SS-SVX10B-EN
49
Start-Up
Compressor Cooling
Compressor output control and protection schemes will function much like on non-SZ VAV units.
Normal compressor HPC and LPC control will remain in effect as well as normal 3-minute minimum
on, off, and inter-stage timers. Also, the condenser fans will be controlled as on non-SZ VAV units.
Cooling Sequence
If the control determines that there is a need for compressor stages in order to meet the discharge
air requirements, once supply fan proving has been made, the unit will begin to stage compressors
accordingly. Note that a 5 second delay will be enforced between the command for supply fan
output operation and the command for compressor output operation. This delay is enforced to
ensure that the supply fan is energized and ramping up to operating speed prior to energizing
compressors.
As the zone cooling demand continues to increase, if additional capacity is required, the supply fan
output will be modulated above minimum speed in order to meet the zone requirements. Note that
the supply fan speed will remain at the compressor stage's associated minimum value until the
control requires additional capacity to meet the zone demand.
As the cooling load in the zone decreases the control will reduce the speed of the fan down to
minimum per compressor stage and control the compressor outputs accordingly. As the
compressors begin to de-energize, the Supply Fan speed will fall back to the Cooling Stage's
associated minimum fan speed but not below. As the load in the zone continues to drop, cooling
capacity will be reduced in order to maintain the calculated discharge air setpoint.
Cooling Stages Minimum Fan Speed
As the unit begins to stage compressors to meet the cooling demand, the following minimum
Supply Fan Speeds will be utilized for each corresponding Cooling Stage. Note that the Supply Fan
Speed will be allowed to ramp up beyond the minimum speed in order to meet the zone cooling
demand. The minimum fan speed for units with 2 stages of DX Cooling will be 58% of the unit's
full airflow capacity. At Stage 1 of DX Cooling the Fan Speed will be at a minimum of 58% and at
Stage 2 of DX Cooling the Fan Speed will be at a minimum of 80%.
Occupied Heating Operation
Occupied Heating operation on units configured with Single Zone VAV control will utilize two
separate control methodologies based on heating configurations. For all "Staged" Heating types
(Electric and Gas), the unit will utilize 100% full airflow during all active heating periods like
traditional Constant Volume units. For Modulating Gas heat units, the unit will have the ability to
control the discharge air temperature to the calculated discharge air heating setpoint in order to
maintain the Zone Temperature to the Zone Heating setpoint.
Staged Heating Operation
For units configured with Mechanical or Auxiliary Heat once the control determines that there is
an active heating capacity request, the unit will energize the Supply Fan and ramp up to full speed.
The control methodology during Active Heating will be identical to traditional Constant Volume
units; heating stages will be energized/de-energized to meet the Zone Heating demand. Note that
all Electric staging sequences will be identical to Constant Volume units.
Unoccupied Cooling and Heating Operation
For SZ VAV units, the unit will control Heating, and Cooling, as during Occupied periods using the
normal heating and cooling Single Zone VAV algorithms. In Unoccupied periods the unit will utilize
setback setpoints, and Auto fan mode operation as on normal Constant Volume units.
Failure and Overriding Conditions
Certain failure and overriding conditions require special handling of the Supply Fan Speed on units
configured with Single Zone VAV. See below for a list of these conditions:
