Ducted Returns
For a central forced air furnace and air conditioning system to function properly, the HVAC distribution system must include adequate supply and return registers. These components are crucial for circulating conditioned air throughout the house and returning stale air to the furnace for reconditioning. Inadequate return air pathways can lead to pressure imbalances, drafts, and temperature differences between rooms, causing discomfort and causing the HVAC equipment to work harder.
A well-designed return air strategy is vital for energy-efficient homes, which have lower airflow requirements due to its air tightness. Each room should have a clear return air path to the air handler, except bathrooms and kitchens to prevent odor spread. While individually ducted returns for each room are costly and space-consuming, a central return strategy is more feasible. Central returns should be installed in hallways or stairwells, with at least one per floor, using air-sealed, insulated ducts. They only need to be insulated if the duct work travels through unconditioned space.
Building cavities like wall or joist bays should not be used as return air pathways. Fully ducted return systems offer better airflow and easier sealing. To ensure air return from rooms with closeable doors, builders often use door undercuts, jump ducts, or transfer grilles. However, standard door undercuts are insufficient for energy-efficient homes. Jump ducts and transfer grilles are recommended.
To calculate the amount of return air needed, the target return capacity should be twice the volume of the total supply air, with an airflow velocity of less than 500 feet per minute. The grille's net free area should be 1.5 times the cross-sectional area of the return duct. ENERGY STAR standards require that returns maintain a pressure differential between -3 and +3 Pascals when bedroom doors are closed and the air handler operates at the highest design fan speed. A pressure differential between -5 and +5 Pascals is acceptable for rooms with design airflow ≥150 cfm. Bedrooms can be pressure-balanced using transfer grilles, jump ducts, dedicated return ducts, or undercut doors.
At the final HERS inspection, your Rater will pressure test the entire duct system. Optimal sealing is critical, in addition to design and install of a well balanced system.
Figure 1 - Jump ducts, in this configuration, can be used to balance pressure differentials between bedrooms and the main volume of the structure.
Figure 2 - A transfer grill is installed in the wall to connect a closed room with an open area, thus providing an air pathway to the central return air register. The registers are offset to minimize sound and light transfer. Air flows through the uninsulated interior wall cavity. 
Figure 3 - A complete HVAC system includes ducted returns.