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The VVT Master continuously evaluates the system and updates the air source with the most current system
demand. Based on the evaluation, the reference zone can change from one zone to another. The evaluation
process continues until there is no demand from any zone or the 30 minute timer causes a re-evaluation of
the system conditions.
If no heating or cooling is required or the current air source mode is satisfied, the VVT Master calculates the
weighted average of the occupied and unoccupied heating and cooling setpoints. It also calculates a zone
temperature that is midway between the setpoints (occupied or unoccupied based on the system's current
occupancy status). This information, plus the occupancy status, is sent to the air source so that its current
mode is disabled and the unit ceases heating or cooling operation. If the system is occupied, the air source
fan and OA damper, if applicable, operate to maintain proper ventilation.
Air source mode determination
Linked air source modes – In a linked system, the air source determines its operating mode and qualifies
that mode based on its own SAT. The following modes can be sent by the air source depending on its
configuration:
•
OFF – Air source fan is off.
•
WARMUP – Air source fan is on and providing first cycle of heat when changing from unoccupied to
occupied.
•
HEAT – Air source fan is on and providing heat.
•
FREECOOL – Air source fan is on and providing cooling using economizer only.
•
COOL – Air source fan is on, and cooling is provided by economizer and mechanical cooling.
•
PRESSURIZATION – Fire-Life safety override input is active. Air source fan is on providing 100 percent
outside air. Mechanical heating and cooling may be disabled.
•
EVACUATION/SHUTDOWN – Fire-Life safety override input is active. Air source fan is off.
•
VENT – Air source fan is on, economizer providing ventilation without heating or cooling, providing neutral
supply air temperature.
See the air source's installation manual for specific operation.
Local air source modes – If the zone controller is stand-alone, or if linkage communication fails, the zone
controller assumes that the fan is always on and monitors its SAT sensor to determine if the primary air
source is providing heating, cooling, or recirculating air in a fan-only or ventilation mode.
•
HEAT – For Series or Parallel Fan controllers when the zone terminal fan is off or for single duct
controllers: The zone's local heat has not operated for at least 5 minutes, and the SAT is more than 5°F
warmer than the space temperature. If the terminal fan is on, the SAT must be at least 8°F more than
the space temperature.
In all cases, Heat mode is maintained until the SAT drops 2°F below the space temperature.
•
VENT – The zone's local heat has not operated for at least 5 minutes and the SAT is between 65 and
80°F.
•
COOL – The zone's local heat has not operated for at least 5 minutes, the current mode is not Heat or
Vent, and the SAT is less than 65°F.
See Appendix B: VVT terminal modes (page 69).
VVT Zone Controller
47
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