which one of these Fujitsu wall units is the better choice
I’m talking to a Fujitsu installer this week. I plan to ask for two one-to-one hyper heat wall units: one for downstairs, one for upstairs. I’m pretty sure that the 9k btuh wall unit is the proper size for the upstairs load. For the downstairs load, I’m not sure whether to request the 12k or the 15k btuh unit. Originally, I thought the 12k would be best, but after taking another look at the 15k’s COP at 47F (which is basically the average heating season ambient temperature in my area), it seems that the 15k would actually be more efficient.
The short version of my question: given that the average heating season temp in my area is 46F, which is a more important indicator of a mini-split’s average heating season efficiency and energy use, the HSPF rating or the COP at 47F rating?
The longer version of this question:
Climate Zone 5
99% design temp: 11F
Average heating season (September – May) temp: 46F
Manual J heat load at 11F: 31,600 btuh
Manual J heat load at 46F: 11,000 btuh
12,ooo btu Halcyon wall unit
Outdoor Unit #: AOUG12LZAS1
Indoor Unit #: ASUG12LZBS
Heating COP at 47F: 5.34 (at min 3,100 btuh) ; 4.64 (at rated 16,000 btuh)
15,000 btuh Halcyon wall unit
Outdoor Unit #: AOUG15LZAS1
Indoor Unit #: ASUG15LZBS
Heating COP at 47F: 6.06 (at min 3,100 btuh); 4.59 (at rated 18,000 btuh)
My 1,000 sq ft first floor is about 63.5% of the heat load while the 500 sq ft second floor is about 36.5% of the heat load. So with an 11,000 btuh whole house heat load at 46F, the first floor’s load would be around 7,000 btuh and the second floor’s 4,000 btuh. Assuming a perfectly linear drop in COP between the minimum and rated outputs (3,100 btuh to 18,000 btuh) of these two units, my math says that the 47F COP at 7,000 btuh for the 12k unit would be 5.12, while the 47F COP at 7,000 btuh for the 15k unit would be 5.67. So it would seem that despite being larger, and despite having a worse HSPF rating, the 15k wall unit is actually more efficient at my average heating season temperature.
The 15k has some other benefits as well: more cooling output (though not a huge factor since I only use AC around two months a year); an outstanding cooling COP; and more importantly, a slightly higher max heating output at 11F which would better match the 11F design temp load of the first floor – this first floor design temp load is 20,000 btuh, and the max output of the 15k at 11F is 21,250 btuh while the max output of the 12k at 11F is only 17,050. (The 9k more than meets the upstairs design temp load of 11,600 with an 11F max output of 15,700).
In other words, for my 31,600 btuh whole-house design temp load:
option one: the 12k + 9k would produce 17,050 + 15,700 = 32,750 btuh
option two: the 15k + 9k would produce 21,250 + 15,700 = 36,950 btuh
The first option is more closely suited to the whole-house heat load (only 3.6% oversized), but the second option ensures that the first-floor load of 20,000 btuh would be satisfied at 11F; it would also mean having a more efficient unit on the first floor than the first option, which would reduce total annual electricity consumption; and while oversized for the whole-house load, option two is not grossly oversized (16.9%).
Anyway, sorry to go on and on. Any thoughts would be appreciated. Thanks.
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