Ultimate Guide to Plenum Duct Sizing for HVAC Systems

Introduction to Plenum Duct Sizing for HVAC Systems

Proper HVAC performance starts with a component most building occupants never see: the plenum. This central distribution box connects directly to the air handler and serves as the critical junction where conditioned air begins its journey through your ductwork. When sized incorrectly, plenums create cascading problems—excessive noise, uneven temperatures, higher energy bills, and premature equipment failure.

Many facility managers struggle with undersized plenums that generate whistling sounds and hot spots, or oversized chambers that waste materials and create dead zones where air stagnates.

ASHRAE research shows that velocity-driven noise increases exponentially when plenum dimensions don't match system requirements, while pressure drop compounds throughout the entire duct network.

You'll learn the engineering principles behind plenum sizing calculations, design best practices for both supply and return applications, and the common mistakes that compromise system efficiency. You'll learn the formulas, velocity standards, and dimensional requirements needed to size plenums correctly the first time.

TLDR: Key Takeaways on Plenum Duct Sizing

• Proper sizing maintains optimal airflow while minimizing noise and pressure drop
• Standard supply plenum velocity is 800 fpm; return plenums use 600-700 fpm for quieter operation
• Use Q = V × A formula (CFM = velocity × area) for sizing calculations
• Size plenum depth at 2.5× fan diameter for optimal air expansion
• Avoid undersizing—it causes noise, pressure loss, and poor distribution

Understanding Plenum Types and Functions

Understanding Plenum Types and Functions

Supply Plenum vs. Return Plenum

Supply plenums attach directly to the air handler discharge, operating under positive pressure to distribute conditioned air. They function as expansion chambers that convert the high-velocity pressure from the fan into static pressure, ensuring uniform airflow to downstream ducts. Because they handle turbulent air directly from the fan wheel, supply plenums require careful sizing to prevent noise and pressure loss.

Return plenums connect to the air handler intake, operating under negative pressure to draw air back from the conditioned space. ASHRAE guidelines indicate that properly designed return systems produce sound levels approximately 5 dB below supply levels. Return plenums often house filter racks and connect to cooling coils, so sizing must account for the pressure drop these components create.

How Plenums Fit Into the Duct System

The plenum serves as the starting point for proper air distribution in your HVAC system:

• Primary distribution hub: Connects the air handler to main trunk ducts
• Pressure conversion zone: Transforms velocity pressure into static pressure for efficient distribution
• Takeoff connection point: Provides attachment locations for branch ducts serving individual zones
• System balancing foundation: Proper plenum sizing ensures adequate static pressure reaches all downstream components