Air can’t pass through a dirty or clogged air filter.
Dirty air filters strain your gas furnace by forcing it to work overtime to push out the heated air leading to overheating. The overheating causes the safety sensors to automatically turn off and blow cold air to cool your furnace. To solve this problem, you need to change your air filters regularly. If you replace clogged air filters with new ones in your older gas furnace, but it still blows cold air, you need to check the HVAC system’s pilot light. Older units have a pilot light. This pilot light continuously burns on to light your furnace burners when required. If the furnace is blowing cold air, it could be that the pilot light is out. Try to relight your pilot light to resolve the issue. Modern furnaces have an electric ignitor instead of the pilot light. Turn on and off the igniter to resolve the issue. If it persists, your HVAC unit most likely needs a complete HVAC repair service from a qualified HVAC professional. You can get an HVAC serviceman to do a furnace repair. Your furnace’s blower could also be blowing cold air because of holes in your ductwork. Leaking ducts causes cold air from your basement and attic to get into the vents as warm air goes out through the duct holes. Repair the ducts with holes to resolve the issue. Check your gas furnace to ensure it has enough gas supply or if the gas supply is cut as a safety mechanism. If it’s functioning well, you should inspect your air vents. If they are blocked by drapes or furniture, they could cause your furnace to overheat just like it would with dirty or clogged air filters. This overheating prompts the limit switch to turn off and start blowing cold air. Make sure nothing is blocking your air vents. Finally, check your thermostat setting. Change the settings from ON to AUTO to stop the HVAC equipment from blowing cold air instead of warm air.