in some heating systems, there is a valve that will shut off the flow of coolant through the heater core. you just might look that up and see if that is not your issue.
If the radiator is filled to the top when the engine is stone cold and the coolant recovery bottle is at least 3/4 filled AND your cooling system is otherwise clean, you need a new *heater control valve.
Make sure there's enough coolant in the radiator. That's the stuff that carried heat to the heater. If it needs refilling, there is a leak and you need to fix that right away.
Your cooling system may need to be purged. If there's air in the system, the water will not properly flow through the heater core.
Insufficient information about what the car is and whether it has Climate Control, simple Air Conditioning or maybe just no air con
could be a faulty matrix
I put my heat on the highest and sometimes it goes to cold? Why?
