There are a few potential reasons why your furnace might be blowing cold air. Some of the most common causes include:

  1. Thermostat issues: Make sure that your thermostat is set to “heat” and that the temperature is set high enough. If the batteries are low, it might not be able to communicate with the furnace properly.
  2. Pilot light: If the pilot light is out, the furnace won’t be able to produce heat. You can relight the pilot light following the instructions in your furnace’s manual, or call a professional to do it for you.
  3. Dirty air filter: A dirty air filter can restrict airflow to the furnace, causing it to blow cold air. Change the air filter and see if that fixes the problem.
  4. Blower motor issues: The blower motor is responsible for circulating air throughout your home. If the motor is malfunctioning, the furnace may not be able to circulate warm air.
  5. clogged burner assembly: A clogged burner assembly can result in the furnace not producing enough heat. The burners need to be cleaned regularly to avoid this problem.
  6. Failed limit switch: A failed limit switch is a safety device that prevents the furnace from overheating. If it’s not working properly, it may turn off the furnace before it has a chance to produce warm air.

It’s always best to have professional check and fix any issues, as this would ensure safety and prevent further damages.

Please also note that if you have an electric furnace, the issue might not be with the furnace itself but with the power supply. If the circuit breaker or fuse has tripped or blown, it will prevent the furnace from running.