What is valve authority and why does it matter?

Valve authority (N) is defined as the ratio of pressure drop across the fully open valve to the pressure drop across the entire circuit at design flow: N = ΔP_valve / (ΔP_valve + ΔP_circuit). It directly affects controllability — an authority of 0.5 means the valve characteristic is minimally distorted; below 0.25, the installed characteristic becomes so distorted that equal-percentage valves behave nearly like quick-opening valves. Per ASHRAE Guideline 36, valve authorities between 0.25 and 0.50 are generally acceptable for most HVAC applications, though 0.5 is the recommended target. Belimo Energy Valves and PICCV products address low-authority scenarios by maintaining pressure-independent flow control.