9 Reasons Your Furnace Isn’t Blowing Hot Air

Find and fix the source of the problem to get your furnace working properly.

Air vent cover for room heating/cooling and return furnace air
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The furnace and HVAC system are complex pieces of equipment that work to keep the home warm in the winter and cool in the summer, but due to the complexity of this system, there are many problems that can occur. One issue that regularly comes up is the case of the furnace blowing cold air instead of hot air. Figuring out the exact cause of this problem and how to resolve the issue isn't easy, especially if you aren't familiar with how a furnace works. To help get hot air flowing back into your home, take a look at these common reasons your furnace isn't blowing hot air, and call a professional if the problem persists.

Thermostat Configuration

One of the first things you should check if your furnace is blowing cold air instead of hot air is the thermostat, because this little device controls the temperature and fan on the furnace. The fan setting on the thermostat may be set to "on," which would result in the fan continuously blowing air even when the temperature in the home is not low enough for the furnace to produce heat. Simply switch the fan to "auto" so that it only comes on when the furnace produces heat.

The setting on the thermostat may have also been changed by another person in the home, so before calling an HVAC tech, make sure the settings are correct. Another thing to watch out for is a low thermostat battery. While some thermostats are connected to the home electrical system, many models have a self-contained battery that needs to be changed when it starts to run low. These batteries tend to last a long time, so this isn't normally a problem in new thermostats.

However, a problem that does occur with new thermostats, especially following a DIY thermostat installation, is that the thermostat is not wired correctly or it's not compatible with the current HVAC system in the home. Unfortunately, this can be a little harder to determine on your own because, presumably, the mistake was yours in the first place. If you can't track down the problem, call in an HVAC professional to correct the existing installation or install a compatible device.

Clogged Filter

The furnace is equipped with a filter that helps to trap dust, dirt, and other particles before they can be blown throughout the home, but the furnace filter can become clogged over time. If the furnace is regularly maintained, then the furnace filter should be replaced once every three months, but if you have recently been neglecting the filter, it may be the cause of your cold air problems.

A clogged furnace filter can cause the furnace to overheat. When this happens, the furnace attempts to cool itself down by blowing cool air, leading to cool air being blown through the ducts and out into the home. Like the thermostat, the furnace filter is something that can be inspected and fixed by most DIYers. Just take out the old furnace filter and replace it with a new, compatible filter to immediately improve airflow and help prevent overheating. After installing the new filter, restart the system and check the air coming out of the vents. It will run cold at first because there is still cold air in the ducts, but within a few minutes, the air should become warm.

High Limit Switch Failure

Maintenance is key to a functioning HVAC system and ultimately your comfort level at home. A clogged filter causes the system to overheat, and if the furnace is allowed to overheat multiple times without resolving the problem this can result in the failure of the high limit switch. This, in turn, indicates to the HVAC system that it is overheating and the furnace will respond to this prompt by turning on the fan to cool the system down. Once the high limit switch breaks, you step out of the realm of easy-to-fix issues and need to call in an HVAC technician to replace the switch, though you can still change the clogged filter that caused the problem initially.

Condensate Line Clog

HVAC systems are made up of many different parts that are susceptible to wear and tear over the life of the furnace. The condensate line is one such part. It is used to remove water from the condensing unit, but this line can become clogged. A clog in the condensate line trips a switch in the furnace that prevents the burners from igniting, leading to cold air being blown into the home. HVAC companies are equipped with specialized tools for clearing these lines and once the line has been cleared, they can reset the system so that it is functioning correctly once more. Just keep in mind that regular maintenance can help prevent this and many other issues.

Deficient Fuel Source

A furnace needs some type of fuel to work. Whether that is oil, gas, or electricity depends on the type of furnace. For an oil furnace, make sure to check the tank, because if it is empty then the furnace won't have any oil to burn to produce heat. Similarly, a gas furnace won't be able to generate heat if the gas valve isn't open. Seized gas valves can become a problem over time, so make sure to regularly inspect them to ensure they are in good working order.

An electric furnace isn't likely to be blowing cold air if it isn't receiving power because the fan also runs on electricity. However, if you have an electric furnace and it isn't working then it's a good idea to make sure the furnace is actually turned on and if it is on, then check for a blown fuse or breaker.

Leaking Ducts

One area of the HVAC system that is often overlooked when you are trying to find the source of an issue is the ductwork in the home. The ducts are responsible for carrying the air from the furnace to the rest of the home, but the ductwork is vulnerable to leaks. Leaky ducts don't just lose hot air through these openings: The cold air around the ducts can also get in, leading to cold air blowing through the registers and into the home. An HVAC technician will need to inspect the ducts and fix any leaks that could have been caused by an unhooked duct, old leaks reopening, or ductwork that has fractured because it wasn't screwed together properly.

Pilot Light Problems

When the furnace isn't able to create heat, the issue could be with the pilot light or ignition system. Check to see if the pilot light is currently lit. If it's out, read the user's manual for instructions on how to properly relight the pilot light. Ideally, the pilot light will stay lit after you relight it, but if it goes out then the problem may be related to a thermocouple failure. This repair is difficult for most DIYers to try and tackle, so it's best to contact an HVAC company. However, before calling in a professional, you should also verify that the gas valve is opened and that the pilot light is not too dirty to light the gas. Sometimes just cleaning the pilot light can improve gas flow and get the system working again.

Blocked Burner

Burners can get blocked or clogged over time, preventing fuel from getting through. Without access to fuel, the burner cannot ignite and the furnace can't produce heat, but a blocked burner doesn't impact the fan, so the system will push cool air through the vents and out into the home. Resolve this problem by calling in an HVAC technician to clean the burner and prevent future issues with a regular maintenance schedule.

Dirty or Worn Flame Sensor

Another component of the furnace that can get coated with dirt and grime or become worn over time is the flame sensor. A blocked or damaged flame sensor will continue to turn the burner off, resulting in the furnace blowing cold air into the home. If the sensor is just dirty, then cleaning it will fix the problem, but if it's damaged then it will need to be replaced, which is a job that should be left to professional HVAC technicians.


HVAC systems have many important components that help to manage the controlled burning of fuel within the home. It isn't advised for DIYers to tackle furnace-related problems beyond basic maintenance including reigniting the pilot light, changing the filter, checking the valves, and adjusting the thermostat. All other maintenance and repair issues should be left to professional technicians.