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Control Valve Cv & Authority Calculator
Size control valves for hydronic coils, determine valve authority, and evaluate equal-percentage flow characteristic performance.
Valve & System Inputs
Results
Required Cv
—
GPM / √psi
Valve Authority β
—
ΔPv / ΔPsys
Flow @ 50% Lift
—
GPM
Flow @ 25% Lift
—
GPM (min controllable)
Flow Characteristic Curve (Inherent)
Equal-percentage valves maintain a constant percentage change in flow per unit change in lift — ideal for HVAC coil control where system resistance varies with flow. Linear valves best suit fixed-ΔP applications like bypass lines.
Characteristic Selection Guide
Characteristic
Best Application
Installed Behavior
Authority Needed
Equal Percentage
Cooling/heating coils, most HVAC
Becomes more linear in system — improves controllability
β ≥ 0.25
Linear
Mixing valves, bypass valves, fixed ΔP
Becomes quick-opening if authority low
β ≥ 0.50
Quick Opening
On/off service, safety valves
Most flow near closed position
N/A (on/off)
Inputs, results, and method — opens in Excel or Sheets.
This control valve calculator sizes a modulating valve for a hydronic coil and evaluates how well it will control. Enter the design flow in GPM, the valve pressure drop, the system pressure drop, the fluid, and the valve characteristic; the tool returns the required Cv, the valve authority β, and the flow delivered at 50% and 25% stem lift for the chosen characteristic.
Cv (the valve flow coefficient) is the GPM of water a valve passes at 1 psi drop. Authority β = ΔPvalve ÷ ΔPsystem tells you whether the valve controls enough of the circuit resistance for stable modulation. The inherent characteristic curve shows how flow varies with lift for equal-percentage or linear trim.
Formula & Method
Required Cv
Cv = GPM ÷ √(ΔP ÷ SG)
Valve authority
β = ΔPvalve ÷ ΔPsystem
Equal %
Q(lift) = Qmax × R(lift − 1)
Linear
Q(lift) = Qmax × lift
Cv sizing follows the standard valve flow-coefficient relation used in manufacturer Cv data and ISA-75 valve-sizing practice: Cv = GPM ÷ √(ΔP) for water (SG = 1.0), with the SG term generalizing it to glycol (water 1.00 up to 50% ethylene glycol 1.085). The equal-percentage model uses the inherent relation Q = Qmax · R(lift−1), where R is rangeability (default 50); linear trim uses Q = Qmax · lift. Authority β thresholds (≥0.50 excellent, 0.25–0.50 acceptable, <0.25 poor) follow standard hydronic control practice.
Frequently Asked Questions
How do I calculate the required Cv for a control valve?
Required Cv equals the design flow in GPM divided by the square root of the valve pressure drop in psi, corrected for specific gravity: Cv = GPM divided by the square root of (Delta-P divided by SG). For water SG is 1.0; glycol raises it. Cv is the GPM a valve passes at a 1 psi drop, so this gives the wide-open flow coefficient the valve must provide at design conditions.
What valve authority should an HVAC control valve have?
Valve authority beta equals the valve pressure drop divided by the total system pressure drop. Target beta at or above 0.25 for equal-percentage coil valves and 0.5 or higher for critical or linear-valve loops. When authority is below 0.25 the installed flow characteristic is badly distorted and control becomes unstable, so you should allocate more pressure drop to the valve.
Why use an equal-percentage characteristic for coils?
A coil's heat output is non-linear with flow, and the piping resistance falls as flow drops. An equal-percentage valve gives a constant percentage flow change per unit of stem lift, and when combined with the circuit resistance it produces a nearly linear installed response. This tool models equal-percentage flow as design flow times rangeability raised to the power of (lift minus one).
What is rangeability and what value is typical?
Rangeability R is the ratio of maximum to minimum controllable flow, typically 50 to 1 for HVAC equal-percentage valves. A higher R lets the valve control smaller flows accurately at low lift. This tool defaults to R = 50 and uses it in the equal-percentage flow equation; the 25 percent lift flow is shown as a practical minimum controllable point.
This calculator handles one step. AIM Works runs the complete MEP design workflow — thermal load calculations, duct & pipe networks, equipment selection, code compliance, and an AI design assistant — in one tool.
Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.