter should be changed every three months if the furnace is properly maintained, but if you have lately overlooked the filter, it might be the source of your cold air problems.

Overheating can be caused by a clogged furnace filter. When this happens, the furnace tries to cool itself by producing cool air, which causes cool air to be blown through the ducts and into the house. The furnace filter, like the thermostat, is something that most DIYers can check and repair.

To enhance airflow and help avoid overheating, simply remove the old furnace filter and replace it with a new, compatible filter. Try restarting after installing the new filter and inspect the air flowing out of the vents. Because there is still cold air in the ducts, it will run chilly at first, but after a few minutes, the air should warm up.


Maintaining your HVAC system and, as a result, your level of comfort at home is critical. A blocked filter causes the system to overheat, and if the problem is not resolved, the high limit switch might fail. This signals the HVAC system to the information that it is overheating, and the furnace responds by turning on the fan to cool the system.

When the high limit switch malfunctions, you’re no longer dealing with a simple problem and will need to hire an HVAC professional to repair it, but you may still replace the clogged filter that created the problem in the first place.


HVAC systems are made up of a variety of parts that are subject to wear and tear throughout the course of the furnace’s lifespan. One such component is the condensate line. This pipe is used to drain water from the condensing unit, however, it might clog. A blockage in the condensate line triggers a switch in the furnace, preventing the burners from starting and forcing cold air into the house.

HVAC providers have specialized tools for cleaning these lines, and once they’ve cleaned the line, they may reset the system to get it working properly again. Just remember that regular maintenance can help you avoid this and a slew of other problems.


The ductwork in the home is one part of the HVAC system that is sometimes ignored when trying to pinpoint the source of an issue. The ducts are in charge of transporting air from the furnace to the rest of the house, but they are prone to leaks. Leaky ducts lose more than simply heated air through these holes:

Cold air can also enter through the ducts, causing cold air to blow through the registers and into the house. An HVAC professional will need to check the ducts and repair any leaks that may have resulted from an unhooked duct, existing leaks reopening, or ductwork that has fractured due to improper screwing.


Over time, burners can get blocked or clogged, preventing fuel from passing through. The burner cannot ignite and the furnace cannot create heat without fuel, but a blocked burner has no effect on the fan, so the system will continue to pump cool air through the vents and out into the home. Call in an HVAC specialist to clean the burner and maintain a regular maintenance program to avoid future problems.


The flame sensor is another component of the furnace that can get clogged with dirt and grime or worn out over time. The burner will continue to switch off due to a clogged or malfunctioning flame sensor, resulting in the furnace blowing chilly air into the home. If the sensor is just filthy, cleaning it will solve the problem; but, if it is broken, it will need to be replaced, which is a job best left to the professionals.

If you are experiencing these problems with your furnace, be sure to contact Calvey Heating and Air right away.

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