I'm running models comparing 2018 energy code to 2024 energy code and noticed that when i reduce air changes per hour from 5 to 3, BeOpt actually says the energy costs go up. This happens when i leave every other option the same and ONLY change the ACH from one model to the other. Can anyone explain why reducing the air leakage would cause increased costs? or is there an error in the program?
Thanks!
Thanks for emailing me your BEopt project files. I took a quick look.
To be honest, the changes in energy use between the two files are so small (0.1 MBtu/yr or less) that they are essentially equivalent. And the reason they are essentially equivalent is because there is a mechanical ventilation system (that is being sized per ASHRAE 62.2). As the infiltration rate decreases, the mechanical ventilation airflow (and corresponding fan power) increases to achieve the same total air exchange for the home.
If you were to A) remove the mechanical ventilation system, or B) assign a hard-coded airflow rate of say 100 cfm to the mechanical ventilation system, then you would see the expected decrease in energy from 5->3->1 ACH50.
(Aside: It's worth noting that these three homes do not have equivalent ventilation/comfort. The home with 1ACH50 and a larger mechanical ventilation system is getting more consistent ventilation/comfort throughout the year, whereas the home with 5ACH50 will have more varying ventilation throughout the year -- based on wind speeds and outdoor temperatures.)
The energy impact of decreased infiltration can be reduced through the use of recovery ventilation (ERV or HRV) rather than the conventional exhaust-only ventilation. The fan must run regardless, but a sizable portion of the energy spent conditioning the energy is saved with an xRV.
Asked: 2025-01-08 14:12:42 -0600
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Last updated: Jan 09
