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Why does Zone Air CO2 Concentration output vary by season

asked 2023-08-14 16:08:02 -0500

xchen's avatar

updated 2023-08-15 13:41:08 -0500

I plotted out the hourly "Zone Air CO2 Concentration" output and noticed that the CO2 concentration varies by season, even though the schedules I have do not. image description

Does anyone know the reason why this happens? It's a typical VAV reheat system. My zone has the People object with

0.0000000382, !- Carbon Dioxide Generation Rate {m3/s-W}

Yes,                     !- Carbon Dioxide Concentration
Outdoor CO2 Schedule;    !- Outdoor Carbon Dioxide Schedule Name

Outdoor CO2 Schedule,    !- Name
Any Number,              !- Schedule Type Limits Name
Through: 12/31,          !- Field 1
For: AllDays,            !- Field 2
Until: 24:00,400.0;      !- Field 3

System outdoor air. image description

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The amount of outdoor air changed?

rraustad's avatar rraustad  ( 2023-08-14 16:26:33 -0500 )edit

No the outdoor air does not change by season. And the supply air does not change by season either.

xchen's avatar xchen  ( 2023-08-14 16:27:18 -0500 )edit

Does infiltration change?

rraustad's avatar rraustad  ( 2023-08-14 16:29:21 -0500 )edit

This is a core zone without any exterior surfaces and no infiltration defined.

xchen's avatar xchen  ( 2023-08-14 16:38:23 -0500 )edit

So I changed the system to a 100% OSA, constant volume but still see higher CO2 concentration in summer and lower in winter.

xchen's avatar xchen  ( 2023-08-14 17:58:40 -0500 )edit

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answered 2023-08-20 02:03:45 -0500

Keigo's avatar

I'm interested in the issue and tested with an ExampleFile (V23-1-0) with weather data of Chicago, but the Zone Air CO2 Concentration output did not vary by season. I don't think your issue is a bug in EnergyPlus.

Your system outdoor air is constant, so economizer or nightcycle is not the cause.

Another likely cause is that your model is a typical VAV reheat system. If your system supplies air to multiple zones, your issue could happen. You said the zone is a core zone without any exterior surfaces and no infiltration. The load of the zone is supposed to be low and almost constant. If other zones have large cooling loads in summer, those zones have much supply air. Much outdoor air is supplied to other zones and less outdoor air is supplied to this core zone in summer, which results in higher Zone Air CO2 Concentration in this core zone. For VAV system, the amount of outdoor air to each zone varies even thouth the amount of system outdoor air is constant. Please check the Zone Air Terminal Outdoor Air Volume Flow Rate and System Node Mass Flow Rate of Air Outlet Node for all zones.

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It turns out the cause for the variation is because I had ZoneCrossMixing objects in the model, which mix air across a bunch of zones. After I took those out, the seasonal variation is gone.

xchen's avatar xchen  ( 2023-08-22 16:07:21 -0500 )edit

Mixing with zones with infiltration?

Keigo's avatar Keigo  ( 2023-08-22 18:52:25 -0500 )edit

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Asked: 2023-08-14 16:08:02 -0500

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Last updated: Aug 20 '23