Ideal Air Loads Analysis - Strange results when testing SHGC

I am trying to run a simple analysis to show the impact of SHGC on annual cooling loads. The results were showing that as the SHGC increased, the annual cooling load would increase linearly (as expected) - EXCEPT from 0.25 to 0.30 SHGC. The attached chart illustrates what I am talking about. I tried a lot of things - and kept getting this result. Full disclosure: I am not an Energy Plus wizard.

I have the ventilation air set to 0 and the economizer turned off because I wanted to see total zone cooling loads without any contribution from outdoor air.

I attached a very simplified GH file (sorry I couldn’t figure out how to embed the epw file and too big to upload here; I am using the San Francisco International Airport TMY3 epw file.) I’m at a loss for what could be causing this. Any advice is appreciated! Thanks so much in advance.

(FYI - my Rhino units are set to inches and yes I understand that is bad form :yum:)

SHGC Analysis Test (582.0 KB)


It might be your surface convection. As the solar radiation heats surfaces, convection at the surface will vary with temperature, temperature difference, and changing air properties (amongst other things). Specifically, this might be a case of the exterior/interior air films in your glazing layers heating up non-linearly and resulting in a non-linear cooling load.

Try plotting the sum of the Window Beam Energy and Window Diff Energy output with the Glaz Energy Flow against the same SHGC changes. Both of these outputs are in the Read EP Surface Result component. That way you can separate out the radiation from the conduction/convection in your windows, and confirm if the former is linear and the latter is not.


Thanks for your response. I plotted these glazing surface variables - it’s definitely the Glaz Energy Flow that is driving these strange results. Do you have any advice to make this not happen? I am using the EPWindowMat to define the construction.