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Error PsyRhoAirFnPbTdbW: RhoAir (Density of Air) is calculated <= 0 [-2.17716]

asked 2015-07-20 16:53:16 -0500

apetersen gravatar image

updated 2015-08-31 13:45:40 -0500


I am working on a model that has a fatal error that I am having trouble getting rid of. The error looks like this: PsyRhoAirFnPbTdbW: RhoAir (Density of Air) is calculated <= 0 [-2.17716]

Essentially I think the program is doing a psychometric analysis and calculating a negative air density, which is obviously wrong. Someone using designbuilder had this error and the suggested solution was that the air temperature distribution needed to be changed from "Dynamic Gradient" to "Mixed". Since I searched my IDF for "dynamic" and it was not in my project, I don't believe this is my issue. To give a little more background this error occurs after the model has completed zone sizing, but before the model completes system sizing. This error occurred after I added a unit heater into the model.

Any help would be appreciated!


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If possible submit the input file to EnergyPlus help desk so that development team can take a look into it.

Chandan Sharma gravatar imageChandan Sharma ( 2015-07-21 01:47:44 -0500 )edit

Thank you. I submitted the input file and will let Unmet Hours know if the answer is helpful to the community

apetersen gravatar imageapetersen ( 2015-07-21 09:11:51 -0500 )edit

2 Answers

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answered 2015-08-27 10:25:28 -0500

updated 2015-08-27 10:26:21 -0500

The culprit in this case turned out to be bad u-values for the walls doors of the walk-in coolers in this supermarket model. This resulted in a very large heat removal from the adjacent zones causing a very cold air temperature to be passed to the psychrometric function which tripped the fatal error.

How did we find this?

  1. The full error message included "tdb=[-427.76]" which gives a clue that something is very cold.
  2. The autosized heating supply air flow rate for some zones was very high (e.g. 1107880 m3/s). This gives a clue that this is an envelope or internal gains problem, because the full system is not modeled during sizing.
  3. The refrigerated cases and walkin coolers were the likely cause of the large heating load. Tested this by deleting all refrigeration objects, and the error was gone.
  4. After poring over the details of all the refrigerated cases, @apetersen found the problem in the walkin coolers. Every one of them had these inputs:


255.599352755867,        !- Insulated Surface U-Value Facing Zone 1 {W/m2-K}
255.599352755867,        !- Glass Reach In Door U Value Facing Zone 1 {W/m2-K}
255.599352755867,        !- Stocking Door U Value Facing Zone 1 {W/m2-K}


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answered 2015-08-28 08:25:34 -0500

pow_skier gravatar image

Hello Colleagues,

Yes. I found this to be the case to. When I got that error it ended up because of the U value for the Walkin was all off. It seemed to happen when working in OpenStudio at IP units. I found that by switching back to metric units before I simulate the U=value I assigned would stay and not default to the large 255 number that errors out the program.

I used 0.285 W/m2-K = R-20 (U=0.05 Btu/hr-ft2-R) and it ran no problem.


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@aparker Note that @apetersen also mentioned this problem originated in OpenStudio. Adding a tag.

MJWitte gravatar imageMJWitte ( 2015-08-31 13:44:55 -0500 )edit

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Asked: 2015-07-20 16:53:16 -0500

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Last updated: Aug 27 '15