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Assembly U-factor, SHGC and VT are not calculated correctly

asked 2023-10-25 03:34:40 -0500

Keigo's avatar

updated 2023-10-25 09:06:45 -0500

Envelope Summary reports Assembly U-factor, Assembly SHGC and Assembly Visible Transmittance when the energy model has WindowProperty:FrameAndDivider, but the weird thing is that the same values are reported regardless of the ratio of the window area to the frame area or the shape of the window.

Let me show you one example. I added WindowProperty:FrameAndDivider to an ExampleFile 5Zone_IdealLoadsAirSystems_ReturnPlenum.idf. Besides, I unified all the fenestration surfaces into one construction type for clarity. The model has 6 exterior fenestration surfaces. image description

The Envelope Summary is shown below. All the fenestration surfaces have the same Assembly U-Factor although the the ratio of the window area to the frame area is different. The same is true for the Assembly SHGC and the Assembly Visible Transmittance. Is this a bug? In my understanding, Assembly U-Factor is the area weighted average thermal transmittance of all components including glazing, frame and divider. The percentage of the frame area should have an impact. image description

This question was posted 8 years ago, but can't EnergyPlus still calculate assembly U-factor, SHGC and VT?

The idf file (V23-3-0) and the html file are here.


I made one window (WL-1) smaller, but the Assembly U-factor, SHGC and VT remains the same. I think this is a bug.

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It is possible that the size of the windows is so large that the frame and divider impact is too small to see for the number of digits displayed in the report. You might want to try this experiment with two windows that are much smaller but different sizes.

JasonGlazer's avatar JasonGlazer  ( 2023-10-25 07:02:46 -0500 )edit

@JasonGlazer Please find my UPDATE above.

Keigo's avatar Keigo  ( 2023-10-25 09:07:40 -0500 )edit

Sorry, I am remembering now how this works. The assembly is based on the standard size shown in the "NFRC product type for assembly calculations" field. These are a standard size so it does not vary based on the actual size of the window. So this is working as it is intended, I believe.

JasonGlazer's avatar JasonGlazer  ( 2023-10-25 09:22:44 -0500 )edit

@JasonGlazer I see! Thank you for your clarification. I agree that this is working as intended. It's just different from what I expected.

I tested several cases. The assembly is based on the standard size, so the actual size of the window does not matter, but the actual Frame Width matters. The outermost size of the standard window is fixed, so the window area decreases as the frame width increases.

Keigo's avatar Keigo  ( 2023-10-25 11:09:27 -0500 )edit

1 Answer

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answered 2023-10-25 14:57:08 -0500

updated 2023-10-25 19:16:37 -0500

Hi @Keigo. I'm just sharing a few test results. @JasonGlazer's feedback should be upgraded as the accepted answer.

I added a Frame&Divider object to the 4 windows in the office zone of the US DOE Prototype Warehouse. I compared E+ v23.1 vs v23.2. Same results. I then changed the "CurtainWall" NFRC product type (2.00m x 2.00m NFRC size) to "DoorSidelite" (0.60 m x 2.09m) :

image description

  • Top results : "CurtainWall"
  • Bottom results : "DoorSidelite"

As @JasonGlazer stated, the Assembly U-factor, SHGC and VT remain the same for all 4 windows (regardless of their actual sizes), but they do differ if one changes the NFRC product type.

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Asked: 2023-10-25 03:34:40 -0500

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Last updated: Oct 25 '23