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How much more efficiently does a ductless mini-split perform below peak demand?

etting | Posted in Mechanicals on

My Manual J load calculation for my the 960 sf house I’m building in central Arizona shows a heating load of 13,535 BTU and a cooling load of 11,860 BTU.

I’m debating between two ductless mini-splits:
Panasonic E12NKUA with inverter: 12,000 BTU Cooling 13,800 BTU Heating, 20 SEER 10.6 HSPF
Panasonic E18NKUA with inverter: 17,100 BTU Cooling 20,400 BTU Heating, 18 SEER 8.5 HSPF

The smaller one is less expensive and more efficient at its rated outputs, but the bigger one would easily handle any extraordinarily cold nights. I’ve read here at GBA that a mini-split will run more efficiently than its rated SEER if it’s running at less than its maximum output; I’m not sure whether that would be equally true for HSPF.

At approximately what SEER and HSPF would the bigger mini-split likely perform if it was typically running at 40-60% of its maximum output?

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  1. etting | | #1

    Many thanks for an incredibly helpful response, Dana! I will read all of the resources you cite. The winter design temperature here is 20 degrees. Summers max out at around 105; it's usually in the upper 90s.

  2. Expert Member
    Dana Dorsett | | #2

    It varies by manufacturer & temperature, but the effect is similar as long as the thing has a wide enough modulation range. A set of bench tests of a 1-ton Mitsubishi and 1-ton Fujitsu can be found here:

    See figures 5 & 9 to see the range of efficiencies at different modulations and outdoor temps for heating efficiency, and figures 14 & 18 for cooling mode variations.

    The submittal pages for the Panasonic units seem incomplete, since they don't give nominal capacity numbers only "total capacity", nor do they give the min & max output levels at any one temperature to give a sense of the modulation range:

    What is your location, or your 99% & 1% outside design temps? At "total capacity" of 13,800 BTU/hr at 47F (one end of the HSPF test range) will be higher than it's capacity you might need at your 99% outside design temp if that temp is lower than 47F.

    Look at page 73(pdf pagination) of the technical manual for the E18NKUA:

    In particular, look at the heating capacity graph.

    At the +47F tick it puts out about 6000 watts, or (6000 watts x 3.412BTU/watt-hr = ) 20, 472BTU/hr

    At the +23F tick it's only putting out about 4300 watts, or 14,470 BTU/hr.

    At +14F that drops to 3400 watts, or 11,600 BTU/hr.

    So it kind matters where you live along that curve. There are plenty of locations in central AZ with 99% outdoor design temps colder than Prescott's +20F, which is about the temp at which the E18NKUA would be breaking even with your calculated 13,535 BTU heat load, which would be fine. But maybe not so much if your 99% outside design temp is +8F (like Flagstaff) or +14F (Winslow.)

    Without pulling up the tech manual for the E12NKUA it seems unlikely it would have the heating capacity for any central AZ location. Even the E18NKUA might not really cut it.

    edited to add:

    For yuks I looked it up, confirming expectations see page 85 for the heating capacity graph:

    At +20F the 1-ton delivers about 2500 watts or of heat or ~8500 BTU/hr of heat into a +68F room.

    A cold climate 1-ton mini-split would deliver about twice that at +20F.

  3. Expert Member
    Dana Dorsett | | #3

    The Fujitsu AOU 12RLS2 would cover your loads, as would the newer Mitsubishi FH12NA (more efficient than the FE12NA in that bench test.)

    If you want some margin for colder weather, the 1.25 ton versions aren't bad either, and may be the right choice here:

    Within the Panasonic lineup you'd have bump up to the 2- ton E24NKUA (see p.77 of that first tech manual). It would cover your load down to about +15F or so but it's losing capacity fast with every degree on that end of the temperature curve. Your heat load is about 4000 watts @ 20F, the CU-E24NK will deliver only 4400 watts @ +14F, which is about your heat load at that temp. It's margin, but not a lot of margin.

    By contrast the 1.25 ton -FH15NA or -15RLS2 will deliver ~18,000 BTU/hr (5275 watts) at +5F, which is about your heat load at +5F, which means you would have real margin for those clear cold nights during much cooler than normal weather. They're quite a bit more efficient too.

    The nominal outdoor temp for SEER tests is 95F with an indoor temp of +80F, so you'd have capacity that covered you up to the low 100s with the 1.25 tonners too. You would probably be OK with the 1-ton versions in those series, but with modest ~1.25-1.5x oversizing you gain a bit on average operating efficiency in both heating & cooling modes.

  4. etting | | #4

    Many thanks again, Dana. I'll read everything and take a close look at those better units.

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