Motorized Valve Selection Guide

Introduction

Motorized valve selection is essential for HVAC systems that regulate flow and temperature in heating and cooling applications. Our engineering team has developed this guide based on EN 215 and ASHRAE standards validated through real-world HVAC projects.

These valves combine a valve body with an electric actuator to provide automated control based on thermostat or controller signals. Proper valve selection ensures accurate control, energy efficiency, system stability, and reliable operation.

Understanding motorized valve sizing enables engineers to properly size valves using Kv/Cv coefficients, select appropriate actuators, and ensure proper valve authority for control stability. Our calculations follow industry-standard methods for optimal system performance.

This guide covers the fundamental sizing formulas, Kv/Cv coefficient calculations, valve authority determination, actuator selection criteria, and installation best practices.

Quick Answer: How to Select a Motorized Control Valve?

Motorized control valves regulate flow and temperature in heating and cooling systems through modulating or on/off control. Proper valve selection ensures accurate control, energy efficiency, and system stability.

Core Selection Formula

Flow Coefficient (Kv):

Where:

• $K_v$ = Flow coefficient (m³/hr at 1 bar ΔP)
• $Q$ = Flow rate (m³/hr)
• $\Delta P$ = Pressure drop across valve (bar)

Convert to Cv (US units):

$C_v = K_v \times 1.156$

Additional Formulas

Valve Diameter Selection:

$v = \frac{Q}{A \times 3600} = \frac{Q \times 4}{\pi D^2 \times 3600}$

Where:

• $v$ = Movement velocity (m/s)
• $A$ = Valve cross-sectional area (m²)
• $D$ = Valve diameter (m)
• Target velocity: 0.5-2.0 m/s

Valve Authority:

$\beta = \frac{\Delta P_v}{\Delta P_v + \Delta P_s}$

Where:

• $\beta$ = Valve authority (0.3-0.5 ideal)
• $\Delta P_v$ = Pressure drop across valve (bar)
• $\Delta P_s$ = Force drop across system (bar)

Worked Example

Reference Table

Key Standards

What Are the Main Types of Control Valves?

2-Way vs 3-Way Valves

2-Way Valves:

Function: Control current rate by throttling (partially closing)

Characteristics:

• One inlet, one outlet
• Variable movement, variable force drop
• Reduces total mechanism circulation when closed
• Requires variable speed pump or bypass for installation stability

Applications:

• Zone control in heat system systems
• Fan coil unit control
• Chiller and boiler capacity control
• Terminal unit control

Control Action:

• Normally closed (NC): Opens on power/signal
• Normally open (NO): Closes on power/signal

3-Way Valves:

Function: Divert or mix flow rate between two paths

Types:

1. Mixing Valve: Two inlets (hot/cold), one outlet (mixed)
2. Diverting Valve: One inlet, two outlets (diverted discharge)

Characteristics:

• Constant total stream through valve
• Maintains equipment amperage rate
• No pump interaction issues

Applications:

• Mixing valves: Temperature control (mix hot supply with return)
• Diverting valves: Bypass control, boiler protection
• Constant movement systems

Comparison Table:

Modulating vs On/Off Control

Modulating (Proportional) Control:

Function: Valve position varies proportionally to control signal

Characteristics:

• Smooth, gradual control
• Precise heat regulation
• 0-10V DC, 2-10V DC, or 4-20mA signal
• Actuator positions valve anywhere from 0-100%

Advantages:

• Better comfort (no thermal value swings)
• Reduced wear and tear
• Energy efficient
• Quiet operation

Applications:

• Large thermal system/air conditioning loads
• Critical degree control
• VAV systems
• Chilled water systems

On/Off Control:

Function: Valve is either fully open or fully closed

Characteristics:

• Simple two-position control
• 24V AC, 110V AC, or 230V AC powered
• Fast actuation (5-30 seconds)
• Low cost

Advantages:

• Simple wiring and control
• Lower equipment cost
• Suitable for small loads
• Reliable operation

Disadvantages:

• Heat level fluctuations
• Increased cycling and wear
• Less precise control

Applications:

• Small residential systems
• Simple zone control
• Domestic hot water priority
• Low-cost installations

How Do You Size a Motorized Valve?

