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Why a negative zone volume?

asked 2017-11-09 19:25:30 -0500

maddog56 gravatar image

updated 2017-11-10 07:58:34 -0500

I have this very symmetrical building and EnergyPlus warnings as follows,

* Warning * Indicated Zone Volume <= 0.0 for Zone=THERMAL ZONE: 1ST FLOOR A * ~~~ * The calculated Zone Volume was=-188.78 * ~~~ * The simulation will continue with the Zone Volume set to 10.0 m3.

* Warning * Indicated Zone Volume <= 0.0 for Zone=THERMAL ZONE: 2ND FLOOR A * ~~~ * The calculated Zone Volume was=-188.78 * ~~~ * The simulation will continue with the Zone Volume set to 10.0 m3.

I can set the zone ceiling height to what I know it to be and the warnings go away. Leaving on auto calculate and I get negative volumes. Not a show stopper but just very curious as to what might be at play here.

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woah that's super weird. There is an OpenStudio script in SketchUp that resets origins - try running that and see if you still get the error. Maybe also try deleting the zone floor and ceiling and then redrawing them. You shouldn't be getting negative zone volumes. In earlier versions of EPlus, this would have thrown an error - see this issue on Github.

mdahlhausen gravatar image mdahlhausen  ( 2017-11-09 23:24:18 -0500 )edit

Is there a way to check that the faces are being exported properly & that they are oriented correctly? If the faces are reversed in Sketchup, when the plugin writes the IDF file, it could be essentially pointing the faces the wrong way and resulting in an incorrect volume calculation you could check this my checking what the cardinal orientation of the faces are in the EnergyPlus output tables.

kwalkerman gravatar image kwalkerman  ( 2017-11-10 10:12:52 -0500 )edit

Is this in EnergyPlus 8.8.0?

JasonGlazer gravatar image JasonGlazer  ( 2017-11-10 10:17:36 -0500 )edit

Re @kwalkerman's comment, look specifically at the inside walls for zone 1a and 2a. Noticed they didn't match properly. That could mean they are just not aligned, or could mean one is reversed from expected face normal.

David Goldwasser gravatar image David Goldwasser  ( 2017-11-10 10:20:35 -0500 )edit

EnergyPlus 8.8 changed the zone volume calcs. From my initial experience, it seems like this new code catches more bad geometries, but can fail on complex geometries and it is difficult to understand why. In this particular case, it looks like you've found the issue.

aparker gravatar image aparker  ( 2017-11-10 10:32:58 -0500 )edit

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answered 2017-11-14 14:09:56 -0500

updated 2017-11-14 14:19:50 -0500

For the particular file provided by maddog56, it turned out the some of the surface coordinate were entered in clockwise order instead of counter clockwise order as specified in there GlobalGeometryRules object. But no warning messages were issued to help provide guidance on the issue. To help users in the future, issue 6392 was added.

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Asked: 2017-11-09 19:25:30 -0500

Seen: 609 times

Last updated: Nov 14 '17