Unrealistic Space Load Calculation Results with High Thermal Conductivity Ceiling Material

[TiejunWu]

Issue overview

In the attached example files, there is a simple shoebox building with a single conditioned zone and an unconditioned ceiling space above it. In the first example file, the ceiling material is 19 mm acoustic tile. In the other example file, the ceiling material is 1 mm steel panel.

For the zone, I have an ideal air system with heating setpoint of 18 C and cooling setpoint of 24 C.

When I look at the winter designday (OAT= -17.3 C) ideal air system load , for the case with acoustic tile ceiling, the zone heating load is 7584 W. For the case with 1mm steel panel ceiling, the zone heating load is 0.

For the steel panel ceiling surface, I can see the surface temperature is at 22.86 C. That seems to be heating the zone and keeping the zone temperature at 21.33 C, well above the heating setpoint of 18 C. That causes the ideal air system heating load to be 0.

The only difference between the two case is the ceiling material, the steel conductivity is much higher than the acoustic tile. But I am not sure how that caused the winter designday ceiling surface temperature to be so high. EnergyPlus did not report any warning or error.

Details

Some additional details for this issue (if relevant):

• Platform: Windows 10 64 Bit
• Version of EnergyPlus: EnergyPlus 24.1
• Unmethours link or helpdesk ticket number: NA

Checklist

Add to this list or remove from it as applicable. This is a simple templated set of guidelines.

• Defect file added (1. Acoustic Tile Ceiling; 2. Steel Panel Ceiling)
• Ticket added to Pivotal for defect (development team task)
• Pull request created (the pull request will have additional tasks related to reviewing changes that fix this defect)

[RKStrand]

@TiejunWu This is a known issue in E+. Any surface that has a single material with high conductivity like a metal is going to run into issues due to the CTF calculations having potential stability issues with something that thin and conductive. I don't believe there is a possible solution to this issue proposed inside the code. The best solution is to modify input by trying: an additional layer to give the construction more than just the metal or try a non-mass layer with the R-value equal to the 1mm metal. It's possible that there could be some sort of solution that we could come up with, but there are no guarantees and I'm pretty sure it would not be a simple fix.

@Myoldmopar This is kinda of a high risk and potentially high complexity issue. We've never come up with a solution mostly because we know the problem is the CTF solution and situations like this will lead to that sort of problem. However, I don't recall anyone spending any time trying to solve it and it's also possible that any solution could end up being a "one hit wonder" that doesn't catch all of the ways this could happen. What are your thoughts on this?

[TiejunWu]

@RKStrand Thanks for confirming this issue. Should we use a criteria such as R value less than a certain threshold to decide when to model a single layer construction as a no-mass material? Are there any general guidelines about what the threshold value for R should be?

[RKStrand]

@TiejunWu That's a good question and unfortunately the answer is: I don't really know AND it may depend on the rest of the input file (meaning we might not be able to come up with rules to avoid this). The guideline is simply to avoid constructions that have a single highly conductive layer (like metal). Your better bet is to create a fictitious construction that has similar radiative properties as the metal, defined as a no-mass material, and with a higher R-value than the metal. I realize that isn't much to go on, but without significantly more investment here, it's the best that I can do unfortunately.

[RKStrand]

@Myoldmopar Do you have any thoughts on this defect (see comments/questions made in a previous post here)?

[Myoldmopar]

Hmmm. I feel like we could probably come up with some guidelines, but I don't have anything off the top of my head. Need to go back and check some textbooks, theses and/or dissertations...

If it truly is a material that is "so conductive" that the CTF terms don't accurately catch the behavior, I would suggest the finite difference calculations, at least for that one surface. If you enable it for that one surface, you shouldn't have a significant impact on performance.