Sequence of operation
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 capability and
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, providing cooling using economizer only, and system is unoccupied.
•
COOL
– Air source fan is on, and cooling is provided by economizer and/or mechanical cooling.
•
PRESSURIZATION
– Fire/Life-safety override mode which causes the air source supply fan to operate, and if
equipped, the OA damper to provide 100% outside air. Mechanical heating and cooling may be disabled. Any
linked terminal will open its primary air damper to provide the configured maximum cooling airflow.
•
EVACUATION/SHUTDOWN
– Fire/Life-safety override mode which causes the air source supply fan to stop,
the return fan to operate, and if equipped, the exhaust air damper to open 100%. Any linked terminal will fully
close its primary air damper and disable its terminal fans, if equipped.
•
VENT
– Air source fan is on, economizer providing ventilation without any mechanical heating or cooling and
providing neutral supply air temperature.
See the air source’s installation manual for more specific operation.
Local air source modes
– If the zone controller is stand-alone or if linkage communication fails, the zone controller
monitors the cfm (liters/second) for fan status and 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 minut
es, and the SAT is more than 5∆°F (2.8∆°C)
warmer than the space temperature. If the terminal fan is on, the SAT must be more than 8∆°F (4.4∆°C)
(warmer than the space temperature.
In all cases, Heat mode is maintained until the SAT drops 2∆°F (1.1∆°C) be
low the space temperature.
•
VENT
– The zone’s local heat has not operated for at least 5 minutes and the SAT is between 65°F (18.3°C)
and 80°F (26.6°C).
•
COOL
– The zone’s local heat has not operated for at least 5 minutes, the current mode is not Heat, and the
SAT is less than 65°F (18.3°).
•
OFF
– Terminal airflow is used determine if the primary air source fan is on or off. If the terminal's Damper
Position is greater than 95% and the CFM (liters/second) is less than 10% of
Nominal CFM
* (liters/second)
at 1” (.249 kpa) Velocity Pressure for 1 minute and the CFM (liters/second) Setpoint is not equal to 0, then
the fan is determined to be off. If CFM (liters/second) is greater than 20% of the
Nominal CFM
*
(liters/second) at 1" (.249 kpa) Velocity Pressure, or greater than 50% of CFM (liters/second) Setpoint for 10
seconds (whichever is greater), then the fan is determined to be on. Occupied/Unoccupied Min CFM
(liters/second) must be set for greater than 10% of Max Cool CFM (liters/second).
* Shown on
Properties
page as
Manufacturer’s specified air flow at 1" water column:
for both English and
Metric applications.
English: Use manufacturer’s CFM at 1" water column
46
VAV Zone Controllers
