Valve Flow Coefficient (Cv)

Cv is a valve's capacity expressed as a single number: the US gallons per minute of 60 °F water that flow through it, wide open, at exactly 1 psi of pressure drop. Because loss goes as velocity squared, flow scales as the square root of the drop, so a Cv 25 valve passes 25 × √4 = 50 gpm at 4 psi. Run the equation the other way to size a valve: the required Cv for 100 gpm of 1.1 SG glycol at a 1 bar (14.5 psi) design drop is 100/√(14.5/1.1) ≈ 27.5, so you pick the next standard size up.

The single biggest control-valve error is sizing for full flow at minimum drop. A control valve only controls if it owns a meaningful share of the circuit's pressure: typically 25–50% of the variable loss at design flow. Size it for the tiny drop the pump has left over and the valve becomes a hole in the pipe: it does its entire job in the first 10% of travel, hunts, and wears out its trim in a season. Note also that Cv assumes non-flashing liquid; at high drops choked flow caps the capacity no matter how much more ΔP you apply.