Have HVAC Questions? Ask an HVAC Expert for Answers
Welcome to Just Answer, my name is ***** ***** I will do my best to help you with your issue. If my initial response doesn’t answer your question then let me know and we can continue our conversation.
Furnaces usually blow cold air in heating mode when a limit sensor has tripped. This can be caused by (among other things) a dirty filter.
Like I said there are a number of other possibilities. I can't give you specifics if you can't provide the brand and model number of your furnace. One other thing that most furnaces have in common that could also be causing this is a tripped rollout switch (see attached picture for typical). They are located just outside the burner chamber. Turn the power off to the unit then try pushing the small button between the 2 leads on the switch. If it clicks you have reset it and the unit will fire up when you turn the power back on with any luck. If it doesn't click it wasn't tripped. There may be more than 1 so once you find the first one follow the wire to check for others in that circuit.