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openstudio/energyplus - performance discrepancy among versions

asked 2017-09-07 12:49:05 -0500

Matt Koch gravatar image

updated 2017-09-07 17:04:14 -0500

So, I first noted odd behavior in this here post (, but thought it might be necessary to make this more prominent and give it another, clearer and more dedicated post.

Basically, I am in the process of upgrading from OpenStudio 1.14.0 & EnergyPlus 8.6.0 to OpenStudio 2.2.0 & EnergyPlus 8.7.0. I am looking at a Proposed Building with Air-Cooled Chillers (ACC), Air Handling Units (AHU) and Variable Air Volume (VAV) terminals, and a Reference Building with Roof-Top Units (RTU) and Fan-Powered Box (FPB) terminals. For each, I opened the 1.14.0 .OSM file in OpenStudio 2.2.0 and then saved it as a 2.2.0 .OSM file from there. No other changes were made!

As it turns out, the 2.2.0 proposed building has a slightly different EUI and greatly different unmet hours from those of the 1.14.0 proposed building, even though they should be identical. The difference is even more stark when one looks at mixed air and outdoor air flows for one of the AHU, as well as setpoints and temperature for one of the VAV. This is shown in the images below. Note how the 2.2.0 version cannot even maintain setpoint, even though the equipment summary (i.e. equipment size) is identical for the 1.14.0 and 2.2.0 versions.

A) Proposed Mixed and Outdoor Air

image description vs. image description

B) Proposed Setpoints and Temperature

image description vs. image description

In contrast, the 2.2.0 reference building has identical performance to that of the 1.14.0 reference building, as I would expect. This is shown in the images below.

C) Reference Mixed and Outdoor Air

image description vs. image description

D) Reference Setpoints and Temperature

image description vs. image description

I find this rather striking and am worried about the accuracy of either 1.14.0/8.6.0 or 2.2.0/8.7.0 I should note that aside from a different type of HVAC system, the proposed building has hundreds of intricate shading surfaces where the reference building has none. I would appreciate any insight and can provide the .OSM files if need be. (Per the post mentioned at the beginning, I had already done an .IDF export file comparison, but found nothing striking).

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The release page for EnergyPlus 8.7 shows all the changes made in that release. I would look through the 'Defects Repaired' list to see if anything might be relevant to your model. Perhaps this one, which corrects a problem sizing reheat coils.

Eric Ringold gravatar image Eric Ringold  ( 2017-09-07 13:26:43 -0500 )edit

Yes, I looked there and found nothing that would ring a bell. Also, none of my coils - whether 1.14.0 or 2.2.0 - size to zero, so I doubt this is even applicable? Also, for good measure, I just now reduced the shading of the proposed building to that of the reference building, but left the ACC-VAV vs. the RTU-FPB system in place. I still get the same type of discrepancy, which is characterized by the fact that the unmet heating hours decrease ever so slightly (three hours), but the unmet cooling ours increase massively (factor of seven) when going from 1.14.0 to 2.2.0.

Matt Koch gravatar image Matt Koch  ( 2017-09-07 13:46:44 -0500 )edit

Again, what strikes me particularly odd is how 1.14.0 and 2.2.0 size identically, but during the simulation, 1.14.0 can handle the load, i.e.maintain setpoint, yet 2.2.0 cannot? Seems to suggest to me that in 2.2.0 either loads during simulation are calculated higher than during sizing, or equipment during simulation is not allowed to run all out?

Matt Koch gravatar image Matt Koch  ( 2017-09-07 13:49:39 -0500 )edit

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answered 2017-09-07 15:44:13 -0500

Matt Koch gravatar image

OK, I believe I tracked this down to the chilled water loop.

The following is the CHW loop as it appears in OpenStudio 1.14.0. Notice in particular that the demand side only has the two chillers but no bypass. This is what I want since I am trying to simulate a variable primary only loop.

image description

The following is the CHW loop as it appears in OpenStudio 2.2.0 after opening the 1.14.0 .OSM file from within it. Notice in particular that the demand side still has the two chillers but now it also has a bypass. This no longer simulates what I intend to simulate. The bypass, which appears out of nowhere during the 1.14.0 to 2.2.0 conversion, makes the loop behave totally different.

image description

When I delete this bypass in OpenStudio 2.2.0, then the results almost fall back in line exactly. Still not perfectly, but nowhere near as drastically different as with the bypass in place. For example the unmet heating hours are spot on identical, the unmet cooling hours are 1.67 hours higher for 2.2.0 than for 1.14.0. I can live with that a whole lot better than if they were seven times higher. Still, a small nagging doubt remains - why is 8.7.0 even slightly different than 8.6.0 - shouldn't all bugs that affect something so fundamentally standard as a chilled water loop have been ironed out by this high a software revision?

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Asked: 2017-09-07 12:49:05 -0500

Seen: 145 times

Last updated: Sep 07 '17