How does a heater control valve work in a car?

A heater control valve is simply a water valve which allows more or less water to circulate through the heater core, depending on how much the valve is opened.

When the engine warms up it heats coolant (mostly water) in the water jacket of the engine. To keep the engine from overheating, the water pump circulates water through the radiator once the engine temperature reaches the set-point of the thermostat.

When it's cold outside the engine STILL reaches a temperature that allows you to make use of the engine heat when you open the water valve of the heater.

Heater control valves take many forms but basically they all do the same thing; they open to allow heated engine coolant to circulate through the heater core or they close to stop the coolant from flowing.

Heater control valves can be manual (cable controlled or even lever controlled), vacuum (which allows the engine vacuum to control the position of the heater control valve based on where you set the control knob) or electrical which allows for greater flexibility and even allows the auto manufacturer to connect the heater controls to a computer or other electronic control device.