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How does ZoneCrossMixing object in zone affects sizing?

asked 2023-11-08 11:37:04 -0600

xchen's avatar

updated 2023-11-09 08:06:05 -0600

I have ZoneCrossMixing objects in both the source and receiving zones with a delta temperature of 0 (i.e. mixing occurs regardless of the relative air temperature). Does the ZoneCrossMixing object affect the zone equipment sizing and the system sizing, e.g. ATU airflow in a VAV system, AHU sizing (both zones under the same AHU)? If so, how? I was not able to find any documentation for this.

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answered 2023-11-08 11:57:48 -0600

The easiest way to tell is to run a file with and without the object and check the zone/system sizing details in the eio file. I ran the example file 5ZoneNightVent2.idf and then again without the ZoneCrossMixing objects and do see a very small difference in zone loads and zone design air flow. Even though the ZoneCrossMixing objects mixed between Space2-1 and Space4-1 this change had a small impact on all zones (less than 1 Watt or 0.03%). Red is the original file and yellow is the file without the ZoneCrossMixing objects.

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System sizing will also be affected when zone design air flow rates change.

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Thanks rraustad. What is the zonecrossmixing airflow you used in this exercise, and how much higher or lower is it compared to the airflow supplied by mechanical conditioning airflow? I assume in your case, there is only natural ventilation and no mechanical ventilation? In my model, I notice that because I have high zonecrossmixing airflow rates in the zones, e.g. 5-15ACH, the hourly outdoor air intake of the AHU is increased compared to a similar model without the zonecrossmixing objects. So I was wondering if the high zonecrossmixing airflow affects how the zone supply airflow is sized.

xchen's avatar xchen  ( 2023-11-08 13:37:03 -0600 )edit

This is easy to test with your file. Just remove the ZoneCrossMixing objects and compare the eio results with the original file. As for my results above, you can look at the example file 5ZoneNightVent2.idf to see what values the ZoneCrossMixing object used.

rraustad's avatar rraustad  ( 2023-11-08 14:05:06 -0600 )edit

Thanks rraustad. I will try look into the example file. I also want to know how this is calculated in the engine, something similar to the steps outlined in the Engineering Reference. Is there any documentation for that?

xchen's avatar xchen  ( 2023-11-08 18:01:56 -0600 )edit

If you search the Engineering Reference for "ZoneCrossMixing" you will see the equations.

rraustad's avatar rraustad  ( 2023-11-08 19:35:25 -0600 )edit

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Asked: 2023-11-08 11:37:04 -0600

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Last updated: Nov 08