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With the a/c on can you control the temperature that way? If you can't control temperature at all then you have an issue of some sort with the blend door. THe blend door is the door that controls temperature by directing air through the evaporator and heater core. If the blend door isn't working it could be the door itself is stuck in the heater box, it could be broken, the actuator that controls it may be bad, or it could be a wiring or control head issue.
If you are able to control temperature when it is on a/c but you have no heat then you may have a restricted heater core. Over time the cores can become restricted and prevent coolant flow through them so heat output lessens or goes away. If the heater core is restricted you can usually flush it out and restore heat at least partially, often you can bring it back to normal with just a flush.
If I turn the temperature all the way cold (to 60), it blows cold air. If I turn it all the way hot (to 90) it gets moderately warmer but not that much.
What you would need to do is remove the hoses from the heater core inlet, and it's best to then use a few feet of bulk hose that you can push into each tube. Then you'll want to run water from a garden hose through the hoses in each direction alternately until the water runs clear in both directions with no cloudiness.
Then you can reinstall the heater hoses at the firewall, bleed the cooling system and see if you then have better heat. To bleed the cooling system you'll need to leave the radiator cap off until the engine reaches operating temperature and no more air comes up.
It may be something that you'd rather have a shop handle in that case. You shouldn't pay more than an hour worth of labor time to have the heater core flushed.