When to use: Use to size a two-way control valve on a hydronic coil. The required flow coefficient Cv = GPM × √(SG / ΔP) sets how much the valve must pass at its design pressure drop. For good controllability, the valve should take 25–50% of the variable-circuit pressure drop — this is the valve authority N = ΔPvalve / ΔPtotal. Aim for N ≥ 0.5; below 0.25 the installed characteristic distorts and the valve hunts. Pick the smallest standard Cv at or above the required value to avoid oversizing.
This tool calculates the required flow coefficient (Cv) for a two-way hydronic control valve and evaluates valve authority, helping engineers select the correct valve size for cooling coils, heating coils, and heat exchangers in HVAC systems. Proper Cv sizing and valve authority are critical for stable DDC control loop performance.
The flow coefficient Cv is defined as the flow in US GPM of water at 60°F that produces a 1 psi pressure drop across the valve at full open. For a liquid control valve, the required Cv is: Cv = Q × √(SG / ΔP), where Q is the design flow in GPM, SG is the specific gravity of the fluid (1.00 for water, 1.04–1.07 for glycol solutions), and ΔP is the design pressure drop across the valve in psi.
Valve authority N measures how much control the valve has over the circuit flow: N = ΔPvalve / (ΔPvalve + ΔPcircuit). A valve with authority below 0.25 has a severely distorted installed characteristic — the valve must travel nearly to full open to pass even 50% of design flow, and control loops hunting and overshoot result. Target N ≥ 0.5 for stable DDC control; most manufacturers recommend N = 0.25–0.5 as acceptable.
ANSI/ISA-75.01.01 (IEC 60534-2-1) governs control valve sizing equations including the liquid Cv equation used in this calculator. ASHRAE Handbook — HVAC Systems and Equipment Chapter 47 provides guidance on hydronic control valve selection, valve authority, and rangeability requirements. ASHRAE 90.1 indirectly governs valve selection through requirements for pressure-independent control valve (PICV) use in variable-flow chilled water systems, which eliminates valve authority concerns by decoupling valve position from system pressure changes.
Always select the smallest standard Cv that equals or exceeds the required Cv — oversizing a valve reduces authority and degrades controllability at partial loads. Valve rangeability (typically 50:1 to 100:1 for globe valves) determines the minimum controllable flow; a valve cannot modulate below its minimum controllable flow without losing control. For glycol systems, the higher specific gravity increases the required Cv slightly compared to water. Pressure-independent control valves (PICVs) integrate a differential pressure regulator and a flow-limiting cartridge, eliminating valve authority issues in variable-flow systems and are increasingly specified on variable-primary chilled water plants.
Enter the design flow in GPM, the design pressure drop across the valve (ΔP_valve), the remaining circuit pressure drop (coil + piping + fittings, ΔP_circuit), and the fluid specific gravity. Select a fluid type to auto-populate SG. The tool reports the required Cv, the next standard Cv from the R5 series, valve authority N, and an authority rating. If authority is below 0.25, increase the valve pressure drop by selecting a smaller Cv or specifying a higher-authority valve position in the system.
Valve authority N = ΔPvalve / ΔPtotal measures the fraction of system pressure drop that occurs across the valve. At low authority (N < 0.25), small valve movements cause large flow changes, making the loop difficult to tune and causing hunting. At high authority (N ≥ 0.5), the installed characteristic closely matches the inherent characteristic, giving the DDC controller smooth, proportional control.
The R5 (Renard series 5) provides standardized Cv values at approximately 60% increments: 0.4, 0.6, 1.0, 1.6, 2.5, 4.0, 6.3, 10, 16, 25, 40, 63, 100, 160. Selecting from this series ensures spare parts availability and compatibility with standard actuators. Always select the first standard Cv at or above the required Cv.
Glycol solutions have higher specific gravity (SG) and higher viscosity than water. The Cv equation accounts for SG: higher SG increases the required Cv by the square root of the SG ratio. A 50% glycol solution (SG = 1.07) requires about 3.4% more Cv than pure water at the same flow and pressure drop. Viscosity effects are generally negligible at the flow rates typical of HVAC coils.
Equal-percentage globe valves are standard for hydronic cooling and heating coil control because their inherent characteristic compensates for the nonlinear heat-transfer relationship between coil water flow and heat output. At low flow, the equal-percentage characteristic provides sensitive fine control; at high flow, it provides less sensitive gross adjustment, matching the coil performance curve.
Two-way valves are preferred in variable-flow chilled water systems because they allow pump speed to vary with load, saving pump energy via the affinity laws. Three-way valves maintain constant flow through the circuit and are used in constant-flow systems or where minimum flow through a chiller must be guaranteed. ASHRAE 90.1 effectively requires variable flow on new chilled water systems above a minimum size.
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