Yet another minisplit sizing question
I’m building a new home in Washington state (Seattle area, Marine 4C) and it should be fairly well built as I’m keeping the design simple, with rectangular dims and minimizing penetrations in roof/wall/floor. The builder claims “energy efficient” methods but they all do. Washington does require fairly progressive standards and a blower door test is standard fare. I won’t have results until the building is done and I’m planning for my mini-split now.
The building is 2800 ft^2, two story, R-21 walls, R-49 roof, R-30 floor, 0.30 windows. I’ve limited door/window area to around 280 ft^2 and I’m not installing a ducted system so there are no issues with duct leakage/pentetration into the shell. The house is on a crawl space, which I will eventually seal and run it with mechanical ventilation under a negative pressure relative to the living space. I’ll of corse mechanically vent into the outdoor heat pump. 🙂
I have considered running duct work to the bedrooms (all upstairs) and running a single panasonic inline fan to continuously circulate air from the bedrooms to downstairs (eventually into the crawl) I’ve considered this for two reasons. First, the main limitation of ASHRA 62.2 with a single point exhaust is the limitation of NOT necessarily pulling the air out of the locations where you need to dilute it. Second, I want to use one, perhaps two mini-splits on the entire structure and I want to promote even temp distribution.
By code I have to use the single zone electrical heaters, which will be my primary heat until the inspectors are gone. Once they are out of the way I’ll install a single mini-split downstairs, a second smaller capacity upstairs. I’m strongly considering the highly efficient Mitsubishi MSZ-FH (probably the 12,000 flavor). Most of our energy use is for heating but our weather is mild, rarely below freezing and when it is for short periods of time.
So…long diatribe here but I’m looking for confirmation on sizing. I think I can get by with the 1-ton downstairs as my primary heat source. The house has opening in the stairwell going up and an open area in the entrance where heat will naturally rise into the building. Upstairs I was seriously considering buying a cheaper Gree unit because it’s primary purpose will be cooling and not as mission critical as the downstairs unit which will be my primary heating source.
Any input or help is much appreciated. I’ve installed mini-splits in two different houses on my own so I’m fairly confident with the install. What I primarily need reassurance on is calculation of needed size.
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