This can happen if the water or coolant level in the radiator is low. There are other causes but this is the most common.
maybe your a/c has problems try to check the a/c maybe its time for maintenance or repair
Heater core changed three years ago,I changed Thermostat Heater still blows cool air.
heater core
low coolant level, malfunctioning thermostat, clogged heater core, malfunctioning heater control valve, malfunctioning climate control programmer,
When it is first turned on it blows cold air. Give it 2 minutes it wll heat up to regular temperature.
This is typical of an engine being low on coolant in the radiator, check your radiators coolant level and fill it.
it blows more air out
it might be the thermostate, or if your prestone goes down your lose pressure in your coulant system but change your t-stat first.
This would indicate that the system is low on coolant when the gauge is hot. Not enough coolant to service the heater core.
First thing to check is open hood and feel heater hoses going into heater core located on r/s of vehilcle if sitting in driver seat.If they are both very hot and feel the same temp,blend door in heater box is not operating correctly.If one heater hose is hot and the other is cooler than chances are the heater core is plugged.In either case a professional would be your best bet for repair.Hope info was helpful.
If it blows warm while idling and cool as you accelerate down the road, the coolant is low and or doesn't have proper mixture for your avg. temperature.
You might have bad actuator that opens and closes airways.