Size a hydronic control valve: Cv, valve authority, and equal-percentage characteristic.
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Valve & System Inputs
Results
Required Cv
—
GPM / √psi
Valve Authority β
—
ΔPv ÷ (ΔPv + ΔPcoil+branch)
Flow @ 50% Lift
—
GPM, installed
Flow @ 25% Lift
—
GPM, installed
Min Controllable Flow
—
GPM, installed, at Cv ÷ R
Flow vs Lift — Installed (Inherent in Brackets)
Bars show the installed flow, with the valve’s authority applied; the value in brackets is the inherent (constant-ΔP) characteristic. Equal-percentage trim gives a constant percentage change in flow per unit of lift, and with typical coil authority its installed curve straightens toward linear — which is why it suits coils. Low authority pushes any trim toward quick-opening.
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 — as an Excel workbook or plain CSV.
Doing this for a whole building? See this step inside a connected AIM Works project — loads, ventilation, equipment, and Design Review in one place.
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 coil and branch pressure drop the valve controls, the fluid, the valve characteristic, and the rangeability; the tool returns the required Cv, the valve authority β, the installed flow at 50% and 25% stem lift, and the minimum controllable flow.
Cv (the valve flow coefficient) is the GPM of 60°F water a valve passes at 1 psi drop. Authority β = ΔPvalve ÷ (ΔPvalve + ΔPcoil+branch) tells you how much the circuit distorts the valve’s characteristic. The curve shows the installed flow at each lift alongside the inherent characteristic.
Formula & Method
Required Cv
Cv = GPM ÷ √(ΔP ÷ SG)
Valve authority
β = ΔPvalve ÷ (ΔPvalve + ΔPcoil+branch)
Inherent
φ(lift) = R(lift − 1) (equal %) or lift (linear)
Installed
Q = Qdesign × φ ÷ √(β + (1 − β)φ²)
Min controllable
installed Q at φ = 1 ÷ R
Cv = GPM ÷ √(ΔP ÷ SG) is the basic incompressible-liquid valve equation — the starting point of ANSI/ISA-75.01.01 sizing. This tool does not check choked flow or cavitation (important for large valve pressure drops or hot water), so confirm the manufacturer’s allowable ΔP. SG comes from the fluid dropdown: Dow typical densities for inhibited glycol at about 45°F relative to 60°F water. The installed curve assumes a constant pressure difference across the valve and its circuit and a valve whose full-open Cv equals the required Cv; real valves are picked a little larger, which lowers authority. Authority bands (below 0.25 low, 0.25–0.5 acceptable, 0.5–0.75 good, above 0.75 wasted pump head) are common design practice and match the AIM Works app’s control-valve check.
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 valve pressure drop plus the pressure drop of the coil and branch piping it controls. Target beta at or above 0.25 for equal-percentage coil valves and around 0.5 for critical or linear-valve loops. Below 0.25 the installed flow characteristic is badly distorted toward quick-opening and control becomes unstable, so allocate more pressure drop to the valve; above about 0.75 the valve burns pump head for little extra control.
Why use an equal-percentage characteristic for coils?
A coil's heat output rises steeply at low flow and flattens near design, and the circuit resistance distorts the valve's own characteristic. An equal-percentage valve gives a constant percentage flow change per unit of stem lift, which offsets the coil's curve and, with typical authority, gives a more nearly linear heat output with stem position. This tool computes the inherent equal-percentage flow as rangeability raised to the power of (lift minus one) and then applies the valve authority to show the installed flow.
What is rangeability and what value is typical?
Rangeability R is the ratio of the valve's maximum to minimum controllable Cv, commonly 50 to 1 or more for HVAC characterized valves. A higher R lets the valve control smaller flows accurately at low lift. This tool defaults to R = 50, uses it in the equal-percentage curve, and reports the minimum controllable flow as the installed flow when the valve is throttled to its full-open Cv divided by R.
Connect the calculation to the rest of the project.
AIM Works brings peak loads, ASHRAE 62.1 ventilation, equipment and plant sizing, compliance checks, Design Review, and engineering deliverables into one connected mechanical design workspace.
Peak loads (RTS or CLTD/CLF) and ASHRAE 62.1-2019 ventilation
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Design Review that cites the check behind every finding
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Duct and pipe distribution sizing is planned for V2 — at launch, keep using this calculator for that step.
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.