CV sizing for water HVAC control valves
Control Valve Sizing and Valve Authority Explained
Matching the valve pressure drop to the coil pressure drop is a common HVAC rule of thumb, but the most important factor for modulating control valve is valve authority. Valve authority determines how well the valve can control flow and maintain stable operation.
What is Valve Authority?
Valve Authority (N) is defined as:
N = ΔPvalve / (ΔPvalve + ΔPrest)
Where:
• N = Valve Authority
• ΔPvalve = Pressure drop across the control valve at design flow
• ΔPrest = Pressure drop of the rest of the controlled circuit (coil, piping, balancing valves, fittings, strainers, etc.) at design flow
An equivalent form is:
N = ΔPvalve / ΔPtotal
Recommended Authority
• N = 0.50 = Excellent control
• N = 0.30 to 0.50 = Generally acceptable
• N = 0.25 = May result in unstable control and valve hunting
Most HVAC designers target a valve authority between 0.30 and 0.50.
Water Valve Cv Sizing Formula
For water systems:
Cv = Q / √ΔP
More generally:
Cv = (Q × √G) / √ΔP
Where:
• Cv = Valve flow coefficient
• Q = Flow rate in GPM
• G = Specific gravity of fluid (Water = 1.0)
• ΔP = Pressure drop across the valve in PSI
Example 1 – Rule of Thumb Sizing
Assume:
• Coil Flow = 44 GPM
• Desired Valve Pressure Drop = 4 PSI
Step 1
Cv = 44 / √4
Step 2
Cv = 44 / 2
Step 3
Cv = 22
Required Cv = 22
Example 2 – Using Actual Coil Pressure Drop
Assume:
• Flow = 44 GPM
• Coil Pressure Drop = 8.5 ft. H₂O
Step 1 – Convert Feet of Water to PSI
8.5 × 0.433 = 3.68 PSI
Step 2
Cv = 44 / √3.68
Step 3
Cv = 44 / 1.918
Step 4
Cv = 22.94
Required Cv ≈ 23
Checking Available Valve Sizes
Suppose the required Cv is 22, but the available valves are:
• 1¼" Valve = Cv 20
• 1½" Valve = Cv 28
Use the following formula to determine the actual pressure drop through each valve:
ΔP = (Q / Cv)²
1½" Valve (Cv = 28)
ΔP = (44 / 28)²
ΔP = (1.57)²
ΔP = 2.47 PSI
1¼" Valve (Cv = 20)
ΔP = (44 / 20)²
ΔP = (2.2)²
ΔP = 4.84 PSI
Which Valve Should You Select?
Using the traditional 3–5 PSI rule of thumb:
• 1¼" valve = 4.84 PSI ✓
• 1½" valve = 2.47 PSI
Based strictly on valve pressure drop, the 1¼" valve would typically be selected.
However, pressure drop alone should not determine the final selection. You should also evaluate valve authority.
Valve Authority Example
Assume:
• Coil Pressure Drop = 3.7 PSI
• Other Branch Losses = 1.3 PSI
• Total Branch Losses Excluding Valve = 5 PSI
1¼" Valve
N = 4.84 / (4.84 + 5)
N = 0.49
Result: Excellent authority
1½" Valve
N = 2.47 / (2.47 + 5)
N = 0.33
Result: Acceptable authority
Practical Considerations
If the valve Cv is too large:
• Lower pressure drop through the valve
• Lower valve authority
• Reduced controllability
• Greater chance of hunting
If the valve Cv is too small:
• Higher pressure drop through the valve
• Better valve authority
• Increased pump head requirements
• Potential inability to achieve design flow if sufficient differential pressure is not available
The best valve selection balances:
1. Required design flow
2. Available pump head
3. Valve authority (target 0.30–0.50)
4. Stable control performance
A slightly undersized valve is often preferred over an oversized valve, provided the pump can still deliver the required design flow.
Remember: Matching the valve pressure drop to the coil pressure drop is a useful starting point, but valve authority is what ultimately determines how well a modulating control valve will perform in a real HVAC system.