If the fan is blowing air out of the vents, check the temperature guage on the instrument panel. If it does not indicate at least 1/2 way up the scale then I would suspect that the thermostat is in need of replacement. Another (but less likely possibility) is that the control "doors" in the heating ducts are not opening or shutting properly and are allowing cold air into the interior instead of heated air.
VENT TO HEATER CORE IS NOT CLOSING................
You are likely in an antarctic region.
You probably have a bad thermostat.
You may be low on coolant, or the thermostat is stuck open, or the heater core is plugged up, or the temp blend door is broken.
An automobile's heater that blows only cold air could either be a faulty thermostat or, more likely, a faulty heater core. A trip to a mechanic or shop that specializes in radiator and cooling system repair should be able to diagnose and repair any problems.
could be thermostat or heater fan if you can hear all the speeds on the blower motor working then it could be your thermostat but your heater core might be plugged as well
I assume your car heater is blowing cold air, when the heater is on. It seems that, you have air on your coolant. that air pockets blocks the cooland to travel through your heater. it may be a sign of a bigger problem such as blown head gasket in the old cars, such acura legend
Low coolant? Wrong thermostat temperature range? Heater core plugged,restricted or airbound? Water pump not circulating coolant? Temperature control cable not adjusted properly?
Mercury Mountaineers have a "blend" door in the heater box. It is probably malfunctioning. You can find more info by searching for Ford + "blend door" or "no Heat"
See "Related Questions" below for a complete discussion on possible causes and more importantly: fixes
thermocouple is bad.
it could be your thermo stat