A heater control valve is a device that regulates the flow of hot water to a car's heater core. The valve is usually located on the engine side of the firewall, near the firewall itself. The purpose of the valve is to regulate how much hot water flows through the heater core. By controlling the amount of hot water that flows through the heater core, the temperature in the cabin can be controlled.
The heater control valve has two positions: on and off. When the valve is in the on position, hot water will flow from the engine to the heater core. When the valve is in the off position, no hot water will flow to the heater core. There are also valves that have a third position: recirculate. This position allows hot water to flow from the engine to the heater core, but it also allows cooledant to flow back into