Pressure regulating heater control valve

In an automotive vehicle having a passenger compartment and a heating system for the passenger compartment that comprises a heat exchanger through which heat delivering liquid is pumped by a pumping means to the extent allowed by a heater control valve, the improvement wherein said heater control valve comprises first, second and third ports via which it is fluid connected with said pumping means and said heat exchanger, said valve comprising a body having a straight bore, said ports having communication with said bore at spaced apart locations along the length of said bore, a first valve seat circumscribing said bore between the location at which said first port has communication with said bore and the location at which said second port has communication with said bore, a second valve seat circumscribing said bore between the location at which said second port has communication with said bore and the location at which said third port has communication with said bore, said second valve seat having an annular shape, a valve element disposed within said bore for selective positioning within said bore between said valve seats for respective coaction with said valve seats to selectively 1) seat on an annular seating zone of said second seat and thereby allow fluid communication between said first and second ports while disallowing said third port to fluid communicate with said first and second ports through said second valve seat, 2) seat on neither of said seats and thereby allow fluid communication between all three ports, and 3) seat on said first seat and thereby allow fluid communication between said second and third ports while disallowing said first port to fluid communicate with said second and third ports, said bore comprising a shoulder that faces away from the location at which said second port communicates with said bore and toward the location at which said third port communicates with said bore, and means acting on said second valve seat for selectively positioning same within said bore comprising means resiliently biasing said second valve seat toward said shoulder and means exposing selected surface areas of said second valve seat that are to either side of said seating zone to fluid pressure differential acting on said surface areas such that when there is less than a certain pressure differential acting on said selected surface areas, said second valve seat is seated on said shoulder, and when there is greater than a certain selected pressure differential acting on said surface areas, said second valve seat is unseated from said shoulder.