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Properly Designed Air Distribution Network
Working Back to the Air Handling Unit
All load calculations begin with the spaces served for which the designer calculates the airflow required. An appropriate terminal device, the register or grill, determines the direction of airflow. If too small, it constricts airflow and causes 'grill noise'. If too large, insufficient 'throw' means short-path airflow patterns that cause hot and cold spots.
Source: Snips Magazine
(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.
The principles of air distribution are discussed in the SMACNA "HVACs 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 lowresistance 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.
Older residential systems have sheet metal ducting wrapped with insulating material. In today's economy, tough, durable flexible ducting is my choice. It has far superior sound characteristics than sheet metal ducting and its installation cost is a fraction of sheet metal. Metal ducting must be separately insulated after the duct has been mounted, sealed and suspended, but flexible ducting comes in 25-foot lengths, pre-insulated and ready to go.
Sizing Flexible Ducting
Source: Hart & Cooley
Flexible duct has many advantages in the HVAC environment. Its ease of use and timesaving (money) speed of installation compared to hard duct is inviting. But using it as a direct size replacement for smooth, galvanized duct is not one of its advantages due to a difference in performance. Because of flex duct's unique corrugated construction and flexibility, there is a higher airflow friction loss compared to the same size smooth-walled galvanized duct. Performance equivalent to hard duct requires a larger diameter flex duct.
Friction loss in straight duct is dependent on the relationships of duct diameter, air velocity in the duct, and duct roughness as major components, and to a much lesser degree on air density. As one can imagine, flex duct with its helical corrugations is going to be much "rougher" or less smooth than galvanized duct. This is especially true if it is not stretched out to the extent possible during installation. Slack duct allows the coils of reinforcing wire to relax, which bunches up the polyester and pushes it into the interior of the core, adding more resistance to airflow.
Sizing charts and calculators for duct sizing are available from many sources. Hart & Cooley has a Sheet Metal Duct Friction Loss Calculator on one side of a slide chart with a Flexible Duct Friction Loss Calculator on the other side that we make available. We also have an interactive flex duct calculator on our web site. Spending a few minutes with these aids can quickly demonstrate the differences between the friction losses for galvanized verses flexible duct. It is worth noting that for a fixed duct diameter, as the velocity in the duct increases, the friction loss increases twice as fast. So if the velocity were to double, the friction loss would be four times greater! A handy rule that is very effective and reliable is to increase the size of flex duct one diameter to neutralize the added friction loss compared to that of galvanized duct for the same CFM.
A further penalty in performance will occur if flexible duct is compressed from its round shape to an oval shape, say by squeezing it into a joist space. Just because it can doesn't mean it should. We do allow for up to approximately a 20% reduction in diameter only if it occurs in one spot, but not over any distance or repeatedly. The friction loss for flex squeezed into an elliptical shape over any distance is severe, and the loss of airflow will be significant.
Cubic feet per minute airflow rate still equals the air velocity times the area of the duct in which the air is flowing. Increasing the area of the duct will slow the velocity of the air and reduce pressure loss. Keep in mind that the long-term system performance will be af fected by the up-front, one-time cost of the flex duct. Increasing flex duct one size to offset its higher pressure loss compared to smooth duct is prudent.