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Heat Balance of a Thermal Zone

asked 2018-08-15 06:45:14 -0500

Vasco Zeferina gravatar image

updated 2019-03-05 06:56:40 -0500

dhollman gravatar image

I'm a new EnergyPlus modeller, and I am analysing the hourly and annual heat balances in different components/levels inside my single room model (6 surfaces 4 windows). I have analysed, explored and widely understood the following heat balances Zone Air Heat Balance, Surface Inside, Sub-Surface/Windows.

Then, I was expecting that the heat balance of the sum of all zone Surface Inside Face Conduction Heat Transfer Rate, Zone Windows Total Transmitted Solar Radiation Rate and Zone Total Internal Radiant Heating Rate would be equal to zero. The difference is significant, and I would like to have some feedback on what heat flows I am missing, please.

I acknowledge that this ignores the interaction of Surface Inside Face Net Surface Thermal Radiation between surfaces/windows.

Thanks in Advance

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@Vasco Zeferina Do you use any mechanical system? How about infiltration?

Avi gravatar imageAvi ( 2018-08-17 00:36:00 -0500 )edit

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answered 2018-08-17 15:11:00 -0500

As mentioned above, this balance appears to assume no HVAC system and no infiltration or ventilation. Surface Inside Face Conduction Heat Transfer Rate is only valid for opaque surfaces, so this is missing conduction through the window which for this particular view of the heat balance should be Surface Window Inside Face Glazing Net Infrared Heat Transfer Rate. Plus, reflected solar and visible internal gains can leave through the window, so you need Surface Window Shortwave from Zone Back Out Window Heat Transfer Rate. And are all of the internal gains 100% radiant? If not then you also need Zone Total Internal Convective Heating Rate. If you haven't found this already, all of the output variables are defined in the Input Output Reference - search on the variable name.

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answered 2018-10-19 10:51:27 -0500

lukanuts gravatar image

image description

Bearing in mind the above mentioned equation (from Szokolay). I have selected the followings for a simple one zone model

Surface Heat storage Rate/Energy (all the surfaces with thermal mass)

(int load-in case of only radiant internal gains) Zone Electric Equipment Total Heating Energy

(Solar heat gains) Zone Windows Total Transmitted Solar Radiation Rate Zone Windows Total Heat Gain/Loss Rate

(convective with no infiltration) Zone Ventilation Total Heat Loss/gain Energy

(conduction through building material and solar conductive heat gain) Surface Average Face Conduction Heat Transfer Rate Zone Windows Total Heat Loss/Gain Rate

(ideal loads) Zone Ideal Loads Zone Total Heating/cooling Energy

So far i haven't manage to demonstrate this balance.

Any suggestions, especially for evaporative heat-loss ? to my knowledge the only variables related to latent heat are the one of ideal loads and ventilation and internal loads.

thanks in advance

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Unless you are using HAMT, there really is no Qe in an EnergyPlus model. Because the latent and sensible do not interact, it is best to balance these separately. For this answer (really a question) and the original post, please share the idf you are starting with.

MJWitte gravatar imageMJWitte ( 2018-10-19 14:51:41 -0500 )edit

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Asked: 2018-08-15 06:45:14 -0500

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Last updated: Oct 19 '18