A furnace blowing cold air is due to a failure of the heating element, according to Hannabery HVAC. In a gas furnace, this may be due to a problem with the pilot light or gas supply, while an electrical furnace could blow cold air if the heating element fails or trips its breaker.
Before a homeowner begins working on a furnace, he should check to make sure the thermostat is appropriately set to heat and that it is set above current room temperature, according to RealEstate.com's "A Guide to Troubleshooting Your Gas Furnace." Whether the furnace is electric or gas, the blower fan may still function if the heating element fails, producing cold air. In the case of a gas furnace, one should check to see whether the pilot light is still lit. If it has blown out, the homeowner should relight it using the instructions provided by the manufacturer. If the pilot light won't stay alight, one should contact the gas company or a repairman to troubleshoot the problem.
If the furnace is electric, one should check to see if the heating element's circuit breaker may have tripped. If resetting the breaker fails to produce heat, it may be due to a damaged heating element. If the breaker trips again, Hannabery advises not resetting it a second time, as it may be protecting the home from a dangerous electrical fault in the furnace.