HVAC airflow balance

What could explain why I am seeing Volume flow rate (Standard Density) at the Supply node lower than the flow rate at the outdoor air inlet? I assume that the volume flow rate downstream of the outdoor air Inlet could only be the same (or higher if mixed with return air) ?

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Outputs show

Outdoor air fraction = 1
Economiser/HX bypass = 0
Inlet flow rate > Supply flow rate

I assume that EnergyPlus does a mass balance in the HVAC air loop. Is that correct? Are there any components in an airloop that can leak?

The results below are mostly ok for the second day. Other AHUs are showing much worse agreement.

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1.7k

TomB

asked 2017-10-08 02:09:22 -0500

4.4k

__AmirRoth__

updated 2017-10-08 09:42:42 -0500

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Zone exhaust fans and zone DOAS may cause this type of difference. The return node flow rate is equal to the sum of the zone supply inlet node flows minus the zone exhaust node flows.

I see where these numbers are coming from (i.e., the 25.243 inlet node flow and 20.460 supply node flow) however, what I don't understand is why this isn't happening all the time since the minimum OA flow is set to 25.243. I would suggest autosizing all the inputs or hardsizing them consistently. In your file the air loop is hard-sized throughout all the way to the terminal units where these are autosized. The total autosized flow rate for both terminal units is 20.46.

 AirLoopHVAC,
   AHU/L21/INT 1,           !- Name
   25.243,                  !- Design Supply Air Flow Rate {m3/s}

 Fan:VariableVolume,
   AHU/L21/INT 1 RAF,       !- Name
   NABERS HVAC,             !- Availability Schedule Name
   0.6045,                  !- Fan Total Efficiency
   600,                     !- Pressure Rise {Pa}
   25.243,                  !- Maximum Flow Rate {m3/s}

Component Sizing Information, Fan:VariableVolume, AHU/L21/INT 1 RAF, Design Size Maximum Flow Rate [m3/s], 20.46023
Component Sizing Information, Fan:VariableVolume, AHU/L21/INT 1 RAF, User-Specified Maximum Flow Rate [m3/s], 25.24300

 Controller:OutdoorAir,
   AHU/L21/INT 1 Outdoor Air System Controller,  !- Name
   AHU/L21/INT 1 Inlet pre HX,  !- Actuator Node Name
   25.243,                  !- Minimum Outdoor Air Flow Rate {m3/s}
   25.243,                  !- Maximum Outdoor Air Flow Rate {m3/s}

 Fan:VariableVolume,
   AHU/L21/INT 1 SAF,       !- Name
   NABERS HVAC,             !- Availability Schedule Name
   0.6,                     !- Fan Total Efficiency
   800,                     !- Pressure Rise {Pa}
   25.243,                  !- Maximum Flow Rate {m3/s}

Component Sizing Information, Fan:VariableVolume, AHU/L21/INT 1 SAF, Design Size Maximum Flow Rate [m3/s], 20.46023
Component Sizing Information, Fan:VariableVolume, AHU/L21/INT 1 SAF, User-Specified Maximum Flow Rate [m3/s], 25.24300

 AirTerminal:SingleDuct:VAV:NoReheat,
   Thermal Zone: Level 14:INT 1:AHU/L21/INT 1 No Reheat,  !- Name
   Autosize,                !- Maximum Air Flow Rate {m3/s}

 AirTerminal:SingleDuct:VAV:NoReheat,
   Thermal Zone: Level 30:INT 1:AHU/L21/INT 1 No Reheat,  !- Name
   Autosize,                !- Maximum Air Flow Rate {m3/s}

Component Sizing Information, AirTerminal:SingleDuct:VAV:NoReheat, THERMAL ZONE: LEVEL 14:INT 1:AHU/L21/INT 1 NO REHEAT, Design Size Maximum Air Flow Rate [m3/s], 11.17615
Component Sizing Information, AirTerminal:SingleDuct:VAV:NoReheat, THERMAL ZONE: LEVEL 30:INT 1:AHU/L21/INT 1 NO REHEAT, Design Size Maximum Air Flow Rate [m3/s], 9.28408