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U value walls and windows

asked 2015-10-27 08:43:42 -0500

J Monteiro gravatar image

updated 2015-11-08 11:05:08 -0500

How can I change the values of superficial thermal resistance in OpenStudio? How can I see information about U values calculation in EnergyPlus?

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It would be my great pleasure if anyone could assist me with the name of EnergyPlus output that reports U-Value and SHGC of a window (already modeled as SimpleGlazingSystem material)? Thanks!

Mohammad gravatar imageMohammad ( 2020-09-03 03:11:29 -0500 )edit

@Mohammad you can find these outputs in the Envelope Summary option for table summary reports defined by the Output:Table:SummaryReports object.

Aaron Boranian gravatar imageAaron Boranian ( 2020-09-03 08:24:08 -0500 )edit

@Aaron Boranian Thank you so much for your hint. However, I need to have such output reported per timestep. In particular, I was wondering if EnergyPlus offers an option to add window U-Value and SHGC as an output through Output:Variable. For instance, do we have an option similar to "Surface Window SHGC"? Thanks!

Mohammad gravatar imageMohammad ( 2020-09-03 14:07:33 -0500 )edit

@Mohammad No, those timestep outputs don't exist. The closest you can get is if you model thermochromic windows then the EIO output file will contain overall properties (U-factor/SHGC/VT) of a thermochromic window at specific temperatures you define.

You could potentially calculate these parameters each timestep in post-processing.

Aaron Boranian gravatar imageAaron Boranian ( 2020-09-03 15:58:33 -0500 )edit

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answered 2015-10-27 10:41:31 -0500

EnergyPlus performs separate heat balances for the outside surface (short-wave solar + long-wave thermal + convection + conduction) and the inside surface (long-wave radiation from surfaces + short-wave radiation from lights + long-wave radiation from equipment + conduction + transmitted solar from outside surface + convection). The links will take you to the Engineering Reference for more info on the equations.

You can customize the surface thermal resistance with multiple surface property objects in EnergyPlus. Some examples of these objects are:

EnergyPlus calculates U-value using the thermal resistance values of each material layer within the surface's construction assembly, the inside surface film resistance, and the outside surface film resistance. To see the end results of the EnergyPlus U-value calculations, you should include the Envelope Summary Table in your list of outputs.

You should also check out the Engineering Reference document for how EnergyPlus handles window heat balance calculations differently than opaque surface heat balance calculations.

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I forgot to mention that if you simulate in OpenStudio, the Envelope Summary table is already included in your outputs. In the "Results Summary" tab at the bottom of the left side of the OpenStudio interface, you can select to view OpenStudio reports or EnergyPlus reports. The EnergyPlus reports are identical to the HTML table reports, while the OpenStudio reports include bar charts and pie charts of energy usage along with the table reports. EnergyPlus results are in SI units while OpenStudio results are in IP units. The envelope table shows U-values for window glazing only, not the assembly.

Aaron Boranian gravatar imageAaron Boranian ( 2015-10-27 11:44:32 -0500 )edit

But can I edit the value of the superficial thermal resistance in OpenStudio? Because for example I can do it in programs like IES?

J Monteiro gravatar imageJ Monteiro ( 2015-10-28 07:16:56 -0500 )edit

In OpenStudio, you can select which algorithm is used for inside and outside surface convection calculations. You can do this from the "Simulation Settings" tab (looks like a gear). It does not appear that you can directly set convection coefficients or other side coefficients from either the "Simulation Settings" tab or the "Constructions" tab in OpenStudio. This will require writing a new measure, as one currently doesn't seem to exist for this application on the BCL.

Aaron Boranian gravatar imageAaron Boranian ( 2015-10-28 10:34:32 -0500 )edit

@Aaron Boranian modeled a polyethylene Surface and according to literature its U-Value would be around the 8 but E+ calculated 3.7. I've changed the inside and outside convection coefficient according to what you said about SurfaceProperty:ConvectionCoefficients (I checked Surface Face Convection Heat Transfer Coefficient at Output:Variable and it had been changed to my intended number). But the u-factor of my surface hasn't changed!!! I've changed the thickness and conduction coefficient but they do not have remarkable effect. what should I do?

taher.ahel gravatar imagetaher.ahel ( 2019-01-07 08:26:14 -0500 )edit

answered 2015-10-27 09:02:26 -0500

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Asked: 2015-10-27 08:43:42 -0500

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Last updated: Sep 03