Control valve selection and authority

A control valve is the device that turns a controller's signal into a change in heating or cooling. If the valve is sized and selected well, a coil responds smoothly and predictably across its full range. If the valve is oversized, poorly matched to the coil or installed in a system with large pressure swings, the coil hunts, overshoots and wastes energy. Many comfort and low delta T problems trace back to control valves.

This guide explains the key concepts, flow coefficient, valve authority and flow characteristic, and shows how to apply them when selecting valves for HVAC hydronic systems.

Flow coefficient: Cv and Kv

A valve's flow capacity is described by its flow coefficient:

• Cv (imperial) is the flow in US gallons per minute of water at about 15.6 C (60 F) that passes through a fully open valve with a pressure drop of 1 psi.
• Kv (metric) is the flow in cubic metres per hour with a pressure drop of 1 bar. Kv is approximately 0.865 times Cv.

For water, the relationship is:

Q = Kv x square root of (delta P), with Q in m3/h and delta P in bar. Equivalently, Q (gpm) = Cv x square root of delta P (psi).

To size a valve, the designer chooses a design pressure drop across the valve at design flow and calculates the required Kv or Cv, then selects the closest standard valve, usually not larger than calculated.

Valve authority

Valve authority describes how much of the total pressure drop in a controlled circuit occurs across the valve itself. It is commonly defined as:

Authority = delta P across fully open valve / (delta P across fully open valve + delta P across the rest of the controlled circuit)

The "rest of the circuit" includes the coil, piping, fittings and balancing valve between the points where the differential pressure is held roughly constant.

• High authority (close to 1) means the valve controls most of the pressure drop, so its inherent characteristic is preserved in operation.
• Low authority (well below 0.5) means the valve has little influence until it is nearly closed, distorting its characteristic. Small changes in valve position near closed produce large flow changes, and the coil becomes hard to control.

A common design target is an authority of at least about 0.25 to 0.5, with 0.5 often cited as ideal in classical guidance. In practice, the valve pressure drop is often set roughly equal to the coil pressure drop, subject to the available pump head.

Authority in variable flow systems

In systems with variable speed pumps and two-way valves, the differential pressure across each branch changes as other valves open and close. A valve near the pump may see much higher differential pressure at part load than at design. This effectively reduces authority and can cause valves near the pump to operate almost closed, with poor control. Pressure-independent control valves and differential pressure control at branches address this. See pressure-independent control valves.

Flow characteristics

A valve's inherent characteristic describes the relationship between stem position and flow at constant differential pressure.

Why equal percentage suits coils

A water coil's heat output rises steeply at low flow and flattens at high flow: the first portion of flow delivers a large share of capacity. An equal percentage valve compensates by giving small flow increments at low stroke and larger increments near full open. Combined, the valve and coil produce a roughly linear relationship between stroke and heat output, which makes control loops easier to tune. See PID control loops and tuning.

The benefit depends on adequate authority. At low authority, an equal percentage valve behaves more like a linear valve, and the combined response becomes non-linear again.

Rangeability

Rangeability is the ratio of maximum to minimum controllable flow. A valve with poor rangeability cannot control small flows, causing cycling at light loads. Characterized ball valves and globe valves designed for HVAC typically offer good rangeability when properly sized.

Two-way versus three-way valves

Two-way valves

Two-way valves throttle flow through the coil, reducing system flow at part load. They are the standard for variable flow systems because they save pump energy and help maintain high delta T. NECB and ASHRAE 90.1 generally require variable flow for larger hydronic systems, which in practice means two-way valves.

Three-way valves

Three-way valves keep flow roughly constant by diverting water around the coil. They are used where constant flow is needed, such as some chiller or boiler loops, end-of-line bypasses, or older constant-flow systems. In variable flow systems they waste pump energy and lower delta T by mixing supply water into return. Replacing three-way valves with two-way valves is a common retrofit, but requires adding variable speed pumping and checking minimum flow requirements at the plant.

Valve types

• Globe valves: traditional choice with good characterization and close-off, common in larger sizes and steam.
• Characterized ball valves: compact, economical, equal percentage characteristic via an insert disc; common for terminal units and small to medium coils.
• Butterfly valves: used in large pipe sizes, often for isolation or two-position duty; modulating butterflies have limited characterization.
• Pressure-independent control valves (PICVs): combine a control valve with an automatic differential pressure regulator, maintaining flow regardless of system pressure fluctuations.

Actuators and close-off

Actuators must provide enough force to close the valve against the maximum differential pressure it will see, called close-off pressure. Valves near pumps in variable flow systems can see high differential pressures at low load. Failure to close tightly causes leak-through, overcooling or overheating and low delta T.

Fail position matters in Canada. Heating valves on coils exposed to outdoor air usually fail open to protect against freezing. Cooling valves often fail closed. Spring-return or electronic fail-safe actuators provide the required fail position on power loss.

A practical selection procedure

1. Determine the design flow for the coil.
2. Establish the pressure drop across the coil and branch piping at design flow.
3. Choose a valve pressure drop that gives adequate authority, often roughly equal to the coil and branch pressure drop.
4. Calculate the required Kv or Cv and select the valve size, typically not larger than calculated.
5. Choose the characteristic: equal percentage for water coils.
6. Check rangeability and minimum controllable flow at light loads.
7. Confirm actuator close-off rating against the maximum system differential pressure.
8. Specify fail position based on freeze and safety considerations.
9. Consider PICVs where differential pressure varies widely or balancing is difficult.

Common valve problems

• Oversized valves: control only in the first few percent of stroke, causing hunting.
• Low authority: valve pressure drop too small relative to the circuit.
• Leak-through: worn seats or insufficient actuator close-off.
• Wrong characteristic: linear valve on a water coil, or equal percentage where linear is needed.
• Excess differential pressure: noise and wear from pump setpoints that are higher than needed.

Field checks during water balancing and controls verification often reveal these problems.

Frequently asked questions

Is it acceptable to install a valve the same size as the pipe?

Usually not. A line-size valve is often oversized for the flow and will have poor authority. Size valves for the required flow coefficient.

Do PICVs make valve authority irrelevant?

PICVs maintain near-full authority across a wide range of differential pressures, which simplifies sizing. They still need minimum differential pressure to operate and must be selected for the right flow range.

Key takeaways

• Size valves by flow coefficient (Cv or Kv) and design pressure drop, not by pipe size.
• Adequate valve authority preserves the valve's characteristic and enables stable control.
• Equal percentage valves suit water coils because they offset the coil's non-linear output.
• Two-way valves support variable flow and high delta T; three-way valves waste pump energy in variable systems.
• Check actuator close-off, fail position and rangeability as part of every selection.