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Is a Return Air Inlet Required in Every Bedroom?

A Building Inspector recently called asking for clarification on the requirement for return air inlets in bedrooms. The HVAC contractor insists a 1” to 1-1/2” space under the door will provide a path back to return air inlets in virtually every residential application. The short answer is no that is not correct. A limited space under a door is ok in some cases, but not all. Failure to provide a low resistance return path from all supply air outlets back to one or more return inlets results in reduced comfort, airflow, and system efficiency.

This is addressed in the following sections out of the International Residential Code (IRC) and International Mechanical Code (IMC). The 2015 IRC, section M1601.1 reads: Duct systems serving heating, cooling, and ventilation equipment shall be fabricated in accordance with the provisions of this section and ACCA Manual D, the appliance manufacturer’s installation instructions, or other approved methods. The 2018 IMC contains similar text for single dwelling units (IMC 603.2). IRC M1602.2 also provides additional direction.

Manual D is the referenced ANSI Standard in IRC Chapter 43 and IMC Chapter 15; therefore, Manual D is an adopted Code by reference. ACCA Manual D section 4-9 states “An engineered, low resistance return path shall be provided from every room or space that receives supply air.” By definition, a low resistance path limits the velocity through a return path opening to no greater than 350 feet per minute (FPM). Please note the 350 FPM maximum air velocity under a door or through a set of transfer grills is one half the recommended 700 FPM velocity for a residential duct. If we need to cut the velocity to half that of a typical residential duct, transfer openings must be at least twice as large as the branch duct serving the supply air outlet. Past and current editions of Manual D include a Door Cut chart, showing the clearance in inches, required between the top of the flooring to the bottom of the door, for numerous CFM flow rates. The chart was added to Manual D as many contractors claim the use of the space under the door for the return path. Please observe in Table A1-2, a master bedroom with 400 CFM of supply air into the room and an attached bath will require a 5.3″ high opening under the door to provide a low resistance path back to a central return. A quick review of the table shows even 100 CFM requires more than a one-inch space under the door. Locating a return in the bedrooms or providing properly sized transfer grills is often the best solution.

Let’s consider the example of the home floor plan in the diagram below. With doors closed, there is a limited low resistance return path back to the central return air inlet. With this return configuration, the home is placed in a condition where the living space is under negative pressure and the bedrooms are under positive pressure. This creates an increased leakage of air to the outside from positively pressured bedrooms; and conversely, an increased leakage (or infiltration) of air from outside into the negatively pressured living space. The return path for at least a portion of the total return air is outside through bedroom walls, with other cold or hot outside air drawn in through living area walls. Failure to provide the required low resistance path not only reduces comfort, but it also increases energy consumption.

Summary and Final Comments:

      • Failure to provide a low resistance path from each supply back to a return air inlet will result in poor comfort, reduced system airflow, and excessive energy waste.
      • The adopted codes require a low resistance return path.
      • Do not locate returns in kitchens unless it is at least 10’ from cooking appliances, or in bathrooms, or similar spaces.
      • Generally, a single 6” round duct with a 4” X 10” boot and register flows about 100 CFM. For rooms with 100 CFM of supply, a door cut of about 1.5” with a 30” door will provide the low resistance path. See the submitted Manual J, D, and S documents for actual design supply and return CFM volumes.
      • CFM volumes greater than 100 CFM will require a return in the room, transfer grills, or an obnoxiously large door cut.
      • For master bedrooms, account for all supply air outlets in bathrooms and closets, including shower rooms, and toilet rooms. Master bedrooms may often have 200-300 CFM in the bedroom area and another 200+ CFM delivered to closets and bathrooms, requiring a 6” to 8” door cut.
      • Please account for the door width when using the Door Cut Chart.
      • Rarely is a return air inlet, or properly sized transfer grills, found in a bonus room over garages. Equally rare is a comfortable bonus room.
      • A word of caution; if the system includes zone dampers, use central returns with transfer grills, not returns in the bedrooms; otherwise, the bedrooms will go negative when the supply is closed off by the zone damper.

Please feel free to respond with questions and comments.

– Dr. Energy

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