Heat pump decision time
I have been holding off making a decision as long as possible in the hopes that a perfect solution would magically show up, but it hasn’t and I can’t hold off any longer. I hired an engineer to perform a Manual D, J and S and the mechanical subs have done the same. The results were pretty consistent. The “consensus” design includes a single-zone system each for 1) family room/kitchen, 2) living room/foyer, 3) second floor lounge; and 1 multi-zone system with a medium static slim duct unit for the bedrooms/conditioned and unvented attic and a ductless wall mount for the basement. The 2 manufacturers I am considering are Mitsubishi and Daikin. Fujitsu and LG are not options as there are no qualified installers in my area. The issue I am having is a common one on GBA, low design loads coupled with units that don’t modulate low enough to keep the system from cycling. This only applies to the bedroom/attic unit in my case. My heating and cooling design loads for the 3 bedrooms and attic combined are 5964 and 7765, respectively. In a multi-zone setup, both the Mitsubishi and Daikin 9k units minimum operating levels are greater than my design loads which should result in cycling and less than adequate dehumidification. In fact, the Mitsubishi Manual S showed that the 9k unit could not handle the latent load. Daikin has better dehumidification capabilities and latent is not an issue with their unit. However, the Daikin installer recommended using their zoned ducting kit so each bedroom and the attic would be its own zone, which would just make the problem 4x worse. The alternative I am looking at is a single-zone for the bedrooms and a single-zone for the basement – or none in the basement, we don’t really have a dire need for AC/dehumidification/heat in dry NJ basements. The 9k single-zone medium static slim ducted unit from Mitsubishi would modulate down to about 4k btu’s and would handle my heating and cooling needs at design loads (I need to confirm this for the Daikin unit). However, neither company offers hyper-heat when their slims ducts are configured for a single-zone. Mitsubishi locks out at -7F, and Daikin’s engineering manual only shows btu output down to 5F, so I am not sure if it locks out or not. Although my design temp is 14, we do get an occasional polar vortex. This would require back-up heat, which I am fine with. I can pull out portable heaters for those really cold days/nights of the year. So with this info, my questions are:
1) Would I be better off with a single-zone setup for the bedrooms/attic?
2) Should I upgrade to a 12k vs 9k unit if the minimum capacities are virtually the same, the dehumidification is better, and the max BTU’s at low single digits would make the need for back-up heat very unlikely? I assume there is a slight energy penalty for this with a bigger compressor, but when I look at btu’s/watt at 5F, it is virtually the same for both the 9k and 12k units.
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