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Understanding HVAC Metering Devices April 21, 2024, 2:20 am

April 21, 2024

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Returns – Passive? Active? Sizing? w/ Adam & Matt

In this episode of the HVAC School podcast, Bryan Orr sits down with Adam and Matt to tackle one of the most misunderstood aspects of[...]

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In this episode of the HVAC School podcast, Bryan Orr sits down with Adam and Matt to tackle one of the most misunderstood aspects of HVAC system design: return air systems. The conversation dives deep into the common myths surrounding returns, particularly the widespread belief that adding returns to rooms will automatically improve comfort and air mixing. The hosts explore why returns are often seen as mysterious – sometimes helping when added to a room, sometimes making things worse – and work to clarify the actual science behind how returns function versus how many technicians think they work.

The discussion reveals a fundamental misunderstanding in the industry about the difference between supply and return air behavior. While supply air creates significant mixing effects through entrainment and the Coanda effect, returns have a much more limited impact on room air patterns. According to Manual T, laboratory tests show that return air intake influence is limited to just a few feet around the grille, even at high velocities. This leads to the key insight that returns serve primarily as pressure relief mechanisms rather than air mixing devices, which explains why their placement and sizing strategies differ significantly from supply air design principles.

The hosts examine real-world scenarios, including Matt’s experience with a two-story home where the homeowner requested an additional return on the second floor despite having adequate return pathways. They discuss the Manual D recommendation for returns on each floor while sharing field experiences where single central returns have successfully maintained temperature differentials within two degrees across multiple floors. The conversation emphasizes that proper system design, including accurate load calculations and right-sized equipment, often eliminates the need for complex return strategies.

A significant portion of the discussion focuses on the practical considerations of passive versus active returns. The hosts lean toward passive return strategies (transfer grilles, jumper ducts, and door undercuts) for their simplicity, cost-effectiveness, and reduced ductwork requirements. However, they acknowledge the challenges of proper sizing – passive returns must be significantly larger than their active counterparts since they rely on minimal pressure differentials rather than mechanical assistance. The conversation also addresses privacy concerns with transfer grilles and the benefits of oversized jumper ducts for noise attenuation.

Key Topics Covered

Common Return Myths Debunked
- The belief that adding returns automatically improves room comfort
- Misconceptions about returns mixing air effectively
- The difference between supply air entrainment and return air influence
Pressure Relief vs. Air Mixing
- Returns function primarily for pressure balancing, not air circulation
- How to measure pressure problems using flow hoods and micromanometers
- The impact of door closure on supply air delivery
Manual D Guidelines and Field Reality
- Manual D recommendations for returns on each floor
- Real-world examples of successful single central return systems
- When to follow guidelines versus when field conditions allow flexibility
Passive Return Strategies
- Transfer grilles, jumper ducts, and door undercuts as alternatives to active returns
- Proper sizing considerations for low-resistance return pathways
- Privacy and noise concerns with different passive return methods
System Design Fundamentals
- The importance of accurate load calculations in reducing return requirements
- How right-sizing equipment reduces airflow and pressure management challenges
- The relationship between static pressure and airflow changes
Zoning Considerations
- Whether to add dampers to returns in zoned systems
- Potential depressurization issues when zones are closed
- Passive solutions for zoned system return air management
Practical Installation Tips
- Avoiding short-cycling by maintaining proper supply-to-return distances
- The risks of undersized return pathways
- Balancing damper selection and sealing strategies
Advanced Concepts
- Destratification strategies using ceiling fans versus high returns
- The impact of conditioned versus unconditioned return air pathways
- Energy efficiency considerations in return system design