Help me understand static pressure in ducted minisplit systems
Tried to piggyback a comment on this thread (https://www.greenbuildingadvisor.com/comment/reply/55726), but I guess the thread is too old and my comment was marked as spam.
I am planning a ducted mini split for the 2nd floor of our home. I have run manual J calculations and find that a Fujitsu 9RLFCD will fit the heat/cool load, assuming I can distribute the BTUs to all the rooms.
In talking with a Fujitsu salesperson yesterday, he was skeptical I could make it work because, according to him, the duct runs can’t be more than 15′ or so from the unit. It is my understanding that there are no rules of thumb when it comes to ducting mini splits, so I am a bit skeptical of this 15′ rule of thumb.
I have done a little research and have determined the heat loads for each room, and what I believe are the CFM requirements to deliver those BTUs to each room. I have found calculators online that turn CFMs into duct sizes, and found the static pressure calculator at Engineering Toolbox. (http://www.engineeringtoolbox.com/duct-friction-pressure-loss-d_444.html) The calculator returns results of static pressure in “inches water” and in “inches water per 100 ft”.
What I don’t know is how static pressure works in relation to a ducted mini split. I see on the submittal sheet for the 9RLFCD, the static pressure rating is 0 ~ 0.36 w.c.. I have read that “wc” equals “water column”. Would “w.c” be the same thing as “inches water”?
If so, does that mean that the ductwork can’t exceed 0.36 w.c.? If I have calculated CFMs and can then find the static pressure of size and length and pipe and all the fittings, I need to make sure they all add up to less than 0.36? Do I have that right?
The good news in my case, the two bedrooms farthest from the unit just need ~50CFM each (if I have done my calculations right), and I have a pretty centralized floorplan, so hopefully it will be possible to get the job done.
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