What is the heater control valve for?
The heater control valve (sometimes called the hot water valve) is used to control the rate at which coolant flows through the heater core. The valve is located on the heater inlet hose. Manually operated systems may use a cable, a vacuum motor or an electric solenoid to operate the valve. via
Why is my car heater taking so long to warm up?
A few things that cause automotive heaters to work less well are low coolant levels in the radiator, a faulty heater valve, or the thermostat is stuck open. Have a certified mechanic replace your thermostat if necessary or diagnose and repair other cooling system issues, such as leaking coolant. via
How can I make my car heater stronger? (video)