Flow Coefficient (Kv/Cv)

The discharge coefficient relates stream rate to stress drop:

Metric (Kv):

$K_v = \frac{Q}{\sqrt{\Delta P}}$

Kv is the electrical stream rate (m³/hr) of water at 15°C that produces 1 bar load drop.

Imperial (Cv):

$C_v = \frac{Q_{\text{gpm}}}{\sqrt{\Delta P_{\text{psi}}}}$

Cv is the movement rate (US gpm) of water at 60°F that produces 1 psi pressure value drop.

Conversion:

$C_v = K_v \times 1.156$

Kv Selection Guidelines:

1. Determine required Kv: Use setup circulation rate and allowable $\Delta P$
2. Select valve with Kv ≥ calculated: From manufacturer data
3. Avoid excessive oversizing: Maximum 1 pipe size larger
4. Verify actual $\Delta P$: Recalculate with selected valve Kv

Example:

• Flow rate: 10 m³/hr
• $\Delta P$ available: 0.4 bar
• $K_v = 10 / \sqrt{0.4} = 15.8$
• Select valve with Kv = 16 or next larger

Valve Diameter Selection

Valve size affects:

• Discharge velocity (noise, erosion)
• Arrangement head loss (energy, control)
• Initial cost

Velocity Calculation:

$v = \frac{4Q}{\pi D^2 \times 3600}$

Recommended Velocities:

• Minimum: 0.5 m/s (prevents stratification)
• Typical: 1.0-2.0 m/s (good control)
• Maximum: 2.5 m/s (noise threshold)

Valve Size vs Pipe Size:

Undersizing: Valve smaller than tube acceptable if Kv adequate and velocity acceptable.

Valve Authority

Valve authority ($\beta$) measures the valve's control effectiveness:

$\beta = \frac{\Delta P_{\text{valve, open}}}{\Delta P_{\text{valve, open}} + \Delta P_{\text{mechanism}}}$

Interpretation:

Design Guideline: Target $\beta$ = 0.3 to 0.5

Low Authority Problem:

• Small valve movement causes large stream change
• Difficult to control accurately
• Hunting and oscillation

Solution: Increase valve $\Delta P$ (smaller Kv or partially close balancing valve).

Example:

• Installation $\Delta P$: 1.0 bar
• Valve $\Delta P$: 0.3 bar
• Authority: 0.3 / (0.3 + 1.0) = 0.23 (marginal, could be improved)

How Do You Select the Right Actuator?

Actuator Types

Electric Modulating Actuators:

Characteristics:

• Analog control signal (0-10V, 2-10V, 4-20mA)
• Variable valve position
• 90-180 second actuation time typical
• Built-in spring return (optional)

Applications:

• Precise temp control
• Large commercial systems
• VAV and hydronic balancing

Electric On/Off Actuators:

Characteristics:

• 2-position control
• Fast actuation (5-30 seconds)
• Low wattage consumption
• Simple wiring

Applications:

• Residential zoning
• Simple control strategies
• DHW priority switching

Floating Point Actuators:

Characteristics:

• 3-wire control (open/common/close)
• Controller pulses actuator open or closed
• Simple, no position feedback
• Used with simple thermostats

Applications:

• Residential and light commercial
• Radiator zone valves
• Small fan coil units

Torque Requirements

Actuator must provide sufficient torque to operate valve against force:

Torque Computation (Simplified):

$T = \Delta P \times A \times r$

Where:

• $T$ = Required torque (Nm)
• $\Delta P$ = Stress drop (Pa)
• $A$ = Valve disc area (m²)
• $r$ = Effective radius (m)

Typical Torque Requirements:

Selection Rule: Choose actuator with torque $\ge 1.5 \times$ required torque (safety factor).

