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# difference Site Outdoor Air Drybulb Temperature and Zone Outdoor Air Drybulb Temperature

Why there is difference between these two?

Site Outdoor Air Drybulb Temperature
Zone Outdoor Air Drybulb Temperature

Site Outdoor Air Drybulb Temperature
26.7135 26.409 26.19375 26.00475 25.8525 25.8735 26.18325 26.99175 28.13625 29.25975 30.25725 31.0185 31.575 31.96875 32.1 31.8795 31.428 30.8505 30.03675 29.23875 28.61925 28.05225 27.55875 27.11775

and

Zone Outdoor Air Drybulb Temperature
27.104459 26.978256 26.866087 26.765204 26.671216 26.593249 26.556566 25.089114 25.0 25 25 25 25 25 25 25 25 25 25 27.502535 27.858049 27.670282 27.519915 27.382448

strange is that difference (Site Outdoor Air Drybulb Temperature - Zone Outdoor Air Drybulb Temperature) is fixed for each hour ...where this come from?

0.00325 0.00325 0.00325 0.00325 0.00325 0.00325 0.00325 0.00325 0.00325 0.00325 0.00325 0.00325 0.00325 0.00325 0.00325 0.00325 0.00325 0.00325 0.00325 0.00325 0.00325 0.00325 0.00325 0.00325

I just run

Simulation Control

Do Zone Sizing, Do System Sizing, Do Plant Sizing, Do Design Days, Do Weather Simulation, Do HVAC SizingSi 1, Yes, Yes, Yes, Yes, No, No

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Did you paste the wrong values? The differences between the two columns of numbers above are not 0.00325.

( 2021-11-29 12:54:49 -0600 )edit

@shorowit you are correct, the difference is varying for this example... but my whole question is about definition and understanding difference between two values

( 2021-11-29 13:00:32 -0600 )edit

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I think the Zone Outdoor Air Drybulb is the Site Outdoor Air Drybulb recalculated to account for the temperture difference due to atmospheric variation, which in EP is based on zone/surface centroid height[1]. Site wind speed is recalculated for the same reason.

I briefly skimmed the I/O reference, but can't find the actual term, but I'm sure it's in there somewhere.

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@saeranv thanks for the reply ... Q1so if my weather data is .epw KOR_SO_Seoul.WS.471080 at Altitude = 87.1 this is where my Site Outdoor Air Drybulb and Zone Outdoor Air Drybulb is recalculated for the height of my walls/building, is 4m so at 2m?

Q2:so in https://bigladdersoftware.com/epx/doc... DOE-2 Model it uses Zone Outdoor Air Drybulb or Site Outdoor Air Drybulb? , Q3 same here? https://bigladdersoftware.com/epx/doc...

( 2021-12-01 02:25:34 -0600 )edit

I can only say that, based on my interpretation of the EP reference, the answer is yes for all three, but caveat: I haven't looked at the code, so I'm going purely off the written documentation. For Q1: the EP ref specifically says "The zone centroid or surface centroid are used to determine the height above ground."

( 2021-12-01 04:20:42 -0600 )edit

For Q2 and Q3 they explicitly mention "Only local outdoor air temperature and wind speed are currently calculated because they are important factors for the exterior convection calculation for surfaces (see Exterior Convection below) and can also be factors in the zone infiltration and ventilation calculations."

( 2021-12-01 04:22:16 -0600 )edit

If there's no explicit exceptions mentioned in the documentation of specific algorithms, I see no reason to assume they would use the site outdoor drybulb rather then use the recalculated drybulb with respect to height.

( 2021-12-01 04:26:01 -0600 )edit