Auto-Zone Control Systems Auto-Zone Plus Systems Installation & Operation (Version 03A) - Manual (Page 17)

Auto-Zone Plus Section 1
Design Guide 1-13
Heating - Partial Load Conditions
Heating difficulties are less common than cooling difficulties. They are similar in nature,
however, and the cures are generally the same.
• Increase the Heating minimum setpoints on as many zones as possible.
• Increase the static pressure setting as high as is practical. Increasing static pressure
does not help if you are using pressure independent operation.
• Bypass to plenum instead of the return air intake if acceptable.
• Do not oversize your equipment.
• Use auxiliary heat in either your VAV boxes or baseboard.
Caution:
When using a VFD for duct static pressure control, the VFD control
must have its minimum speed setting configured to meet the minimum
airflow requirements of the installed heating equipment. This is
especially critical when using indirect fired gas heating since the heat
exchanger could be damaged if the heating airflow falls below the
minimum airflow recommended for the heat exchanger. If the unit is
equipped with electric heat, failure to meet minimum airflow
requirements of the electric heater could cause nuisance tripping of the
electric heaters safety devices. The minimum heating airflow required
for your HVAC unit should be available from your HVAC unit
manufacturer. See your VFD installation manual for information on
setting the minimum airflow.
Auto-Zone Plus has a number of auxiliary heat control options which provide solutions to
most problems. Refer to the Auxiliary Heat Control Options topic near the end of this
section.
Override Conditions
After-hours overrides can produce aggravated partial load conditions in both the heating
and cooling modes. The problem is most commonly caused by a single zone being
overridden for after-hours use. This causes the rooftop equipment to operate for only one
zone. The Auto-Zone Plus system offers an improved solution to this common problem
by allowing a single override to trigger a group of zones via a “global” override. This
allows the system to operate with sufficient load to reduce cycling caused by light load
conditions.