Hi Community
An edit before posting: I might have figured this out while writing my post. See "Possible solution" below. I am posting this to help others with the same problem, and to maybe get some confirmation of my "solution" from those who know more than me.
Sorry for long post but I have been trying to understand this for years. I have Googled and looked at some existing topics on this forum, but can't seem to find anything directly discussing my issue. I will CC this to DesignBuilder support for any comment, but I figure it may be more of an EP question than a DB question.
PROBLEM
Re: EP Slab pre-processor in EP 9.4.0-217a24fc09 with DesignBuilder v. 7.3.1.003 as front-end on MS Windows 11 platform.
I try to model vertical perimeter insulation board placed around the slab-on-ground (concrete surface bed in my local terminology).
For now I am using the old "ground domain"/SLAB pre-proc approach. (My next thing to try will be Kiva, which looks amazing, but I went for the GD approach because it seems more mature/tested.)
I set up a ground domain as per attached screen snips in DesignBuilder. When I have no insulation, everything runs fine. But with vertical perimeter insulation, I get the following from eplusout.err (full .err file attached):
************* Beginning Simulation
** Severe ** Site:GroundDomain:SlabDoEndOfIterationOperations: Out of range temperatures detected in the ground domain.
** ~~~ ** This could be due to the size of the loads on the domain.
** ~~~ ** Verify inputs are correct. If problem persists, notify EnergyPlus support.
** Fatal ** Preceding error(s) cause program termination
...Summary of Errors that led to program termination:
..... Reference severe error count=1
..... Last severe error=Site:GroundDomain:SlabDoEndOfIterationOperations: Out of range temperatures detected in the ground domain.
*************
************* ===== Recurring Error Summary =====
************* The following recurring error messages occurred.
*************
************* ** Warning ** Temperature out of range [-100. to 200.] (PsyPsatFnTemp)
************* ** ~~~ ** This error occurred 30 total times;
************* ** ~~~ ** during Warmup 0 times;
************* ** ~~~ ** during Sizing 30 times.
************* ** ~~~ ** Max=-1058.994812 C Min=-35351.626953 C
As I understand, a numerical method inside the slab pre-processor is going awry with temperatures shooting off to extreme (cold) values instead of converging nicely.
I have this problem on most occasions that I try to model vertical perimeter insulation. I tried changing aspect ratio, domain size etc thinking I was onto something, but ultimately these were proved to not be the critical parameters. I could never find a pattern to sucess/failure of the sims.
POSSIBLE SOLUTION
Looking again at Auxiliary Programs 2023 manual, Section 3.4 Slab configuration Drawing:
The slab configuration used in the slab model is a “slab in grade” model. That is, the slab top surface is assumed level with the outside earth surface. If a “slab on grade” configuration, having the bottom surface of the slab level with the outside earth surface is desired, the best approximation is to use the horizontal insulation configuration. The edge of the slab will have a small thermal resistance due to the two dimensional path through the earth, but the effect is small.
I interpret from this text that the fundamental mode of operation inside EP SLAB is to model slab-in-grade. Although it accepts slab on grade as an input, it seems this option causes SLAB to make some kind of approximation internally(?). This approximation seems to be internal to EP, not done in DB, because in.idf does show:
OnGrade, !- Slab Location (InGrade/OnGrade)
QUESTION
Am I correct that with those approximating parameters set inside EP/SLAB by EP, when I also request vertical perimeter insulation, the combination of parameters makes SLAB prone to instability? Also, perhaps it is not a good model of reality anyway, in such cases. i.e. even if it does run, I should be wary of using the results?
It seems that for my local slab-ON-grade industry norm (South Africa), with below-grade vertical perimeter insulation against the foundation wall (the below-grade wall above footing) I should rather model as slab-IN-grade with the vertical insulation ticked and just accept some physical inaccuracy due to the slab being modelled in-grade instead of on-grade. I have now done this with three different insulation specs, three sims in a row. All of them ran successfully! For me, this sim reliability was unheard of with slab-on-grade. I'd only get a successful run very occasionally, by luck I guess.
Thanks for any comments!
I don't have enough points to attach files, so here they are on OneDrive
