Sizes a two-port HVAC control valve, required flow coefficient Kv, the recommended catalogue Kvs and the resulting valve authority N.
Tender-stage only. Verify final selection against manufacturer Kvs data and IEC 60534-2-1.
For design guidance only. Always verify with a qualified engineer.
About this control valve sizing calculator
This free control valve sizing calculator works out the flow coefficient a liquid two-port control valve needs to pass its design flow at the chosen pressure drop. From the design flow, the valve’s differential pressure and the fluid’s specific gravity it returns the required Kv, recommends the smallest standard catalogue Kvs at or above it, and checks the resulting valve authority. It is aimed at UK building-services engineers selecting modulating valves for heating and chilled-water circuits at tender or commissioning stage.
How the valve Kv and authority are calculated
For incompressible flow the required flow coefficient is Kv = Q ÷ √(ΔP ÷ G), where Q is the volumetric flow in m³/h, ΔP is the valve pressure drop in bar and G is the liquid’s specific gravity relative to water, the incompressible-flow method of IEC 60534-2-1 and the Spirax Sarco water-systems guidance. The recommended Kvs is the smallest ISO 3 (Renard R5) preferred-number step at or above the required Kv. Valve authority is N = ΔPvalve ÷ (ΔPvalve + ΔPrest-of-circuit); Spirax Sarco advises N should be near to 0.5 but not greater, and certainly not below 0.2, so the calculator bands the result as poor below 0.2, acceptable from 0.2 to 0.5, and high above 0.5.
Frequently asked questions
What is the difference between Kv and Kvs?
Kv is the flow coefficient your duty actually requires, the m³/h of water that gives a 1 bar drop. Kvs is the rated, fully-open coefficient printed in the valve catalogue. You pick the nearest Kvs at or above the calculated Kv so the valve is never undersized.
Why does valve authority matter?
Authority sets how much of the circuit’s pressure drop sits across the valve. Too little (below 0.2) and the installed characteristic distorts towards on/off control; too much wastes pumping head. A value of 0.2 to 0.5 keeps the installed characteristic close to the inherent equal-percentage curve.
Does it work for glycol and chilled water?
Yes. Enter the fluid’s specific gravity (water is 1.0, glycol mixes are slightly above 1). The Kv equation corrects for density through the √(ΔP ÷ G) term, so a denser fluid needs a marginally larger valve for the same flow and drop.