Valve Cv Sizing Hydronics
A control valve is rated by its flow coefficient Cv — the GPM of water it passes at 1 psi drop. Cv / flow / ΔP solves the sizing equation in any direction: the Cv you need for a flow at a drop, the flow a known valve passes, or the drop it imposes. Valve authority checks the other half of the job — whether the valve takes enough of the circuit's pressure drop to actually control flow.
Input
Output
Worked example
A coil needs 20 GPM of water (SG 1.0) and the schedule allows a 3.3 psi drop across the control valve at design flow.
- The sizing equation:
Cv = Q × √(SG ÷ ΔP). - Plug in:
Cv = 20 × √(1.0 ÷ 3.3) = 11.0. - Pick the cataloged valve whose rated Cv sits at or just above 11 — slightly under is fine; significantly over costs you authority (next tab).
Flip Solve for to read the other way — how much flow a known-Cv valve passes at a given drop, or what drop a valve imposes at a target flow. The metric coefficient rides along: Kv ≈ 0.865 × Cv (Kv is m³/h at 1 bar).
Input
Use any consistent pressure unit — authority is a ratio, so the units cancel. The "rest of the branch" is everything whose drop changes as the valve strokes: the coil, the balancing valve, the fittings on that controlled circuit.
Output
What authority means
A control valve only controls flow if it claims a healthy share of the circuit's pressure drop. Authority β = ΔPvalve, open ÷ (ΔPvalve, open + ΔPrest) compares the drop across the wide-open valve to the total drop across the part of the circuit that varies with the valve. Low authority means the valve does almost all of its throttling in the first sliver of stroke — the loop hunts and the rest of the travel does nothing.
| β | Control quality |
|---|---|
| ≥ 0.5 | Good — installed characteristic stays close to linear. |
| 0.25 – 0.5 | Marginal — distorted, front-loaded stroke. |
| < 0.25 | Poor — little real control, prone to hunting. |
An equal-percentage valve is the usual pick precisely because, fighting a circuit's rising resistance, its installed characteristic lands near linear — but only when authority is high enough to begin with. Oversizing on Cv is the quiet killer: a valve far bigger than the duty barely cracks open and authority collapses.