| 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) |
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.
| 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.