C_v is the flow coefficient of any hydronic device (i.e., pipe, elbow, tee, valve) but is most commonly found on valves. The term C_v can be thought of in simple terms as the flow rate at which the device creates a 1 psi drop (ΔP) across the valve. In other words a control valve might have a C_v = 10, the valve will have a 1 psi pressure drop (ΔP) when 10 gpm is flowing through the valve.

PES recommends choosing valves that have a C_v about twice the desired flow. An example of this choice, a valve with a C_v=30 and a flow rate of 15 gpm means the pressure drop (ΔP) across the valve will be 0.25 psi.