Carrier VAV Zone II Installation And Startup Manual page 60

Sequence of operation
VVT Systems
A VVT Master determines system operation by prioritizing heating and cooling requirements from all the zones
based on their occupancy, demand, and damper size.
The VVT Master continuously scans the system to determine if any zones are occupied. If any zones are occupied,
the VVT Master evaluates the occupied zones' heating or cooling demands to determine the following:

The system mode. The mode is:
Cooling if the number of occupied zones with cooling demands exceeds the number of occupied zones
○
with heating demands, and the demand is greater than or equal to the number of configured Linkage
Callers.
Heating if the number of occupied zones with a heating demand exceeds or is equal to the number of
○
Linkage Callers.

The reference zone, the zone with the greatest demand for the system mode
If no zones are occupied or no occupied zones require heating or cooling, the VVT Master performs the evaluation
described above for the unoccupied zones.
The VVT Master then sends the following information to the air source:

The setpoints and zone temperature from the zone with the reference zone

The system occupancy status

Most open damper position from any zone

RH and CO
The air source then sends to the VVT Master:

The air source mode

Supply air temperature

Outside air temperature (if applicable)

Static pressure (if applicable)
The air source verifies the mode by comparing its supply air temperature to the space temperature received
through Linkage. See the air source documentation for operation and parameters used to verify its mode. This
verification determines if the desired air source mode is being provided. For example, if the VVT Master requests
heating but the air source does not have heat or its heat has failed, the verification indicates this. The actual
current mode is sent to the zones so that they can control accordingly.
The system remains in that mode until all zones of that demand are satisfied or until the System Mode Reselect
Timer (30 minute factory default) causes a forced re-evaluation of the system. If there is no demand for the
opposite mode or the demand is smaller than the current mode, the reselect timer is reset to 30 and the current
mode continues until all zones are satisfied or until the reselect timer expires, repeating the process. If there is a
demand for the opposite mode, the VVT Master sends the new reference zone's space temperature and setpoints
for the opposite mode to the air source and restarts the reselect timer. The air source re-evaluates this data and
then attempts to provide the air required by the new information. The amount of time it takes to switch modes is
determined by the air source's operating parameters.
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

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.
VAV Zone II Controllers
Installation and Start-up Guide
values (if applicable)
2
56
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