eZV-440 Application Guide
Edition 2.6
Page 65 of 84
For electric duct or electric baseboard outputs set up as time-proportional, the minimum heat
output % is ignored by the algorithm to prevent overheating, even when the minimum heat output is
greater than 0. The maximum heat output % still applies to honour the system's upper safety limits.
For hydronic heat outputs, both minimum and maximum heat output % are used.
For programmable controllers, if custom output objects are used, the pulse has a 100% duty cycle.
The output is directly controlled by a GCL program and the custom AV objects are used to
modulate the pulse.
Discharge Air Temperature
The discharge air temperature (DAT) is the duct temperature downstream from any local heating
or cooling stages that supply the air to that space. This section applies to a space with a DAT
sensor and ducted heating or cooling stages.
The heating or cooling output is limited by the discharge air limited controller that maintains a
maximum limit above the CurrentHeatSetPoint (AV800) in heating, and a minimum limit below the
CurrentCoolSetpoint (AV801) during cooling. The maximum limit is calculated by adding the
discharge air temperature high limit setpoint differential (DATHiLimitSetpointDiff) to the
CurrentHeatSetPoint, and the minimum limit is calculated by subtracting the discharge air
temperature low limit setpoint differential from the CurrentCoolSetpoint.
Limiting the ducted heating and cooling stages ensures the temperature differential between space
air and supply air is not large enough to cause hot air to build up near the ceiling. Limiting heating
and cooling outputs also encourages cold air to spread out via the diffuser and mix well with the
room air. Cold air that drops down without mixing can create complaints of cold drafts.
Fan (VAV)
This section describes the fan sequences supported by the VAV algorithm.
Single Speed Parallel Fan Sequence
A VAV parallel fan will run or start in the following cases:
l When the fan is set to run constantly as determined by its ConstantFanModes setting.
l During Occupied mode (includes Unoccupied Override), when the central air handler is
providing primary air to the space, the parallel fan will turn on when the airflow setpoint drops
below the parallel fan flow set point (ParallelFanFlowSetpoint AV131). This ensures there
is enough airflow to adequately mix the primary air in the space and prevent cold duct air
from being dumped into the space during times of low cooling load operation.
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