List of EnergyPlus mysteries and our explanations for them

EnergyPlus is the main engine used for energy, sizing and comfort calculations in Sefaira. 

Sometimes customers get results that are counter-intuitive or just plain weird. When this happens, we firstly make sure that we haven't sent the wrong information to EnergyPlus. Assuming we've sent the right inputs, we do our best to explain what's happening and why. 

This article shares some of the things we've discovered helping customers. Some of the answers are hypothetical, especially if we aren't able to chase down an official line from the EnergyPlus developers.

If you have any thoughts about the answers, feel free to note them at the bottom of the article!

Increasing SHGC increases heating sizing

It makes sense that increasing the Solar Heat Gain Coefficient of the glazing would increase cooling equipment sizing but why would it increase heating equipment as well??? This response curve was observed by a customer:


Although the heating coil load only goes up a small amount, there's a clear upward trend based on SHGC.

Importantly the following happens for heating sizing on a winter sizing day (at least for this project with the hours of proposed operation):

  • There is no solar gain
  • There are no internal loads

Theoretically this should mean SHGC has no impact on heating sizing.

However there is residual heat that has been absorbed by the building mass during the previous day that re-radiates into the space. We think it's possible that what is observed is that you have a slightly higher rate of heat being radiated back out through the glass at night where the SHGC is higher. This means that you then in-turn will have slightly higher heating needs during the day because the mass cools down a little bit more if more heat can escape through the glazing.

What do you think?

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