Mismatch in outside surface heat balance in EnergyPlus

asked 2018-08-22 12:07:59 -0600

updated 2018-08-25 11:08:11 -0600

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).

When I am analysing the surface outside heat balance, there is often a mismatch in the balance: image description

The difference is significant especially when the value of the term Surface Outside Face Convection Heat Gain Rate (q''conv), is large.

Why do this difference occur? Is it for convergence reasons? The algorithm in some occasion skip to not match this heat balance?

Thanks in advance.

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answered 2018-08-23 15:10:14 -0600

MJWitte

9740 ●6 ●10 http://www.gard.com/

These should always balance to near-zero. I added the following output variables to example file RefBldgStripMallNew2004_Chicago.idf:

Output:Variable,*,Surface Outside Face Convection Heat Gain Rate,timestep; !- Zone Average [W]
Output:Variable,*,Surface Outside Face Net Thermal Radiation Heat Gain Rate,timestep; !- Zone Average [W]
Output:Variable,*,Surface Outside Face Solar Radiation Heat Gain Rate,timestep; !- Zone Average [W]
Output:Variable,*,Surface Outside Face Conduction Heat Transfer Rate,timestep; !- Zone Average [W]

I chose a couple of surfaces and found that the sum of these four variables is always near zero (residuals on the order of 1E-05 or smaller). You have to be careful with the sign convention of Surface Outside Face Conduction Heat Transfer Rate - it is the opposite of qko in the equation above.

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Did you analyse the whole period? Which periods did you analyse?

I did the same thing as you, I summed the 4 variables, in 7357 hours sum was zero, and 1451 hours was significantly superior to 0. During this periods, the term Convection Heat Gain Rate is 100 times superior than the normal convection rate. It seems that this term is causing this difference.