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Community and Q&A

Fujitsu vs Mitsubishi

Daniel S | Posted in General Questions on

I currently have a 10 year old Fujitsu multi-zone ducted system cooling my 2nd floor (the first floor is cooled with a combination of ductless heads from a separate outdoor unit).

The outdoor unit is an AOU36RML1 (36k BTU) and is joined to a 12k ducted unit in the upper attic that cools two bedrooms and two 12k ducted units in the kneewall, which cools a finished attic area (they are both controlled by the same thermostat and each are ducted to two registers that are all in a line about 25 ft long via flex duct).

The outdoor unit crapped out, so I’m looking to replace the whole system. My cooling load appears to be about 13k in the zone currently cooled by the two 12k units and 10-11k for the two bedroom zone.

So, I think I can pretty clearly get away with a smaller system, either the AOU24RLXZ (likely with 18k and 9k indoor units) or the MZX-3C24NA (15k and 12k indoor units). I have quotes from a couple contractors who get good ratings – $10k for the Fujitsu system and $11.5k for the Mitsubishi system. Wondering what you would recommend – is Mitsubishi still considered superior in quality to Fujitsu? Will one system be more efficient? It’s not clear to me whether the indoor units for these multi-zoned ducted systems have turn-down capability like with the single-zone systems or if they are just on-off, in which case I imagine the Mitsubishi that is able to fit the load a little closer might be better.

Another question is, will the smaller blower on the Mitsubishi units be a problem here? 25 ft with flex duct covering 4 registers seems like it might be on the edge of what’s possible. If it turns out the external static pressure is a little too high, how bad of a problem is that? Would the Fujitsu be a safer option, given that it has almost double the static pressure to work with?

I’d love to have been able to put in wall units or ceiling cassettes, but the space doesn’t really accommodate those.

thanks for your thoughts!

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Replies

  1. User avatar
    Dana Dorsett | | #1

    We would need a lot more information about your climate zone and the exact models (including the cassettes/heads) to make educated guesses about which is more efficient.

    Are the older ARU12 mini-duct cassettes not compatible with newer Fujitsu compressors?

    It's possible (even likely) that a single dedicated Fujtisu -18RLFCD (or maybe a 12RLFCD) could heat & cool the whole attic with a bit of reconfiguring of the ducts, and would have much higher efficiency and comfort (due to the wide modulation range) than any multi-split solution. How big is the attic, and what sort of R-values are you looking at?

    http://portal.fujitsugeneral.com/files/catalog/files/18RLFCD1.pdf

    There's an article in yesterday's Greentech Media blog about a 1200' house in the east bay (east of San Fransico) retrofited with a single 18RLFCD to heat and cool the whole shebang (and it's oversized for the actual loads.)

    https://www.greentechmedia.com/articles/read/what-does-it-take-to-electrify-everything-in-your-home#gs.uVJxYlA

    In that one they mounted it vertically blowing into a short plenum with four flex ducts coming out of it:

    https://uploads.disquscdn.com/images/2ffa6e108a7ded9f51130ff14126239b275b1244b7d53138beb63b4182d68f13.jpg

    It looks like the ducts were soffited to keep them inside of conditioned space, below the ceiling level:

    https://uploads.disquscdn.com/images/7843213f27734395e6ede8ea696552a8eafd3a2dd7f62c2b61241bb23189a293.jpg

    A 25' run of flex duct could be an issue for the Mitsubishi units or not, but the installer would be on the hook for making it work, even if it means replacing some of the pre-existing duct work.

    From a quality point of view it's a moving target- both Mitsubishi and Fujitsu are first-tier vendors. The amount of local support can make or break any of them. If there are 25 Fujitsu installers within a 50 mile radius of your house and only 5 Mitsubishi installers it's probably safer to go with Fujitsu (and conversely.)

  2. Daniel S | | #2

    Thanks, Dana. Sadly, the old ARU models don't communicate with the newer outdoor unit according to Fujitsu tech support. At least Mitsubishi allows compatibility +/- one generation. I'm in the DC area, so pretty humid, but the house is partly shaded. I don't think a single indoor unit would work for the space since the two bedrooms are part of an addition and the upper attic overlying them doesn't communicate with the kneewall of the finished 2nd floor attic rooms. The ceiling over the finished attic is insulated with dense-packed cellulose and the kneewall has batts between the rafters, so that ductwork is in a conditioned space. The overhead attic over the bedrooms is R49 insulated with cellulose, but I imagine the ducts could be buried in the cellulose, so maybe not exposed to as high temps.

