Auto-Zone Control Systems Auto-Zone Basic Systems Installation & Operation (Version 02C) - Manual (Page 17)

Auto-Zone Basic Section 1
Design Guide 1-11
Step #4 - Room Air Motion/Diffuser
Selection
Air motion is a consideration for occupant comfort. The selection of diffusers for an
Auto-Zone Basic Control System requires more care than a constant volume system due
to varying flow of air into the zones. Slot diffusers are recommended due to their superior
performance at low airflows. Because the zone airflow is variable volume, lower cost
round or rectangular diffusers that were satisfactory for constant volume may prove
unsatisfactory with an Auto-Zone Basic Control System. These diffusers may result in
"dumping" of the cold air at low flows in the cooling mode and insufficient room air
motion at low air flows in the heating mode. Although high air motion in the heating
mode can be undesirable, a slot diffuser with a high induction ratio generally helps to
reduce room air "stratification" when the heating comes from a ceiling diffuser. Linear
slot diffusers should be properly selected for the airflow and "throw" suited to the
specific installation or zone.
Additional factors to consider in diffuser selection are sound level and throw at design
flow. Generally, multiple diffusers will result in lower sound levels in the space, but this
must be balanced with the additional hardware and installation costs. It is commonly
recommended that slot diffusers be located near the perimeter or outside wall with the
airflow directed into the room. Consult your diffuser supplier or catalog for proper
diffuser sizing and location.
Series fan boxes may be used instead of zone dampers where higher induction rates are
desirable. If the heat loss on perimeter walls is high, such as large areas of glass, the use
of Series Fan Boxes may be indicated to maintain higher induction rates to offset
“downdrafts.” If the heat loss is greater than 275 BTUH/LINEAR FOOT, you should use
high quality slot diffusers next to the outer wall with the airflow directed inward to
counteract downdrafts during heating. Serious downdraft problems occur when heat
losses exceed 400 BTUH/LINEAR FOOT. In such case, both high induction diffusers
and series fan boxes are recommended.
Step #5 - Bypass Damper Sizing
Using a load calculation program, the bypass damper should be sized to give you the
maximum CFM of air to be bypassed, typically 60 to 70 percent of the HVAC units rated
capacity. Bypass Dampers can either be round or rectangular depending on building or
job requirements. Use the appropriate round or rectangular damper selection table to
determine the correct damper size for your application. To size the damper, select a
damper from the table based on calculated bypass CFM and a maximum velocity
between 1750-2250 FPM. When determining the bypass duct size, be sure to take into
account any transition fittings and associated pressure drops. (See Table 1-1: Round
Damper Selection or Table 1-2: Rectangular Damper Selection.)