Cooling Load Calc for Sizing a Ductless Minisplit
Last winter, I received a heating & cooling load calc for my house. For cooling, the design temp was set to 93F and the indoor temp to 72F, however the ASHRAE design temp for the Detroit area is actually 87F, and I’ve read that indoor temp for cooling load calc should be set to 75F. In other words, the HVAC guy was erring on the side of oversizing, which apparently is very common.
However, I’ve also read that for an inverter-driven system such as ductless minisplits, it’s OK to oversize a bit because the system can run lower than the rated capacity, and in fact is more efficient when running closer to 50% (?) of rated. (Obviously, it can be too oversized if the minimum capacity exceeds the load.)
With those two things in mind, I’m wondering… should this ductless system design be based on the actual 1% temps for Detroit, or the more extreme temps (0.1%?)? I don’t know which would be better in terms of both comfort and efficiency. Do the ductless system ratings already take design temps into account? For example, I noticed that Mitsubishi’s 9K unit is rated for 9, but has a max capacity of 12K. This also happens to correspond with my upstairs load calcs… roughly 9K at ASHRAE design temps, and 12K using more extreme temps. Does that mean the 9K model would have peak performance, yet still be able to handle the extreme days? Or would it be better to use the 12K model and not have the 9K’s inverter hitting max capacity (i.e. lower efficiency) on the high temp days?
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