The differential pressure regulating valve is a control valve that opens and closes automatically to maintain a near constant differential pressure across the inlet and outlet ports. Hydronic systems are typically designed to operate at a certain differential pressure across the components. The differential pressure can increase or decrease as pump flow goes down or up respectively. A common reason for the pressure variation is due to the characteristics of the pump.
Zone valves begin to close and the total flow through circulating pump decreases.
With a typical pump, the head pressure drops with flow. Off design condition, which is most of the time, the pump will be delivering more pressure than required. The requirement of the valve is to bypass enough flow so that the differential pressure does not increase to objectionable levels. So in a residence there could be one zone operating and the pump, typically single speed, will be operating at a differential pressure higher than when all zones are open. In this case a differential valve that can handle all the flow except for one zone is required.