When to use: Use to size a two-way control valve on a hydronic coil. The required flow coefficient Cv = GPM × √(SG / ΔP) sets how much the valve must pass at its design pressure drop. For good controllability, the valve should take 25–50% of the variable-circuit pressure drop — this is the valve authority N = ΔPvalve / ΔPtotal. Aim for N ≥ 0.5; below 0.25 the installed characteristic distorts and the valve hunts. Pick the smallest standard Cv at or above the required value to avoid oversizing.