How does heater control work

Usually 1 of 2 reasons. Some cars have a water control valve which makes hot water go through the heater core. The water valve is operated either by cable or Vacuum control.

Feel the 2 water lines from the heater core when the engine is hot. They should both be hot with the heat turned on. If one is cooler than the other, it is a water control valve problem.

If both lines are hot, then you have a problem inside the car and under the dash. The air flaps in the duct work are not changing the flow of air. You could have a slipped cable by the heater flap or a Vacuum pull-down is not working. Operate the heater control and see if the heater cable moves anything or not.

Feel your heater hoses. If both are hot, the core is circulating. If only one is hot-poor or no circulation of the core.
If the heater core is circulating, the temperature blend door may not be working to let blower air pass across the heater core, and into the vents. If vacuum operated, watch the vacuum valves as you move heater controls-you may spot the problem. Check that the vacuum line from the engine to the control switch is working and still connected. If not operated by vacuum, the controls may have cables to the heater case or may be electrically operated. You'll just have to scope it out. Good luck.

There are little doors under the dash that direct the air to either the vents (blows at your face), the defrost (blows at the windshield), or underneath (blow to your feet area) - these doors are controlled by what's called actuator/s and they are usually controlled by vacuum or cables - when the air won't change directions it can be either the vacuum switch, the vacuum line disconnected, cable disconnected or the actuator itself can be stuck or bad...

You also have a heater valve that allows the heated coolant to enter the passenger compartment into a heater core (small radiator to allow heat to transfer into the air when the blower blows air across the core) - this can also be vacuum controlled or cable controlled or electronically controlled - if it malfunctions then you may not get any heat into the passenger compartment - you will have to determine if the valve is not getting the signal to open or is bad and will not open - good luck :O)

There are little doors under the dash that direct the air to either the vents (blows at your face), the defrost (blows at the windshield), or underneath (blow to your feet area) - these doors are controlled by what's called actuator/s and they are usually controlled by vacuum or cables - when the air won't change directions it can be either the vacuum switch, the vacuum line disconnected, cable disconnected or the actuator itself can be stuck or bad...

You also have a heater valve that allows the heated coolant to enter the passenger compartment into a heater core (small radiator to allow heat to transfer into the air when the blower blows air across the core) - this can also be vacuum controlled or cable controlled or electronically controlled - if it malfunctions then you may not get any heat into the passenger compartment - you will have to determine if the valve is not getting the signal to open or is bad and will not open - good luck :O)

First thing you want to do is check the heater control valve on the engine side of the firewall attached to one or both of the heater hoses. Coolant must pass through it for the heater to work. If the valve is good, make sure you are getting flow through the heater core (you can run a garden hose through it) "blend door" is inside the heater box and is moved either by a small vacuum chamber (looks a lot like the vacuum advance mechanism on older chevy distributors). or it is moved by a cable. Whichever you have, it can be found on the heater box under the dashboard.
Easiest way to check them is to watch the heater box and with the engine running move the heater control Whichever type of actuator you have it should move the arm for the door when you do that. If you find one that does not work (for vacuum ones only) use a hand held vacuum pump and try to make it move. If it does, it's good if not, you either have a bad dash control or disconnected vacuum supply.
If cable type, find out why the cable isn't moving.

Depends on the model and year, but most controls are cable operated.
Some are vacuum operated.
If they all don't work, then I would suspect a cracked vacuum line, to the controls from the intake manifold, or after the control and before one of the vacuum servos, for a servo that is on. Turning everything off is a good way to tell if the leak is before or after the controls, is something starts working.
These are some of the common types of heater water valves, if you want to see the difference between cable and vacuum operated types.
http://www.dkk.com.cn/SPro7-1.html

It's possible the "heater control valve" is not working. Could be a cable or vacuume line. What ever controls it.
Also the temp controls, behind the dash, ARE controled by vacuume. Could be a line is kinked or come off.
Hopefully the heater core is not stopped up....

not sure if your controls are vacuum operated or cable, either way you need to remove the controls and inspect that hoses are not off or split, {do the **** test} and if cable is off that will be obvious what you need to do, Hope this helps, Good luck

The heater bypass valve is bad.It's located on the heater hose under the hood near the firewall.It looks like a little 3in.diameter black disc with a vacuum line attatched to it

make sure you have vacume too the control diapram