    We are not upstairs all that often during the daytime, so, although I recognize it is less efficient than running the unit 24/7, we would probably turn it on only when we are in the room - since overall, it would probably use less energy than leaving it running at a set temp. Thus, I'm ok with some oversizing so that it can get to temp somewhat quicker.

  3. Daniel S | | #3

    Relevant model numbers for the ducted units, for what it's worth:

    Fujitsu 9k unit: ARU9RLF, 18k unit: ARU18RLF
    Mitsubishi 12k unit: SEZ-KD12NA4R1.TH, 15k unit: SEZ-KD15NA4R1.TH

    As someone mentioned on another minisplit thread, there's a concern that the ducted indoor units on the multi-zone systems don't have individual turndown, unlike the ducted units on single-zone systems. If that's true, I imagine there's more benefit to be gained by going with the Mitsubishi, since the 15k is less oversized for a 13k cooling load than the Fujitsu 18k unit.

  4. User avatar
    Dana Dorsett | | #4

    Unless this is a gia-normous attic that equipment is all extreme overkill. You're probably looking at an actual load/area ratio of about a ton per 1500' of conditioned space, and in DC a significant fraction of that load is latent load (humidity). Oversizing by 3x to allow cooling it quickly doesn't usually handle the latent loads nearly as well as longer, more continuous operation, and it uses more power since it's running the compressors at the highest speed/lowest efficiency. Letting the systems modulate in the lower 1/3 of the range to keep the space from overheating in the first place usually uses a lot LESS energy due to the efficiency being as much as 5x higher at minimum speed earlier in the day when the outdoor temps are lower than at max speed in the late afternoon/evening when outdoor temps are higher.

    Take a look at Figure 14, p19 (p27 in PDF):

    https://www.nrel.gov/docs/fy11osti/52175.pdf

    When it's ~82F outside and running at minimum speed the older 1-ton Fujitsu has a COP of 9. When it's ~87F outside and running at max speed (as it would on a recovery ramp) it's COP is about 4, less than half the efficiency. So even if keeping the place cool when you're not there moves 1.5-2x more BTUs out of the house than it does for a quick cool-down it still uses less power than the quick cool-down mode.

    There may be rare exceptions to prove the rule, but as a rule it'll use few kwh and do better latent load management if it's right sized and allowed to modulate in a "set & forget" approach than oversized with quick PM cool-down. It'll be more comfortable at a higher temp too, since the wall and ceiling temperatures will remain cooler and radiate less on to the humans than a quick cool-down of the ai temp after allowing it all to soak at a higher temp all afternoon.

  5. Daniel S | | #5

    Ok, thanks Dana.

    So here are my measurements:
    Zone 1 (all underneath an R49 attic): Bedroom 1 (125 sq ft), Bedroom 2 (140 sq ft), Bathroom (35 sq ft) = 300 sq ft
    Zone 2: Hallway (100 sq ft), Playroom attic (200 sq ft), front room of attic (125 sq ft) = 425 sq ft (300 sq ft of which are under dense-packed cellulose)

    So it does seem like even using a very conservative 1 ton per 500 sq ft that the AOU18RLXZ with a 9k and 12k head would have enough capacity. I guess one question is, how much does the rule of thumb for cooling load change between the 1st and 2nd floor?

  6. Daniel S | | #6

    And one other question - how much of a difference do you think getting the low-temp version (AOU18RLXFZH) would make over the AOU18RLXFZ? In addition to better low-temp performance, it seems like the SEER is dramatically improved (for ducted, SEER of 19 vs 16 for the regular version), although the EER is only marginally different (12.4 vs 12.1). Not sure which number is ultimately more important in efficiency.

  7. User avatar
    Dana Dorsett | | #7

    Rules of thumb suck. (Maybe they're for thumb suckers? :-) )

    A ton per 500' is a reasonable rule of thumb for the cooling load leaky tarpaper shacks with no insulation and leaky single pane windows. Rather than a "very conservative" estimate for your cooling loads, it's a RIDICULOUS estimate of your cooling loads likely to oversize by more than 2x, maybe even more than 3x, and that is to your comfort & cost detriment!

    Even the very worst houses (probably low/no attic insulation with a lot of west facing windows pushing high peak gains late in the day) do better than that. Take a look at the graphics in this bit o' bloggery:

    https://www.energyvanguard.com/blog/air-conditioner-sizing-rules-of-thumb-must-die

    The worst house (out of 40 careful Manual-Js) had a ratio of a ton per 624', the average was a ton per 1431'. Unless those rooms have big west facing windows you're probably going to be closer to the ton per 1400' mark. Even careful Manual-Js usually overshoot the mark by 10% or more. You have ~725' total and a likely peak load of less than one ton, even if you kept it at a chilly 65F.

