chamber 42 in the case of a heat exchanger economizer line 28 would be connected to the outlet of the heat exchanger which is connected to the compressor economizer port and is at economizer pressure at start up spring 39 will tend to bias differential piston valve member 34 onto its seat 30 thereby blocking flow between oil separator 14 and condenser 18 because chamber 42 is connected to the economizer 20 pressure will build up as the system 10 comes to operating equilibrium with the discharge of the compressor 12 being supplied to oil separator 14 the pressure will rapidly build up at port 14 1 and act on differential piston valve member 34 against the bias of spring 39 causing it to open the bias of spring 39 will insure a sufficient pressure in the oil separator 14 before valve member 34 of valve 16 opens chamber 42 is at economizer pressure so that the differential pressure p d p e depends upon economizer pressure as well as discharge pressure since economizer pressure is more variable over the operating envelope than suction and discharge pressure the differential pressure opposing the bias of spring 39 is able to fully open valve 16 over a larger portion of the operating envelope referring specifically to fig3 the present invention using the economizer pressure rather than the suction pressure as a component of the differential pressure produces modulation over the portion of the operating envelope defined by l m n l this results in the portion of the operating envelope defined by k l n o r k free of throttling and the attendant losses that would be present if suction pressure was used.