Auto-Zone Control Systems Auto-Zone Plus Systems Installation & Operation (Version 03A) - Manual (Page 23)

Auto-Zone Plus Section 1
Design Guide 1-19
Step #4 - Room Air Motion/Diffuser
Selection
Air motion is a consideration for occupant comfort. The selection of diffusers for an
Auto-Zone Plus 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 air flows. Because the zone air flow is variable volume, lower cost
round or rectangular diffusers that were satisfactory for constant volume may prove
unsatisfactory with an Auto-Zone Plus 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 air flow 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 and 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.
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: Damper Sizing Chart)