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Eplus Conduction Rate and Uvalue of the wall check

asked 2018-10-01 04:55:04 -0500

lukanuts's avatar

updated 2018-10-01 09:48:33 -0500

Dear all,

I am performing a simulation assuming the following parameters

  • SurfaceConvectionAlgorithm:Inside,TARP
  • SurfaceConvectionAlgorithm:Outside DOE 2 -Heat Balance algorithm: ConductionTransferFunction
  • Zone air heat balance :AnalyticalSolution

Energy plus version 8.9

I have assessed the uvalue of a wall without surface thermal resistances because are both calculated by Eplus itself per each timestep.

Assuming that my wall has an U value of x W/m2K and i want to check if this value is the actual one calculated by the software, should I have to subtract from

Surface Avg Face Conduction Rate - Surf Outside HG Rate - Surface Inside HG Rate

and then divided it by the difference in temperature btw indoor and outdoor and the overall area? ( case without take into account conductive solar gain)

Following the previous for a wall of 0.98 W/m2K after the permutations i get a result that is almost two times. Why?

Furthermore If i divide the Surface Avg Face Conduction Rate by the difference in temperature btw indoor and outdoor and the overall area (without solar conductive gain) i don't get a result that is equal to the sum of the above mentioned uvalue plus the outside and inside convective coefficients .

Did I forget something?

I am very concerned because I want to check manually the reliability of the results and i still don't get any correct Uvalue, thus leading to higher thermal loads.

Thanks in advance

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answered 2018-10-12 07:30:42 -0500

If the surface construction has any thermal mass, this calculation will be nearly impossible unless you force a steady state condition. The winter design days that are included in the ddy files are typically constant temperature with no sun. Run a sizing period simulation for a winter design day and you should be able to match the heat flow with a UADeltaT calculation. You can use the indoor to outdoor deltaT if you include the convection coefficients, or use the surface inside and outside face temperatures with just the wall conductance (no convection coefficients).

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Why it is impossible? if i clear identify all the components of the heat balance, why can’t I get the U value (no sunlight and removing the superficial resistances)?

at the end of the day If I remove the convective rates and the heatstorage one i should obtain the correct amount conductive transfer rate to be divided by the delta btwn indoor and outdoor surface temperature).

Am I correct?

lukanuts's avatar lukanuts  ( 2018-10-17 03:16:03 -0500 )edit

Surface Average Face Conduction is not a fundamental result, it is an estimate. Same is true for Surface Heat Storage Rate. The descriptions in the Input Output Reference hint at this with the phrase "... in a nominal way." Heat storage within the surface is buried in the conduction transfer history terms and is never directly calculated in the heat balance. So, that's why it's best to use a steady-state case to begin with.

MJWitte's avatar MJWitte  ( 2018-10-17 16:27:57 -0500 )edit

Ok clear. So I can either use the summary tab from IDF file or the design days. thanks

lukanuts's avatar lukanuts  ( 2018-10-19 10:30:42 -0500 )edit

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Asked: 2018-10-01 04:55:04 -0500

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Last updated: Oct 12 '18