My heater is blowing cold air

Check coolant level. If low top it up and find the leak.
Assuming the fan is working, warm up the vehicle to operating temp , turn up the temp, choose defrost, and chech the hoses in amd out of the heater(firewall ) and see if they are hot to the touch. Oops! Wrong Section! Thanks Northern Heat!

Is this an automotive problem? If this is a central furnace problem, I have no idea what the problem could be based on the information you have provided.