    Take the time to run the load numbers with an only Manual-J-ish calculator like http://loadcalc.net/ and use AGGRESSIVE rather than conservative assumptions on R-value, air tightness, etc. Be sure to include aggressive numbers on shading factors too. Even with aggressive assumptions it will generate numbers that have some built-in margin. You don't need or want 100% margin. It would be good to oversize by no more than 30% (which may not even be possible for the 300' zone.)

    Pay attention to the heating numbers too, since you may be better off using mini-splits for heating too.

    Using individual mini-splits rather than multi-splits is usually/often both cheaper and more efficient than a multi-split approach but let's get the load numbers first. A single -9RLFCDs can deliver 12,000 BTU/hr at 80F indoors, 95F outdoors, but can throttle back to 3100 BTU/hr at those temps. A single 12RLFCD can deliver 13,600 BTU/hr of cooling, and throttle back to 3100 BTU/hr. A pair of them would most likely be EXTREME overkill, but a single will still have some room for modulation.

    It's pretty clear that a multi-split can not work efficiently here. The minimum speed output of most multi-split compressors is too close to your likely half-ton-ish to 3/4 ton-ish peak cooling load. The 18RLXFZ (and -H) has a minimum cooling output of 6100 BTU/hr:

    http://www.fujitsugeneral.com/us/resources/pdf/support/downloads/submittal-sheets/18RLXFZ.pdf

    Are you absolutely SURE it can't be done with a single mini-ducted heat pump, and some high-throw registers?

  8. Daniel S | | #8

    Thanks, Dana. Unfortunately, I can't see how we could accomplish this with one unit without doing some surgery on the walls that we are not willing to so. So, I think the 18k indoor unit is our best bet. I think my biggest question at this point is how actual energy savings we would be likely to realize by going with the low heat version with the higher Seer but similar EER. The installation cost difference is about 900.

  9. Daniel S | | #9

    Sorry, 18k Outdoor unit, not indoor unit.

  10. User avatar
    Dana Dorsett | | #10

    Two minisplits is your better bet than a ridiculously oversized multi-split.

    You might consider doing zone 2 with a ductless head in the playroom rather than ducts to the tiny front room and mostly-irrelevant hall. Either a Mitsubishi FH06NA or FH09NA will modulate down to about 1600 BTU/hr if it's on it's own compressor (but not if married to an oversized multi-split with a half-ton minimum compressor output.)

    http://meus1.mylinkdrive.com/files/MSZ-FH06NA_MUZ-FH06NA_ProductDataSheet.pdf

    http://meus1.mylinkdrive.com/files/MSZ-FH09NA_MUZ-FH09NA_Submittal.pdf

    Have you run the room load numbers with loadcalc.net yet?

  11. Daniel S | | #11

    Thanks, Dana. Ductless head in the playroom is a no-go due to a low/sloping ceiling. To go with anything but a drop-in replacement for the current ducted system would be aesthetically and functionally unappealing, although I acknowledge that it would allow a superior turndown ratio. I do kind of like the idea of doing two ducted single-zone systems, but I worry that would be considerably more expensive than one multi-zone system, would result in an extra condenser outside, and is more more bit of equipment that can break.

  12. User avatar
    Dana Dorsett | | #12

    A floor unit instead of a high-wall type works well with sloped-ceiling kneewall situations. The 3/4 ton Mitsubishi KJ09 modulates down to 2300 BTU/hr, well below the 3100 BTU/hr of a 9RLF Fujitsu mini-duct, and probably below the design cooling load of that room:

    http://meus1.mylinkdrive.com/files/MFZ-KJ09NA-U1-MUFZ-KJ09NAHZ-U1_ProductDataSheet.pdf

    It takes up less than 2 square feet of floor area ( less than 1% of a 200 square foot room.)

    Fujitsu's 9RLFF doesn't modulate below the 3100 BTU/hr of their mini-duct solution, but more efficient (even without accounting for duct losses) :

    http://www.fujitsugeneral.com/us/resources/pdf/support/downloads/submittal-sheets/9RLFF.pdf

    Mitsubish's' SUZ/SEZ 3/4 ton mini-duct unit only modulates down to 4800 BTU/hr, which is probably more than the design cooling load of either zone.

    https://ductlessamerica.com/pdf/SEZ-KD09NA4_SUZ-KA09NA_Submittal.pdf

    It's adding another compressor, but it's often (even usually) less expensive to go with two separate units than a multi-split. Yes it's more equipment that can break, but when one breaks you'll still have AC in the other space.

    Run the load calculator- seriously!

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