Return Air Transfer Duct/Pass Thru Calculator

Methodology

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A "Transfer Duct Calculator" helps mechanical engineers, designers, architects, and contractors size transfer ducts to meet building codes. Properly sized ducts increase occupant comfort by balancing air pressure, improving equipment efficiency by reducing energy use, and ensuring predictable and reliable HVAC system performance. Additionally, it helps ensure compliance with standards, avoiding costly rework.

 

The 2023 Florida Mechanical Code, Section 601.6 is the basis of the design of this calculator.  Since the supply air duct size is often unknown, this calculator assumes that the supply duct was designed with a static pressure of approximately 0.11 in. w.c. The minimum return duct was estimated to be 1-1/2 times the estimated supply duct size.  The transfer grilles are selected by ensuring the the grille's Ak, or Grille Outlet Area [ft^2], is greater than the following: 

 

Ak >=  CFM  x 50 in^2 /100 CFM * (1 ft/12 in)^2 

 

See code section references below:

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2023 FBC, Section 601.6 Balanced return air.  Exceptions:

1. Transfer ducts may employed to achieve air balance by increasing the return transfer 1-1/2 times the cross-sectional area (square inches) of the supply duct entering the room or space it is serving and the door having at least an unrestricted 1-inch undercut to achieve proper return air balance.

2. Transfer grilles shall use 50 square inches (of grille area) to 100 cfm (of supply air) for sizing through-the-wall transfer grilles and using an unrestricted 1-inch undercutting of doors to achieve proper return air balance.

3. Habitable rooms only shall be required to meet these requirements for proper balanced return air excluding bathrooms, closets, storage rooms and laundry rooms, except that all supply air into the master suite shall be included.

 

2024 IMC, Section 601.5 Return air openings.  #4
Return and transfer openings shall be sized in accordance with the appliance or equipment manufacturer's installation instructions, ACCA Manual D, or the design of the registered design professional.