Commonly used on automatic temperature control (ATC) systems. This is a pulse width modulated (PWM) solenoid valve operated by the ATC control module. A second solenoid is used on dual zone systems.
Vacuum operated heater valves are used on systems with a temperature blend door. This valve remains fully open for all temperature settings except the coldest setting (max AC) when it closes.
Normally used on systems without a temperature blend door, cable operated heater valves can be manually positioned from fully open (max heat) to fully closed (no heat) or anywhere in between (regulated heat).