Tighter homes use more energy?

beopt
infiltration
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I am running some energy code analysis simulations and BEOpt 3.0.1 is showing unexpected results.

When running a parametric analysis comparing air tightness at ACH 7, ACH 5, ACH 2, and ACH 1 the simulations tend to show the 5ACH home using LESS energy than the 2ACH home.

This is the opposite of what I saw when running BEOpt 2.8 where the tighter the home got, the less energy it used. What has changed with the new version of BEOpt where a leaky home now uses less energy than a tight home? It doesn't make sense to me.

I'm running this single family home model in climate zone 5B and using the default exhaust ventilation as the mechanical setting.

843

asked 2024-06-18 15:23:36 -0500, updated 2024-06-18 15:30:45 -0500

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I'm a design builder user and primary work on commercial projects, but also do some single family home residences. For context, most of my projects are in Vermont. Despite the climate, I show large cooling loads when the building envelope is very well insulated and tight. I show cooling dominant geothermal loads on most projects (added compressor and pump heat to the ground loop).

I'm not familiar with BeOPt and the ventilation requirement.

Here are some questions and comments for you to consider:

is there air conditioning? is your model showing cooling dominant?
If so, you may be benefiting from infiltration when there are cooling loads and the outdoor conditions are mild (40F-60F) do you have large south facing windows? do you have skylights? what is the window SHGC? are you modeling window shading(interior or exterior)? are you modeling natural ventilation for cooling?

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Greg Estep

answered 2024-06-19 09:38:36 -0500

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There is a standard AC and forced air gas furnace in the model. What confuses me is that BOTH the cooling and the heating usage goes UP as the infiltration goes down in the latest version. The really confusing thing for me is why this is the reverse of the exact same scenario in BEOpt 2.8.

If I take two identical models, one in 2.8 and one in 3.0.1 and the only thing I change is the ACH, and I run a parametric analysis, the energy use goes down as the home gets tighter in 2.8 and it goes up as the home gets tighter in 3.0.1

DW (2024-06-20 13:10:46 -0500) edit

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After some digging, it seems that it has to do with how the ventilation fan is functioning. When the home is tighter, the exhaust fan uses more energy. I've also found that there is an incremental cost associated with going to lower ACHs that can be configured in the option manager.

By using the Central Fan Integrated System CFIS, I was able to get clearer results for what I wanted (a home without a forced supply or exhaust system that has fresh air simply ducted into the return). Thank you.

843

answered 2024-06-20 17:00:37 -0500