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How can I use the Slab Preprocessor when the floor is not insulated ?

asked 2019-02-24 06:19:14 -0500

tassimo4.0 gravatar image

updated 2019-03-02 12:39:55 -0500

Since the ground site temperature is a key factor in the simulation of residential buildings and small areas, I'm wondering if the Slab preprocessor is useful in predicting accurate GS-temperature of the non-insulated ground surfaces. And how can I use it?

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answered 2019-03-01 13:12:57 -0500

Instead of the Slab preprocessor, I would recommend using the Foundation boundary condition for a ground-contact surface in EnergyPlus. Instead of pre-calculating ground-contact temperatures in Slab, this option uses Kiva. There are three key benefits to this:

  • Kiva can take EnergyPlus inputs, pre-calculate ground-contact temperatures, then provide ground-contact temperature results as inputs to the EnergyPlus simulation. This is more integrated than running Slab to get temperatures as its own process, then taking outputs from Slab to provide as inputs to EnergyPlus.
  • Kiva can calculate ground-contact temperatures every time step. This is much more accurate than using the monthly average ground temperatures calculated by Slab.
  • Kiva allows you to simulate insulation placement inside or outside of footing walls that EnergyPlus normally can't handle due to its requirement of continuous material layers spanning entire surfaces.

You can read more about Kiva in Unmet Hours posts here and here.

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@Aaron Boranian This looks really good! Are there any shortcomings of this method as compared to the Slab and Basement preprocessors or the Site:GroundDomain:ETC objects?

Slab Basement can only handle rectangular domains, and choosing a floor boundary as a slab perimeter or slab core are a bit granular. Basement took 23 mins to run the example file. The Site:GroundDomain:ETC objects all had "undisturbed" ground temperatures in their descriptions, so do those really apply to ground under buidlings???

Kiva doesn't even ask for data that can be calculated from the weather file.

mldichter gravatar image mldichter  ( 2019-03-01 13:35:24 -0500 )edit

@tassimo4.0 Please change the accepted answer to @Aaron Boranian's. Kiva definitely seems like the way to go!

mldichter gravatar image mldichter  ( 2019-03-01 13:36:58 -0500 )edit

thanks for the answers !

tassimo4.0 gravatar image tassimo4.0  ( 2019-03-01 14:01:06 -0500 )edit

@tassimo4.0 By the way, you might want to just use the Kiva software directly instead of using it through energyplus if you don't need energyplus.

mldichter gravatar image mldichter  ( 2019-03-05 17:30:18 -0500 )edit

answered 2019-02-27 10:25:32 -0500

mldichter gravatar image

updated 2019-03-01 11:43:35 -0500

I'm wondering if the Slab preprocessor is useful in predicting accurate GS-temperature of the non-insulated ground surfaces

I'd say yes. Slab is accurate. Even if you don't have a layer in the slab that is specifically meant for providing insulation, concrete still provides some insulation and Slab will run.

And how can I use it?

That is not as easy to answer. Slab is quite old, and I think it's still written in Fortran.

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Your best bet is to use the Slab objects from an example file with a similar building to yours. You can find example idf files that use slab objects from this file in the energyplus example files folder.

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If you really want to dig into Slab, you can look in the Input Output Reference and Auxiliary Programs documentation pdfs. However, as you can see from the screenshot of the object links file, some of the Slab objects no longer have example files. I've had great difficulty trying to figure out how to use Slab in the past. I think Slab has gotten so old at this point that, while it works and is accurate, the people who wrote the code, and thus truly understood all the Slab objects, are retired. Good luck!

Post Script
I found more ground temperature modeling objects.

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The SITE:GROUNDDOMAIN:BASEMENT, SITE:GROUNDDOMAIN:SLAB, and related objects don't seem to be part of the Slab or Basement preprocessors. I ran the example files and I think they operate during the energyplus simulation, rather than the preprocessors which calculate the Site:GroundTemperature:BuildingSurface object and then energyplus runs.

Post Post Script
After looking at the available objects much closer, it looks like the SITE:GROUNDDOMAIN:BASEMENT and SITE:GROUNDDOMAIN:SLAB use models of the ground for "undisturbed" ground temperatures, which means temperatures of soil that have not been disturbed by construction. "They are not considered appropriate for calculations of building losses."

Then Slab is only implemented for rectangular slabs too, but it's used in models with zones having trapezoidal floors. Perhaps the shape of the slab refers to the entire building footprint, but that is not clear from the documentation.

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I'm very grateful to you, sir mldichter. Thank you so much for taking your time writing this exhaustive answer. What do you mean by 'old'? Are there any other easy to use applications for modeling residential building's ground temperature ?

tassimo4.0 gravatar image tassimo4.0  ( 2019-02-27 14:49:41 -0500 )edit

@tassimo4.0 Old as in the Slab code was written a long time ago.

As for modeling, I don't know of any other easy way to calculate ground temperatures from an energyplus model. Also, I don't think Slab actually runs during the normal energyplus simulation. Instead, Slab uses the materials and geometry of the slab to calculate the temperatures for the Site:GroundTemperature:BuildingSurface object and energyplus uses that object during the building simulation.

Using Slab really isn't that bad if you find a good example. Figuring out all the objects from scratch would be though.

mldichter gravatar image mldichter  ( 2019-02-27 15:24:53 -0500 )edit

Thanks a lot. Sincerely,

tassimo4.0 gravatar image tassimo4.0  ( 2019-02-28 04:34:31 -0500 )edit

@tassimo4.0 You're welcome. And please mark the answer as accepted.

mldichter gravatar image mldichter  ( 2019-02-28 09:22:24 -0500 )edit

@tassimo4.0 By chance I'm working on some building models that I need to run the Slab and Basement preprocessors for and came upon some more useful information. When I looked at the example files links, I didn't realize that there are only two example files. LgOffVAVusingBasement.idf and 5ZoneAirCooledWithSlab.idf are the only example files. So if you have an unusual building that doesn't match either of those, I guess you'll have to dig into the documentation.

mldichter gravatar image mldichter  ( 2019-02-28 18:07:38 -0500 )edit

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Asked: 2019-02-24 06:19:14 -0500

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Last updated: Mar 01 '19