(Editor's note: The following is taken from the Sheet Metal and Air conditioning Contractors' National Association's "Residential Comfort System Installation Standards Manual," seventh edition.)

A residential heating and air conditioning system is only as efficient as its air delivery component. The quantity and velocity of air movement within space and the proper mixing of supply air with space affect comfort levels.

Supply air should be directed to the sources of greatest heat loss or heat gain to offset their effects. Registers and grilles for the supply and return systems should accommodate all aspects of the supply distribution patterns such as throw, spread and drop; also, the outlet and return grille velocities must be held within reasonable limits. Any noise generated at the grille is of equal or greater importance than duct noise. The diagrams show recommended grille and register locations.

Supply air outlets

The principles of air distribution are discussed in the SMACNA "Hvac Systems - Duct Design" manual and the American Society of Heating, Refrigerating and Air-Conditioning Engineers' "ASHRAE Fundamentals." In residential system design, simplified methods of selecting outlet size and location generally are used.

Supply outlets fall into four general groups, defined by air discharge patterns: horizontal high, vertical non-spreading, vertical spreading; and horizontal low. The chart below lists the general characteristics of supply outlets. It includes the performance of various outlet types for cooling as well as heating, since one of the advantages of forced air systems is that they may be used for both heating and cooling. However, no outlet type is best for heating and cooling.

The best outlet types for heating are located near the floor along outside walls and provide a vertical-spreading air flow, preferably under windows, to blanket cold areas and counteract cold drafts. This arrangement, called perimeter heating, causes mixing of the warm air supply with both the cool air from area of high heat loss and the cold air from infiltration which prevents drafts.

High sidewall outlets should deliver the air horizontally or slightly upward during cooling. The throw of a high sidewall outlet should be equal to or not over 30% more than the distance between the outlet and the opposite wall (or effective obstruction to a free air stream) of the room.

The best outlet types for cooling are located in the ceiling and have a horizontal air discharge pattern. For year-round operation, the correct choice of a system depends on the principal application. If heating is of major equal importance, perimeter diffusers should be selected. The system should be designed for the optimum supply velocity during cooling. If cooling is the primary application and heating is of secondary importance due to mild winters, ceiling diffusers will perform most satisfactorily.

Return air grilles should be located in hallways, near entrance doors, or on inside walls to ensure a low resistance return air path between every room and the return side of the blower cabinet. Return air systems use either central or multiple grille locations.

A central return occupies a minimal amount of space with a short return duct, creating a small return-side pressure drop. In multilevel homes, a central return should be installed on each level. A multiple return system provides for a return opening in every major room and transfer grilles for secondary rooms.

Return air grilles should not be located in bathrooms, kitchens, garages, utility spaces, a space used for storage of fuel or flammable materials, or a confined space in which a draft diverter or draft regulator is located or to which combustion air is supplied.

Return air grilles shall be sized to return 100% air being supplied with air velocities not to exceed 4000 fpm face velocity in order to minimize system noise.

(For more information on SMACNA's "Residential Comfort System Installation Standards Manual" or to order a copy, write SMACNA, 4201 Lafayette Center Drive, Chantilly, VA 20151-1209; also call (703) 803-2980; fax (703)803-3732. For online ordering, go to www.smacna.org.)

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