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Change in energy flow through envelope vs internal gains

asked 2021-11-10 12:50:28 -0600

Screamindaemon gravatar image

updated 2021-11-11 09:56:07 -0600

Good morning all,

I'm trying to back calculate the simulation results of a test project, mostly for my own learning purposes. I've come across an oddity that isn't covered (from what I can tell) in ASHRAE's handbook/fundamentals.

The building is a simple block of 100'X100'x10'. The walls have R-20 effective insulation, Roof has R-30 effective insulation, and the ground is full under-slab insulation of R-7.5. There is one window on the east facing of 100'x2.97' with a SHGC of 0.7.

When the building is occupied, or there are solar gains through the window, I am seeing a significant change in the energy through the ground, and a slight change of the energy through the walls. The internal dry bulb temperature is not changing at all. Internal radiant temperature is changing slightly, but not enough to justify the large changes in wall and ground loads. My ground surface tempteratures are changing and correlate somewhat well to the changes, but no linearly, and my wall surface temperatures do not correlate at all.

If we use heat loss of q = UA(t1-t2) for the walls and q = FP(t1-t2) for the ground, and the internal temperature is not changing with loads, is there anything that I'm missing that would explain these dramatic changes in energy?

I have attached an image of the first 10 days of the year with as many environmental details and loads that I could fit.

I am at a bit of a loss about what is happening. I suspect this is a surface temperature issue, but I don't know how to solve it in that case.

Energy Balance

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answered 2021-11-11 14:13:44 -0600

Sounds like you're calculating conduction only. Are you also quantifying convection and radiation? Are you incorporating infiltration?

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answered 2021-11-17 19:29:15 -0600

Screamindaemon gravatar image

Hi, thanks for your reply.

Yes, my calculations are including conduction, convection, and radiation from loads and external ventilation.

I have found an answer to part of my question. The Radiant Time Series method from ASHRAE now strongly agrees with my wall and roof calculations. However, that method does not provide any information for ground heat transfer.

Is there any information regarding incorporating RTS with EnergyPlus' Ground Heat Transfer/Slab program or Kiva? Or since the changes in ground energy transfer strongly correlate with the inverse of internal gains (solar, metabolic, lighting, ventilation) can I assume a ground heat loss through slab with the negative of the internal gains? It doesn't look like it's a perfect inverse, but perhaps it's close enough?

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Asked: 2021-11-10 12:50:28 -0600

Seen: 99 times

Last updated: Nov 17 '21