First check the water level. If the water level is low or very low, you may have air in the radiator. If you have a small leak, you will have enough water to cool the engine but not enough to heat the heater core. Adding water to the radiator may not be enough. Air in the heater core equals no hot air in the cabin. This problem is so bad in some engine configurations that some cars go as far as to actually have a 'bleeder' at the front of the engine, by the upper radiator hose/thermostat housing (Dodge, BMW, etc.). The heat selector lever/ switch in the cabin may be defective (electrical) and may not open the hot water valve enough, or even be disconnected (mechanical). Some (Ford and foreign cars) use vacuum to operate the valve and those lines may be cracked, worn, pinched or just leaking. You could also have a clogged heater core. These are 3 possible items you could start with, in that order. The thermostat could be At Fault, but this is a give away. If the thermostat is stuck open or missing and you drive the car for a distance, the engine will take a while to warm up then overheat, and the cabin heat will follow the engine temperature. If the engine overheats, you need to take care of that motor more than worry about the heat in the cabin. As you said, you're not asking what to do with your overheating car. You would have taken care of that motor by now and taken care of your heater problem with it.
if the heater does not blow out warm air change the setting.
heater on at idle and will not blow warm air. when moving it blows the warm air from the vents. what will cause this
Isn't a heater supposed to blow warm air?
It could be the thermostat on the engine. If the engine does not warm up adequately the heater will never blow hot air.
engine not warm, low coolant, or stuck thermostat
Low on coolant. Stuck open thermostat. Poor circulation (plugged heater core).
Faulty thermostat. Plugged heater core. Temp blend door malfunction.
Your thermostat is most likely bad. It controls the flow of warm water into your heater core which warms your air. Its a common problem in winter and an easy fix.
Heater core may be plugged . Flush the cooling system, also check thermostat.
Poor coolant circulation from a plugged up heater core. Try flushing the heater core and replacing the thermostat.
it is a heater. heat. it blows heat. warm air