Auto-Zone Control Systems Auto-Zone Plus Systems Installation & Operation (Version 03A) - Manual (Page 15)

Auto-Zone Plus Section 1
Design Guide 1-11
Low Supply Air Temperatures
Under lightly loaded conditions much of the supply air may be bypassed back into the
return air side of the system. This bypassing will result in the lowering of the supply air
temperature, which may result in the supply air temperature reaching the low temp safety
limit. If the supply air low temp safety limit is exceeded, the control system will “cut-off”
the mechanical cooling to protect it from damage. Excessive cycling of the mechanical
system will result if this condition persists. Comfort may also suffer if the system cannot
run long enough to satisfy cooling demands.
A number of things can be done to reduce this problem. Some of these things depend
upon the type of installation.
• Avoid oversizing the unit. Do your load calculations carefully. Since Auto-Zone Plus
directs the heating or cooling to the zones which require it, you may find that you can
use a smaller unit in many cases. Oversizing is the number one cause of excessive low
supply air temperature cycling.
• Increase your cooling minimum airflow or damper position settings to allow more air
during cooling operation. Be careful to avoid settings which are so high you cause
over cooling of the spaces. Find a compromise position.
• Bypass the air into the plenum instead of into the return air intake. Be careful if you
use this method since you may get “dumping” of cold air from your return air grilles.
This method works best with plenum returns, do not use this method with ducted
returns unless you have carefully considered the consequences.
• Increase your static pressure setpoint to help reduce the amount of air being bypassed.
Be aware of increased noise levels and the cost of operation if you use excessive static
pressures. This will not work if you are using pressure independent zone controllers,
since they will maintain a constant flow of air to the zones regardless of duct static
pressure. This technique will likely cause over cooling of the spaces due to increased
airflows at minimum positions.