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Unexpected temperature results from custom EPW

asked 2024-05-03 13:46:37 -0500

gspahr's avatar

updated 2024-05-03 14:29:24 -0500

So I was trying out a simulation with a custom made EPW to evaluate the thermal resiliency of a building: upon a dramatic change in ambient temperature I want to look at how much time passes before the internal temperature drops 63% before settling to a final temperature (essentially a time constant for thermal lag evaluation).

  1. Through Ladybug Tools I modeled a simple and unconditioned 100 m2 house, I removed every possible heat gain from its program. I made 12 variations with changing EPS thickness (0, 1, 5, 10 cm) and brick thickness for both internal and external walls (2, 20, 40 cm).
  2. Through Elements I wrote a custom EPW with no global nor direct nor diffuse radiation, no infrared sky radiation, no wind. All of January sits at a steady 25 C, then February suddenly changes to a steady -5 C (30 C temperature drop). Relative humidity is steady 50% for both months. My simulation is only 2 months long so the rest of the year is not important.
  3. I simulated the house model with differing insulation and thermal mass levels. I'm interested in looking at the zone temperature changes after the aforementioned ambient drop in temperature.

Here comes the unexpected part. I was expecting for the initial temperatures to be much closer to 25 C, and the final ones to be closer to -5 C, instead every single case had a differing temperature. For the inital stage I thought that some heat is radiation out into the sky, which would make sense, since the baseline with no insulation (black line in graph below) experiences a lower zone temperature, however the same case also settles at a much higher temperature than the case with the higher insulation thickness (green line). I tried further increasing the insulation thickness and got temperatures that go even lower than outdoor temperature. For extra information, the sky temperature is 10.99 C in January and -23.61 C in February.

Please help me understand what is happening. Here are both the IDF and the EPW files.

image description

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answered 2024-05-06 09:15:04 -0500

PmP's avatar

updated 2024-05-06 09:15:33 -0500

Here is a diagnosis of the issue. 2 things are happening:

  1. You don't have a Ground temperature object, so it defaults to 18oC and your floor (face 0) has undesired heat conduction.

  2. The really cold sky during february is making it have lots of heat loss, that's what offsets the ground heat transfer and it just "coincidentally" seems like your balance is almost steady state with no heat transfer, when it's actually 2 heat fluxes balancing out.

This can be seen in the surface heat transfer variable:

image description

The ground temperature can be controlled with the ground temperature object, here is your .idf with a modification so the ground is at 25 in January and -5 in February:

Your problem is that now no surface balances the roof and the zone temperature is lower than the outside air because of the way to cold sky, which I don't know how to manually modify nor do I know if it would simplify things so much that the study would be academically solid or not.

In any approach that you decide, I wish you luck in your simulations, PMP.

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First of all thank you very much for taking the time to check out my post.

After you pointed out about the balancing fluxes the issue became more obvious. I did some testing with an adiabatic floor: I am now getting the expected results where a higher degree of insulation means higher zone temperatures than the ones where there is no insulation at all.

The main issue with my former file was the insulation was applied to the floor construction, which reduced the heat flux from the ground after the temperature drop. Some more testing is needed to try to cancel the radiative sky flux.

gspahr's avatar gspahr  ( 2024-05-10 09:06:02 -0500 )edit

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Asked: 2024-05-03 13:46:37 -0500

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Last updated: May 10