Control Signals

0-10V DC (Most Common):

• 0V = Valve closed (or open for NO valve)
• 10V = Valve fully open (or closed for NO valve)
• Linear relationship: 5V = 50% open
• Used with DDC controllers and modern thermostats

2-10V DC:

• Similar to 0-10V but 2V minimum (easier fault detection)
• 2V = fully closed, 10V = fully open

4-20mA:

• Amp loop signal
• 4mA = fully closed, 20mA = fully open
• Immune to voltage drop over long wire runs
• Common in industrial applications

Floating Point (3-Wire):

• Open, Common, Close terminals
• Controller pulses actuator open or closed
• No position feedback
• Simple wiring

On/Off:

• Powered = actuated (NC valve opens, NO valve closes)
• De-energized = spring return (if equipped) or unpowered position

Application Guidelines

Heating Systems

Zone Control with 2-Way Valves:

Application: Individual room or zone thermal reading control

Valve Selection:

• 2-way modulating (NC - normally closed)
• Size for zone electric current rate
• $\Delta P$ = 0.2-0.4 bar typical
• 0-10V control from zone thermostat

Equipment Considerations:

• Variable speed circulation pump with differential load sensor
• Bypass valve or pressure value relief if constant speed pumping unit
• Valve authority ≥ 0.3

Boiler Protection with 3-Way Mixing:

Application: Maintain minimum boiler return heat

Valve Selection:

• 3-way mixing valve (DN equal to boiler connection)
• Mix hot supply with cool return
• Modulating actuator, 90-120 second stroke
• Thermal value sensor in mixed discharge

Control Logic:

• Cold return? Open bypass (more return water recirculated)
• Hot return? Close bypass (more supply water to infrastructure)

Cooling Systems

Chilled Water Coil Control:

Application: Control chilled water movement to AHU or FCU AC coil

Valve Selection:

• 2-way modulating valve (NC)
• Size for design circulation and 0.3-0.5 bar $\Delta P$
• Fast response (60-90 second actuator)

Considerations:

• Prevent freeze-up: Minimum circulation speed interlock
• Humidity control: Valve position vs fan speed coordination
• Valve authority critical for stable control

Chiller Capacity Control:

Application: Modulate chilled water discharge through chiller evaporator

Valve Selection:

• 2-way valve on chiller supply or return
• Large Kv required (high stream rates)
• Coordinate with chiller manufacturer

Alternative: Chiller often has built-in capacity control; external valve may not be needed.

Zone Control

Residential Hydronic Zones:

Typical Setup:

• 4-8 zones with individual thermostats
• 2-way zone valves (on/off or modulating)
• Single circulator with I value check or bypass

Valve Selection:

• On/off actuators (24V AC common in residential)
• Fast acting (10-30 seconds)
• End switch to start circulator when any zone calls
• Sweat or threaded connections

Commercial Zone Control:

Typical Arrangement:

• Multiple zones with VAV or fan coil units
• 2-way modulating valves
• Variable speed pumps with $\Delta P$ control
• BMS integration

Valve Selection:

• Modulating actuators (0-10V or 4-20mA)
• Flanged or threaded connections
• Position feedback for BMS

Installation Best Practices

Valve Orientation

Recommended:

• Horizontal pipes, actuator upright: Easiest service access
• Vertical pipes, movement up: Assists closing for NC valves
• Avoid actuator below pipeline: Difficult access, moisture ingress risk

Circulation Direction:

• Mark flow rate direction arrow on valve body
• Incorrect discharge direction: Increased $\Delta P$, noise, poor control
• Some valves are bidirectional (check manufacturer data)

Piping Arrangement

Upstream Requirements:

• Minimum 5 $\times$ duct diameters straight run before valve (for accurate control)
• Strainer upstream to protect valve seat
• Isolation valves for service

Downstream Requirements:

• Minimum 2 $\times$ piping diameters straight run after valve
• Avoid elbows immediately after valve (causes turbulence)

Example: DN25 valve

• Upstream straight: 5 $\times$ 25mm = 125mm minimum
• Downstream straight: 2 $\times$ 25mm = 50mm minimum

