AVDO self-acting constant ow controls are used to maintain a minimum ow rate (e.g. through a low water content gas boiler) or to control di erential pressure in a heating system. The closing of radiator or zone valves due to reduced heating demand may result in low ow and increased system di erential pressure (potentially dead heating pumps). AVDO valves open with a falling system load and allow ow through a bypass line whenever the di erential pressure through the valve exceeds set point; ensuring a minimum ow rate or a system di erential pressure at minimum system load.