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# CBECC-COM Room/Zone needs to have at least 3 surfaces

Can someone please tell me where can I find the documentation for CBECC-COM or EnergyPlus explaining this requirement?

I'm using CBECC-COM for California T-24 simulation. And CBECC-COM requires all zones (including unconditioned zones) have at least 3 surfaces (and one of them needs to be the floor).

I'm trying to understand whether this is a CBECC-COM requirement or an EnergyPlus requirement. I also want to understand how CBECC-COM or EnergyPlus treats these 3 surfaces and how would the surface area affects the calculation. Does CBECC-COM use those three surface to construct a geometry?

I looked up the California ACM and the CBECC-COM manuals but didn't find anything. Thanks.

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To address the first part of this question, I believe this was added for the simplified geometry route where EnergyPlus was failing to converge and they found that having more mass would improve convergence. These surfaces will have an effect on the calculation, especially if adjacencies are defined, but going into much more detail than this will make the answer overloaded. A short example would be if you had an interior wall with an adjacency then during the simulation the heat transfer between both spaces and the interior partition is computed.

If you are pursuing the detailed geometry approach then a geometry will be constructed, but if using the simplified approach, which EnergyPro uses, then surfaces have no spatial awareness of each other and thus no shading effect.

The Title 24 non-residential ACM defines what does and doesn't need to be defined in the model. 5.5.10 "Heat Transfer between Thermal zones" is suggestive that partitions should be modeled with adjacencies defined, but it isn't that clear.

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thanks. The problem I have is that with all zones need to have at least 3 surfaces, the "simplified" geometry method is not that simplified at all. Sometimes it might just take less time to do the detailed geometry. Wish there is better way to solve the convergence problem.

( 2016-02-26 03:27:18 -0500 )edit

@XChen, I agree. I use the detailed geometry approach for all models. The simplified approach also can't perform exterior shading correctly nor can it account for daylighting meaning your project must install all prescriptive controls rather than just mandatory, or if you're installing more controls beyond prescriptive you won't get that benefit in the simplified approach.

( 2016-02-26 11:13:13 -0500 )edit