Wiring and Control

Load Wiring:

• Follow local electrical codes
• Use proper wire gauge (typically 18-22 AWG for low potential)
• Protect low electrical potential control wiring from high V value lines

Control Signal:

• Shield 0-10V and 4-20mA signals if near high EMI sources
• Maximum wire run: 100m typical for 0-10V (check manufacturer specs)
• Use terminal blocks for easy troubleshooting

Labeling:

• Valve tag: Zone name, valve size, Kv value
• Actuator label: Capacity electric tension, control signal type
• Wiring: Color code or label each wire

Conclusion

Proper motorized valve selection is essential for effective HVAC control. Use our Motorized Valve Calculator for instant Kv sizing based on your flow requirements. Export your valve sizing calculations as a professional PDF report for documentation and engineering review.

Correct Kv sizing ensures adequate flow rate control without excessive force drop, while valve authority determines control stability and performance. Selecting appropriate actuator type (modulating vs on/off) and control signal matches infrastructure requirements and control strategy.

Key takeaways:

• Find required Kv based on discharge rate and available stress drop
• Target valve authority of 0.3-0.5 for good control
• Select valve size based on Kv, not just channel size
• Choose actuator with adequate torque and correct control signal type
• Install with proper orientation, piping arrangement, and isolation valves
• Consider fail-safe position (NC vs NO) based on safety requirements

Following these guidelines ensures reliable, efficient, and stable temperature control in heating and cooling systems.

Key Takeaways

• Calculate required Kv based on flow rate and available pressure drop using $K_v = Q / \sqrt{\Delta P}$. The Kv coefficient determines valve size selection from manufacturer catalogs.

• Target valve authority of 0.3-0.5 for good control. Valve authority is calculated as $\beta = \Delta P_{\text{valve}} / (\Delta P_{\text{valve}} + \Delta P_{\text{system}})$ and determines control stability and performance.

• Select valve size based on Kv, not just pipe size. Valve sizing must match the calculated Kv requirement, which may not necessarily match the pipe diameter.

• Choose actuator with adequate torque and correct control signal type. The actuator must provide sufficient torque to operate the valve and match the control system signal (0-10V DC, 24V AC, or 230V AC).

• Install with proper orientation, piping arrangement, and isolation valves. Valve installation affects performance, maintenance access, and system operation.

• Consider fail-safe position (NC vs NO) based on safety requirements. Normally closed (NC) valves are used for energy conservation, while normally open (NO) valves are used for critical flow requirements.

Further Learning

• Circulation Pump Guide - Sizing pumps for HVAC systems
• Expansion Tank Guide - System pressurization
• Heat Loss Guide - Building load calculations
• Circulation Pump Calculator - Interactive calculator for flow and head calculations

References & Standards

Primary Standards

EN 215 Thermostatic radiator valves - Requirements and test methods. Provides specifications for control valve performance, sizing, and testing requirements.

ANSI/ISA-75 Control Valve Sizing Equations. Provides standardized methods for calculating Kv/Cv coefficients and valve sizing for control applications.

Supporting Standards & Guidelines

ASHRAE Handbook - HVAC Systems and Equipment Chapter on Control Valves. Provides comprehensive guidance on valve selection, sizing, and installation for HVAC systems.

CIBSE Guide H Building Control Systems. Provides detailed information on valve selection and sizing for building automation systems.

Further Reading

• ASHRAE Technical Resources - American Society of Heating, Refrigerating and Air-Conditioning Engineers resources
• [Manufacturers' Catalogs] - Valve Kv values, actuator specifications, and installation guidelines vary by manufacturer

Note: Standards and codes are regularly updated. Always verify you're using the current adopted edition applicable to your project's location. Consult with local authorities having jurisdiction (AHJ) for specific requirements.

Disclaimer: This guide provides general technical information based on international HVAC standards. Always verify calculations with applicable local codes and consult licensed professionals for actual installations. HVAC system design should only be performed by qualified professionals. Component ratings and specifications may vary by manufacturer.