VAV MODULATING HOT WATER REHEAT VARIABLE SPEED FAN
TEMPERATURE CONTROL
When the zone temperature is greater than the cooling temperature setpoint, the controller will enter
the cooling mode. The primary airflow rate will be varied between its minCoolFlowStpt (A227), and its
maximum Flowrate setpoint maxCoolFlowStpt (A228) as required to maintain the zone at the cooling
temperature setpoint. Once the zone temperature falls below the cooling temperature setpoint, it will
exit the cooling mode.
When the zone temperature is between the cooling and heating temperature setpoints, it will be in
Deadband or Vent Mode. The controller will maintain the minCoolFlowStpt (A227).
If the activeAirflow (A250) is below the minHeatFlowStpt (A230), the heat will be disabled.
In heating mode, the controller will maintain the minHeatFlowStpt (A230).
When the zone temperature falls below the heating setpoint, the controller will enter the heating mode.
• If no discharge air temperature sensor is configured:
o As heating demand rises from 0-100%, the stages of heat shall be enabled to maintain
zone temperature setpoints.
o As heading demand rises from 51%-100%, the fan speed setpoint shall reset from
minFanSpeed (A237) to maxFanSpeed (A238).
• If discharge air temperature sensor is configured (this meets ASHRAE Standards):
o As heating demand rises from 0-50%, the discharge air temperature shall be reset
upwards, to a maximum of 20 deg above zone temperature. The stages of heat shall be
enabled to maintain the discharge air temperature setpoint.
o As the discharge air heating demand rises from 51-100%, the fan speed setpoint shall reset
from the minFanSpeed (A237) to the maxFanSpeed (A238).
o The duct coil shall control the VAV discharge air temperature to the setpoint. The duct coil
shall not directly control zone temperature.
Typically, the modulating heating valve may be controlled by either AO-14 or DO-09 and DO-10 in a tri-
state control mode where DO-09 drives the hot water valve open and DO-10 drives it closed. There are
additional options available such as having AO-14 as the first stage modulating heat and AO 15 as the
second stage modulating heat.
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