Are my sizing calculations correct? Two-zone v. single-zone?
Hello, GBA Community,
About three years ago, you provided helpful insights as I tried to determine the most efficient HVAC unit for our 1400 square foot home.
If you have time, I’d love to get your perspective on some additional analyses that I’ve done. (See attached.) Do the attached numbers seem accurate?
(1) On the heating side, we’re currently using about 13.5 MMBTUs per year. However, when doing a crude heat load calculation paired with a 1.4x sizing factor, it looks like we only need a 1-ton unit. Do my numbers seem right?
(2) On the cooling side, I used some equations referenced by PSU’s College of Earth and Mineral Sciences (https://www.e-education.psu.edu/egee102/node/2106, example 4) to compare the cost of our current system to new ones using different SEER ratings. When doing so, I kept the “time of usage” constant in this equation to make my comparisons (similar to keeping MMBTUs constant when looking at heating systems). Am I using the proper logic here?
Finally, I’m curious about your thoughts of running a ducted mini-split air-handler combo (12,000 BTU/1-ton) to cover my upstairs (700 square feet, 3 rooms/1 full bath), and a non-ducted mini-split (9000 BTU/.75 ton) to cover my downstairs (700 square feet/2 large rooms). It seems like we might realize some cost savings by utilizing a multi-zone set-up; however, it would be slightly oversized in comparison to just using one single-zone ducted mini-split air-handler for the entire home (18,000 BTU / 1.5 ton). What would you say is better in this scenario? A two-zone system, or a system that’s more accurately sized?
Thanks in advance for your time and insight.
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