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What capacity rating should be used for sizing ductless heat pump?

James Reider | Posted in Mechanicals on

The easily obtainable heating capacity information for ductless heat pumps are generally listed as Btu/h at 47 degrees and Btu/h at 17 degrees. These don’t appear to line up with the expanded capacity charts which can (sometimes) be found with some digging on manufacturers websites. Let’s look at the Fujitsue ASU9RLS3:

It’s capacity is readily reported as:
7,0000 Btu/h @ 17 degrees
12,000 Btu/h @ 47 degrees

However, if I look up the expanded heating capacity chart, it reports the Total Capacity as (assuming 70 degrees indoor temperature):
16,000Btu/h @ 14 degrees (the closest on the chart to 17 degrees)
22,000Btu/h @ 47 degrees

Both of these numbers are significantly higher than what is indicated on the spec sheets from suppliers.

In some previous posts about sizing, I recall Dana Dorsett specifically referencing these charts when sizing and using the Total Capacity numbers. What is the difference here and which numbers should I use for sizing?

Hypothetical Space:
Manual J calls for 16,000 Btu/h. 15 degrees is design temp.
If I go by first set of numbers, the 9RLS3 is too small and even the 15RLS3 won’t be enough.
If I go by the expanded capacity chart and Total Capacity number, the 9RLS3 would be more than enough.

What am I missing? Thanks in advance.

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    As I understand it...

    The "nominal" capacity must be available at +17F, though it may be more. It is not always the maximum capacity at +17F, though it can be, depending on the vendor & model.

    The "rated" capacity is the modulation level at which it's efficiency is tested. The 9RLS3 can deliver over 20,000 BTU/hr @ +47F, but they test it at +47F the chosen nominal output level.

    "Total capacity" is what it can deliver running flat-out, never pausing for defrost cycles.

    If your Manual-J is 16K @ +15F and the capacity charts say it'll deliver 16K @ +14F you're cutting it really short since the total capacity doesn't make allowances for defrost cycles. That may not matter if you live in a SUPER arid location (Atacama desert, mayhaps?) but you'll really be better off with the 1-ton even in a dry climate, or the 1.25 ton in a location humid enough that it sometimes snows at +15F outdoor temps.

  2. James Reider | | #2

    Thank you. The difference between nominal and total capacity makes sense. I am actually in a more humid climate (PNW) and design temps here are closer to +24F. I chose that number because it was right on the edge of the capacity chart for the 9RLS3 and was interested in how that would impact a recommendation. Follow up question if you have time:

    I see a lot of Daikin units installed around here (PNW). But after comparing both the capacity tables, it seems like they are very poor performers relative to the Fujitsu units:

    Daikin FTX24 has total capacity of 13,790 Btu/h @ +14F outside
    Fujitsu 15RLS3 has total capacity of 21,600 Btu/h @ +14F outside

    So the 1.25 ton Fujitsu would more than meet demand of the above scenario but the Daikin 2 ton would not. Plus the Daikin would only be able to modulate down to 7,800 Btu/h vs the 3,100 Btu/h of the Fujitsu.

    Apart from perceived support or reliability of the brands is there any case to be made for choosing a Daikin over a Fujitsu in a heating dominated climate?

    Thank you again for your time and input.

  3. Expert Member
    Dana Dorsett | | #3

    In the PNW, anywhere the western side of the Cascades lower than 1500' of altitude just size it according to the nominal output, forget the extended capacity tables. Yes, you go through more defrost cycles in the foggy dew PNW than in drier places, but you have a lot more max-capacity at 24-25F than you do at +17F too.

    It's true that below 20F the Mitsubishi & Fujitsu cold-climate mini-splits will outperform simpler designs like Daikin's standard line-up, but that doesn't mean you can't use Daikin given your modest design temperatures. Daikin tends to under promise and over deliver relative to their spec sheets, and they are definitely a first-tier vendor.

    Local support (particularly distributor support) is important. If the thing ever starts misbehaving you don't want to have to debug it over the phone with the tech support in CA, then have to wait for parts to be shipped from Taiwan. If there are 5x more installers of one vendor than another in your area, there is usually better support for those products.

    When specifying a mini-split for a relative Kitsap County WA a handful of years ago it seemed as if Mitsubishi had 2/3 of the market tied up, with Fujitsu a distant second, and Daikin an "also ran", well back in the pack along with LG, Samsung, et al. (Things may have changed, probably have.) Only one Fujitsu installer bothered to show up & put in a bid, and they quoted the 1-ton -12RLS2 higher than competing bids for larger Mitsubishi units such as the GE15 and FE18. She ended up with a Mitsubishi FE18 (which predates the FH series), installed by one of the bigger installers (paying slightly more than the rock-bottom low bid from a 3-4 person company 40 minutes away). It has been completely trouble-free, but there are easily a dozen certified Mitsubishi installers within an hour's drive of her place. If it ever has issues she won't have much trouble finding Mitsubishi certified techs to work on it, or find repair/replacement parts.

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