Valve Flow Coefficient (Cv) is the flow capability of a control valve at fully open conditions relative to the
pressure drop across the valve. It is defined as the volume of water (GPM in the US) at 60°F that will flow
through a fully open valve with a pressure differential of 1 psi across the valve. It is useful to know how to
calculate Cv because it is the standardized valve sizing and selection method for control valves used
throughout the HVAC industry.
Simple equation used to calculate Cv is
Q = Flow in Gallons per Minute
G = Specific gravity of fluid (estimated as 1 for water systems)
ΔP = Differential pressure over valve (delta P) – stated in psi
With this knowledge, we can quickly identify that a valve with a Cv of 10 will flow 10 GPM of water at 1 psi
pressure drop across the valve without any math. A practical example, if we wanted to calculate Cv to
select a valve for 10 GPM flow with a required ΔP of 4, first find the square root of the ΔP √ 4=2, and use
that to divide the flow of 10 GPM 10/2 = 5, which results in a Cv requirement of 5. The master
specifications of a job plan will typically list the differential pressure to use when calculating Cv. In the
absence of this information, a ΔP range for selecting control valves for water coils of 3-5psi has evolved.
When selecting valves, choose the closest to the calculated Cv, typically rounding down unless a
specified maximum ΔP of a project is exceeded. An oversized valve may cause control issues such as
hunting or poor heat transfer (delta T) across a coil due to overflow. Conversely, an undersized valve may
not provide enough flow and exceed available ΔP.
Understanding and Calculating Cv
The Valve Flow Coefficient (Cv) represents the flow capacity of a control valve under fully
open conditions relative to the pressure drop across it. Cv is defined as the number of gallons
per minute (GPM) of water at 60°F that will flow through a fully open valve with a 1 psi
pressure differential.
Knowing how to calculate Cv is essential because it is the standardized method for valve sizing
and selection across the HVAC industry.
Cv Formula
Where:
Q = Flow rate in gallons per minute (GPM)
ΔP = Pressure differential across the valve (psi)
G = Specific gravity of the fluid (for water, G ≈ 1)
Example Calculation
If a valve has a Cv of 10, it will pass 10 GPM of water with a 1 psi pressure drop—no math
required.
For example, to determine the Cv for 10 GPM of flow with a required ΔP of 4 psi:
- Find the square root of ΔP: √4 = 2
- Divide the flow by this value: 10 ÷ 2 = 5
Therefore, the required Cv is 5.
Practical Guidelines
Project specifications often list the differential pressure to use when calculating Cv.
If this information is unavailable, a common rule of thumb for water coil control valves is
a ΔP between 3 and 5 psi.
When selecting a valve, choose the size with a Cv closest to the calculated value,
typically rounding down, unless doing so exceeds the project’s maximum ΔP.
An oversized valve can cause control issues such as hunting or poor heat transfer (low ΔT) due
to excess flow. An undersized valve, on the other hand, may restrict flow and create excessive
pressure drop.