A heater control valve (sometimes referred to as a hot water valve) is a component of both the heating and air conditioning (HVAC) system and the engine cooling system. The device diverts some of the hot coolant (antifreeze) from your engine, through a heater hose, and to the heater core inside the passenger compartment. The heater core absorbs heat from the engine coolant and a fan blowing through the heater core pushes warm air through the cabin vents.
The heater control valve moderates the amount of heat blowing into the cabin in response to your selections on the climate controls. When you choose the heat function, the valve opens to allow coolant into the heater core and you get warm air. When you turn off the heat, or turn on the AC, the valve remains closed.