You should expect your furnace to blow cool air only at the beginning of a heating cycle before the heat exchange process picks up. However, if your system has been running for at least half an hour or longer and it's still blowing cold air, you may have a problem. Note that this can be frustrating, especially on a cold evening when you need your heating unit the most. As such, you may want to learn why your system is blowing cold air and see if you can resolve the issue. However, it is best to call an HVAC expert to check your system and repair underlying malfunctions. Here are three reasons why you will experience cool air from the furnace. 

How Is the Thermostat Setting?

Start by checking the thermostat setting and ensure it's on AUTO. Note that if someone accidentally changed it to the ON position, the fan will constantly blow cold air even when the heating unit is on. If this is the case, your unit will end up causing you unnecessary discomfort. The good news is that in such a case, you can simply shift the setting on the thermostat and ensure that it only blows heated air into the home.

When Was the Last Time you Cleaned the Sensors?

The flame sensor is a safety device on your furnace, and it constantly checks whether the burners on the heater are on. If they're not, it will shut them off to prevent fires. Also, if you're running your system without checking the condition of the air filter, environmental contaminants will get to the flame sensor and make it dirty. When this happens, the sensor will become defective in determining if the burners are on or off. In addition, it might switch them off when the unit is in the middle of a heating cycle, resulting in a home with cold air blowing out of the registers. In this case, it is best to reach out to a heating professional to clean up your dirty air sensors because they know how to do it safely. 

Is the Pilot Light On?

Another possible reason the unit blows cold air in the home is when the pilot light goes out. This is because the thermocouple shuts off the gas supply to the burners when the light goes out. As such, no heat exchange happens, meaning the system will blow cool air. The good news is you can fix this by re-lighting the pilot light near the bottom of your furnace.

Those are the common reasons your system might blow cold air into the home. However, while you can troubleshoot some issues, such as a wrong thermostat setting, some complex issues are better left to the HVAC contractor. Moreover, they will troubleshoot all possible reasons your system might malfunction, repair them quickly, and restore your unit's efficiency and lifespan.

Speak to an HVAC contractor to learn more.