Orientation Error

After running a series of orientation tests on a proposed office building scheme in Fairfax, VA, I noticed a less than 2% shift in the energy use intensity between an E-W axis and a N-S axis. This appreared extremely perplexing, so I ran the same test on a very generic 60'x120' rectangular office building, placed it in Dubai, and received the same results. I'm confused. Orientation should have a dramatic effect here. Any idea where I might be going wrong?

Sean, Timothy, and Greg,

Thanks for running Vasari’s energy analysis feature through its paces and providing us feedback. Your findings are correct for the current zoning limitations in Vasari today. The team has been surprised by the number of users diving into the analysis features in Vasari and asking for more. A big validation that we need to accelerate these capabilities to meet industry acceptable practices.

Determining why several of you are seeing low energy differences between different orientations is a science project in itself, but I think I can provide an explanation. I suspect that the zoning is the main culprit. Vasari currently has limited automatic zoning capabilities, and some of the models I have seen created by users either have one zone or two zones (one perimeter and one core) per floor. The underlying analysis engine used, DOE-2.2 (eQuest), assumes fully mixed air for each zone, which can reduce HVAC loads for zones that have simultaneous heating and cooling loads. For instance, during a summer day, the “cooler” zone sides will passively “cool” the hotter zone sides through the “mixing” prior to the HVAC system seeing the hotter zone side’s full load. With industry acceptable zoning per ASHRAE 90.1 Appd. G guidelines, this effect would be minimized, and I would expect a noticeable (5-10% savings) for orientation changes in high aspect ratio floor plates with equal WWR for each façade.


The net affect is that GBS/Vasari can’t really give any feedback on the building form and how the big yellow object in the sky effects the energy performance? If that’s the case then how is this tool useful to determine and test against different options, which has always been the hook. It seems like you could simply do a exterior wall to floor area ratio and come up with the information you would be getting from GBS.


I have had a similar observation with this. I did another test of Vasari, modeling three completely different schemes, keeping all the energy settings similar and the building volume as constant as possible. I was expecting to see which building design was the most energy efficient from a form perspective. No difference. I reviewed this with some of my energy modeling experts and we have concluded that Vasari does not take into account incident solar radiation or direct solar gains in calculating heating and cooling loads. :( I am disappointed to have discovered this.

Changes to energy results from orientation changes depend upon many different factors: including the widow-to-wall ratio; zoning of the model; glazing characteristics; presence of window shading devices; other envelope characteristics; location; building type (and therefore with high internal loads vs a building more sensitive to climate because it does not have heavy internal loads); etc. In some cases the changes will be minimal and in some cases much more dramatic.

If you want to post your model, we can take a look at it to verify that everything is working as expected.