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Difference between defined and calculated air flow

asked 2016-04-22 07:05:28 -0500

JM's avatar

updated 2016-04-22 08:05:48 -0500

Hey, I have modeled my building with ZoneVenitlation:DesignFLowrate objects in each thermal zone since I have only exhaust fans. The supply air is directly provided by the outdoor air.

In the ZoneVenitlation:DesignFLowrate objects I have set the type of ventilation to 'Exhaust', defined the Design Flow Rate, the schedule, the efficiency, and the pressure rise. I kept the default values in the other fields (e.g. constant term coefficient, temperature term coefficient, etc.)

I did not define the 'Design Specification Outdoor Air' since this is only used to size the HVAC systems, but I do not have any air loops under the HVAC Systems tab. (Please correct me if I am wrong)

When I run the simulation now and look at the Results summary I can see in the e+ 'HVAC Sizing Summary' report the 'Calculated Design Air Flow' and the 'User Design Air Flow'. Both values show me the same value because I have set the sizing factor to 1 but these Air Flows are not the one I have specified in the ZoneVenitlation:DesignFLowRate object. The values for the flow rate in the results summary are approximately 3 times greater than defined and also vary with each zone even though I have defined the same airflow for each zone.

So my question is what the reason for this could be and how I can fix it?

Below you can see the values I have defined and and the values as shown in the results summary.

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answered 2016-04-22 09:38:42 -0500

The flow rates shown in the 'Zone Sensible Heating' (and Cooling) subtables of the HVAC Sizing Summary are the results of the Zone Design Loads calculation. These are 'ideal' flow rates calculated from the zone load, the heating thermostat set point and the supply air temperature inputs in the Sizing:Zone object. They are used to size any auto-sized air systems serving the zone, and do not reflect any hard-sized zone HVAC component inputs.

The flow rates are calculated whether the zone has direct air supply or not, if the program is set to 'Do Zone Sizing Calculation' (which you can set in the Simulation Settings tab under 'Simulation Control'). In your case, since you don't have auto-sized systems supplying air to your zones, the calculated flow rates should not be used in the simulation.

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To build on @Eric Ringold's answer, the ZoneVenitlation:DesignFLowrate impact will show up under the "Simple Ventilation" column in the "Outdoor Air Summary" Section of the EnergyPlus or OpenStudio html file.

David Goldwasser's avatar David Goldwasser  ( 2016-04-22 10:38:08 -0500 )edit

Thank you Eric and David! I appreciate your help!

JM's avatar JM  ( 2016-04-22 10:44:56 -0500 )edit

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Asked: 2016-04-22 07:05:28 -0500

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Last updated: Apr 22 '16