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
Have a question that you want us to answer on the podcast? Submit your questions at https://www.speakpipe.com/hvacschool.
Purchase your tickets or learn more about the 7th Annual HVACR Training Symposium at https://hvacrschool.com/symposium.
Subscribe to our podcast on your iPhone or Android.
Subscribe to our YouTube channel.
Check out our handy calculators here or on the HVAC School Mobile App for Apple and Android