Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
There is a lot packed into your question.
First of all, the mini-split capacity was set at 25 kBtuh for the existing home. I don't know how that was chosen, but when you specify the size of the equipment, then the sizing methodology (ACCA or otherwise) will not come into play. If you want to use ACCA sizing (or MaxLoad sizing, which is intended to prevent any unmet load), you need to autosize the equipment. The text you quoted "...while taking into account the heat pump's reduced capacity at the design temperature." pertains to the MaxLoad sizing methodology, not ACCA.
Second, the 25 kBtuh heating capacity is the rated capacity, so it occurs at an outdoor temperature of 47F. BEopt currently assumes a heating capacity retention (heating capacity at cold temperature compared to heating capacity at 47F) that is representative of models on the market. This obviously doesn't help you specify a unit that is higher than average, but in the next version of BEopt, we will be offering the option of specifying the heating capacity retention. That said, note that if you were to increase the heating capacity retention, the heat pump COP must get worse in order to achieve the selected rated performance (SEER/HSPF); that is a confusing concept for many users.
I don't know if I've fully addressed your questions, but hopefully it helps clear some confusion.
