if it is a indoor gas heater check the filter ref 2 owners man or if it is in a car maybe themostat
Get the air out of your heater core.
You should check the thermostat to see if it is operating corectly, or your system may need to be perged.
Check your coolant levels, and check your thermostat - it may need replacing. Also check your heater hoses to ensure they're not leaking.
Check antifreeze level, then thermostat next, heater core last
The most common reason a automobile heater blows cold air is the water level is low. Check the water level and fill to the appropriate level.
Check coolant level in radiator Check to see if it is "airbound" Remove heater hose to see if there is fluid running through it Bad thermostat
check the thermostat dude may need replacing
Check the coolant level. Usually low coolant levels will cause this.
You are in the beginning throws of blend door failure. Check www.heatertreater.net for more information on diagnosing and fixing the problem.
Check coolant level? Heater core plugged or restricted? Heater core airbound? Bad thermostat? Water pump not circulating coolant? Heater control cable not adjusted properly?
Check coolant level
Add coolant if it's low, change the thermostat if it's bad, and/or check for a restriction in the cooling system such as the heater core.