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General Advice for Sizing Problems with VAV Systems
- Make sure
Zone Heating Design Supply Air Temperature
inSizing:Zone
matches theMaximum Reheat Air Temperature
in theAirTerminal:SingleDuct:VAV:Reheat
assigned to this zone. If the sizing temperature is higher than the maximum reheat temperature, then during operation the terminal isn't allowed to operate at the temperature it was designed for, meaning that it can't provide the designed heating capacity. - Make sure that the
Design Loop Exit Temperature
in theSizing:Plant
for the Hot WaterPlantLoop
matches theWinter Design Day Schedule
in theSetpointManager:Scheduled
assigned to your Hot WaterPlantLoop
. If everything is sized for high temperature water but then operated at a lower temperature, sometimes the equipment can't meet the load. - Make sure that the
Rated Inlet Water Temperature
andRated Outlet Water Temperature
of theCoil:Heating:Water
Reheat Coils match theDesign Loop Exit Temperature
andLoop Design Temperature Difference
in theSizing:Plant
for the Hot WaterPlantLoop
. This ensures that the coils and their plant loop are sized with the same temperature and delta-T.
So for your particular IDF:
- Change
Zone Heating Design Supply Air Temperature
from 40C to 35C to match max reheat - Change
Design Loop Exit Temperature
for Hot Water Loop from 82.2C to 67C to match HW setpoint - Change
Rated Inlet Water Temperature
for Reheat Coils from 82.2C to 67C to match HW setpoint - Change
Rated Outlet Water Temperature
for Reheat Coils from 71.1C to 55.9C to match HW delta-T
Result: Time Setpoint Not Met During Occupied Heating drops from 1145.00 to 234.33