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E+, Huge heat transfer between floor and ground

asked 2018-09-27 00:40:15 -0500

Alexis's avatar

updated 2018-09-28 05:28:43 -0500

My issue is, I have a big heat transfer between my room and the ground.
I have set the "site ground temperature :building surface" at 24 C for all the months. I have tried Slab which give me approximately the same temperature (around 24 C).
I am surprised because the temperature in my room is also 24 C and when I check the result in zone loads compenent, I have - 4000 W.
The surface of my room is 270 m2. I find this a bit much. My floor is only concrete (20cm), no insulation. I think, I have set all the parameters correctly (U value = 3.75, conductivity = 2, ...). What make sense for me is HT = 270 x 3,75 x delta T (surface x U x difference of temperature).

I did something wrong ? What are the habits usually when we set the parameter for ground temperature in E+ ? Or this value make sense for you and I miss something ?
Thank for your help

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@Mantra Could you post a link to your model?

Avi's avatar Avi  ( 2018-09-27 11:35:31 -0500 )edit

Here is the link :
I put my slab model as well

Alexis's avatar Alexis  ( 2018-09-27 20:34:13 -0500 )edit

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answered 2018-09-29 12:42:53 -0500

updated 2018-10-01 11:37:45 -0500

Mantra, I'm updating my answer with some new info that might help you.

1) In the first figure, you can find the outputs related to the "floor's interior surface temp.", "floor's exterior surface temp", "VRF cooling load", and "floor's heat transfer rate through conduction". I performed the simulation based on Jodhpur weather file since I didn't have Bali's weather file.

2) As you can see, the cooling peak load (blue graph) occurs around May 25th, where the corresponding interior surface temperature is around 26.5C.

3) The yellow line shows that the outside temperature is constant (24C) throughout the year.

4) The second figure shows a simple calculation based on "heat transfer rate" and the "interior surface temp" exported from the first figure to an Excel sheet. It shows that the U-value is almost constant and it is about 10 W/m2k. The fluctuations are due to the changes in the air film's properties; however, the overall U-value is still very high.

So, it shows that the calculations are correct and whatever is causing the big numbers, is related to the material properties or to be more accurate the conductions. image description

image description

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My u value is around 10 without film and 3.5 with film (included my rsi and rso). My conductivity is 2 for my slab (only concrete).

What i dont really understand is How i Can get - 4000W. Because heat transfer = surface (270) x différence of température (0.5 or 1) x Uvalue (3.5 with film according to my enveloppe summary).
I have a problem of scale here.

Alexis's avatar Alexis  ( 2018-09-30 04:46:23 -0500 )edit

The formula and units are correct. I'm not sure how you would have 0.5 or 1 temperature difference. The Surface temperature that you have set for your model for all the seasons (24C) is the exterior temperature. Your interior temperature is set to be 20C (check ALL OBJECTS IN CLASS: HVACTEMPLATE:THERMOSTAT). So the difference in your temperature is about 4C during the heating days.

Ehsank's avatar Ehsank  ( 2018-09-30 11:48:26 -0500 )edit

I have changed this parameter, I put 24 C for the heating thermostat but I still have my -4000 W of heat transfer.
In my calculation, the delta T is equal to 4 C. So it means, the ground temperature is around 20 C.
I double checked my value, seems to be correct or consistent. I am blocked, Any other clue ?

Alexis's avatar Alexis  ( 2018-09-30 22:38:12 -0500 )edit

Ok thank you, I will assume that this value is correct because the inputs are corrects but I am still very surprised by this value, I did many simulations (not with E+), I never got this value for the floor.

Alexis's avatar Alexis  ( 2018-10-01 05:28:40 -0500 )edit

Just out of curiosity, what is the typical number you get for the heat transfer through the floor during the peak cooling load? And under what temperatures?

Ehsank's avatar Ehsank  ( 2018-10-01 09:25:29 -0500 )edit

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Asked: 2018-09-27 00:40:15 -0500

Seen: 543 times

Last updated: Oct 01 '18