Unexplained CO2 dilution with inter-zone air mixing

[matteomerli]

Issue overview

As I posted here, I’m currently observing unexpected CO2 reductions when connecting occupied zones to empty spaces through inter-zone mixing.

I recreated the problem in a simplified model consisting of one occupied room and an empty volume next to it. I tested the model both with and without air mixing, and tried three different sizes for the adjacent volume: 3 m³, 15 m³, and 150 m³. The fresh air enters only through infiltration (no natural or mechanical ventilation).

The chart shows the CO2 concentration in the occupied room on a typical afternoon.

It seems strange to me that the CO2 concentration is highly diluted even when connected to a small fresh air volume (3 m³), and that the differences between 3 m³, 15 m³, and 150 m³ are not as significant as the difference between no air mixing and air mixing with a small volume.

I’m wondering if there might be an issue in how E+ simulates the CO2 transfer between zones, potentially leading to an overestimation of the mixing effect, even if the CO2 transfer due to interzone air mixing seems to be correctly accounted for in the zone air CO2 balance algorithm.

Here's the IDF: idf

Details

Some additional details for this issue:

• Tested with EnergyPlus 23.2.0 and 24.2.0 (simulation timestep = 12)
• Air mixing tested using both ZoneMixing and ZoneCrossMixing objects (same result)
• Similar post: Indoor CO2 variation with inter-zone mixing vs no mixing

Checklist

Add to this list or remove from it as applicable. This is a simple templated set of guidelines.

• Defect file added (list location of defect file here)
• Ticket added to EnergyPlus Defect Complexity (Github Project)
• Pull request created (the pull request will have additional tasks related to reviewing changes that fix this defect)