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Heating airflow sizing for AirTerminal:SingleDuct:VAV:Reheat

asked 2018-11-26 13:20:14 -0500

jt1234 gravatar image

I have a 100% OA airloop with several dozen zone level AirTerminal:SingleDuct:VAV:Reheat objects with hot water reheat coils. My issue lies in the design heating airflow that is being passed to the terminal unit and subsequently used to size the heating coil. My heating coils are undersizing because they're seeing a calculated airflow as opposed to my user-defined heating airflow.

The terminal unit objects have a 'Scheduled' Zone Minimum Air Flow Method and a corresponding fractional schedule assigned. I have the target cooling and heating flows hardcoded at the zone (Flow/Zone). The zone appears to see the correct heating flow (based on HVAC Sizing Summary), the terminal unit is passed the correct Maximum Air Flow Rate, but the heating coil is passed the incorrect heating airflow. I have replicated this issue in a test model with a min flow schedule at 1.0 (100%) during all operating hours. The correct airflow is passed with both the 'Constant' and 'FixedFlowRate' input method so this appears to be specific to only the 'Scheduled' method.

This is being constructed in OpenStudio, but it appears everything is being passed as it should to EnergyPlus. My hunch is that this is an EnergyPlus issue, but its difficult to tell for sure.

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answered 2018-12-26 15:08:16 -0500

From the Input Output Reference for AirTerminal:SingleDuct:VAV:Reheat for "Minimum Air Flow Fraction Schedule Name":

The reheat coil operates only when the damper is at this minimum flow rate when Damper Heating Action is set to Normal (the default). This field is used if the previous field is set to Scheduled. If the previous field is left blank (and the field Maximum Hot Water or Steam Flow Rate is set to autosize), then the air flow rate usedfor sizing normal-action reheat coils is the average of the minimum and maximum values in this schedule. The air flow rate used for reheat coil sizing is reported with other component sizing information as “Reheat Coil Sizing Air Volume Flow Rate.”

Is this consistent with the results you are seeing? If you have specified a heating design air flow rate which is larger than this, then it sounds like the Damper Heating Action should be "Reverse" or "ReverseWithLimits".

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Asked: 2018-11-26 13:20:14 -0500

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Last updated: Dec 26 '18