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Community and Q&A

Can’t decide between two mini-split models!

Dave H | Posted in Energy Efficiency and Durability on

Hi all –

The HVAC goes in this week, and I’m excited! This is a converted garage that will be used as a video studio, so it’s not a living space but needs to be comfortable when needed. It would rarely be used in the middle of the night, but it could happen. The garage has been air sealed and insulated and I’ve had a manual J which said I need:

20,503 BtuH of heating
7756 BtuH of cooling

The installer recommended an 18RLB Fujitsu model, which sounds good. However, I learned that at our elevation (5280) Fujitsu said I should remove 17% of the output. That makes me wonder if I should consider a ‘high heat’ model by Fujitsu.

The 18RLB does:

nominal cooling 18k
min/max cooling 3100 – 19,000
nominal heating: 18k
min/max heating: 7,000 – 20,000

If I consider the 17% reduction due to altitude, it seems to me that this unit might struggle a bit on those coldest days. Reading the Fujitsu site, I saw the 15RLS3H which offers:

nominal cooling 14.5k
min/max cooling 3100 – 18,400
nominal heating: 18k
min/max heating: 7,000 – 23,900

The 15k unit is a bit more expensive, but also has a higher HSPF, SEER, and EER. It’s very slightly higher on HI (45/45 vs. 42/43) but nominally quieter on quiet mode (26/27 vs 26/27).

If I am understanding correctly, after the altitude correction the 15k unit would be 19,991 which is just a bit lower than what the manual J indicates but very close, but it would also be a bit more efficient. The 18k unit is a bit cheaper, but after altitude correction it’s not even close to the required heating output.

Meanwhile there is an 18k high-heat unit that does 7000-23,000 of cooling and 7,000 to 29,000 of cooling, and 7,000 to 29,000 of heating. This seems a bit overpowered on both cooling and heating so it seems a poor fit.

This is like a puzzle – trying to find the right size! I really don’t need the unit to be able to run well at -15 below, which is evidently the big draw of the high heat models. It’s rare we even get below 5 degrees here. But, the 15k high heat unit seems like it might be the overall best unit.

How to decide? thank you! – Dave

P.S.- I do understand that the manual J is never 100% accurate, and there are tons of factors. But, still I hope to make the best decision even taking that ambiguity into consideration.

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Replies

  1. D Dorsett | | #1

    It's impossible to advice without knowing your 99% outside design temp, and whether you intend to keep the place at temp 24/7 or if it's just occasional use. "...rare we even get below 5 degrees..." and "....not a living space but needs to be comfortable when needed..." are hints, but more clarity would be useful.

    If it only rarely gets below +5F, you should be looking at the 15RLS3, not the 15RLS3H, which costs $100-150 more than the non-"H" flavor. It's basically the same heat pump, but without the pan heater.

    http://www.admorhvac.com/admor-files/submittals/ASU15RLS3.pdf

    Unlike the 18RLB, the 15RLS3 has a rated output all the way down to -5F.

    The electricity rates might also be a factor, given that the 15RLS3 is quite a bit more efficient than the 18RLB at 13,400 BTU/kwh instead of 10,600 BTU/kwh. That's 2900 BTU more heat per kwh, or 27% more heat per dollar. Price-wise the hardware is within $200 of each other at internet sites, and it wouldn't take many heating seasons to make up any price difference if you're keeping the space at temperature most of the time.

  2. Dave H | | #2

    Hi Thanks for your answer.

    I don't know exactly the 99% temp, but I thought that it was built into the manual J so I didn't mention it. I am in Denver CO 80205 if that helps. I also see, in the manual J:

    - outside db -3 F
    -inside db 72 F
    - design TD 75 F

    As for use, it's hard to say exactly but maybe I could describe it like this:

    - this will be a workspace that is used pretty much every day, during weekday business hours, at a minimum.it will frequently be used weekends during the days, and 2 or 3 days a month probably until 9 or 10 pm

    - I need the room to be comfortable, which means around 75, much of the time. The reason is that we're doing photoshoots and some times they go late into the night, say until 4am. That is rare, but it happens say 15 times a year that we go late into the night. So, I'd like to be able to rely on the unit to keep the place in that range most of the time.

    - That said, I think it would be OK if there were a few days where I needed to bring in a radiator. Say, for 5 or even 10 days a year it might get slightly chilly at night, but if I brought in one of my electric heaters and that did the trick that would be enough for me.

    Is that clearer? I know it's easy to say 'I need this place to be 75 degrees 24/7 no matter what, but that isn't really the case. Exceptions, if they are don't happen to often, can be ok.

    thanks!

  3. User avatar
    Dana Dorsett | | #3

    Denver's 99.6th percentile temperature bin is ~0F, and the 99th percentile is +6-7F in town- it's only -3F out at the airport. But since you're insisting on 75F and the Manual-J was done assuming 72F design temp, the calclulated load is comparable to the 99.6% heat load number, since the temperature differences is the same (75F-0F= 72F- -3F).

    Since the 18RLB doesn't even have a rated capacity at your outside design temp, cross it off the list. The real choice is between the 15RLS3 and the 15RLS3H. The wintertime moisture levels are low, and even in January the daily mean temp doesn't drop below 32F for weeks on end, so the likelihood of enough defrost ice building up in the pan to damage the unit are pretty low, which means the non-"H" 15RLS3 would be a reasonable choice.

  4. Dave H | | #4

    Thanks for that informative post.

    It's interesting how the 18k unit produces more cooling, but less heating than the 15k unit. It does look like the 15RLS3 does seem like it fits the bill. Is the only difference between that and the 15RLS3H the base/pan heater? If so, it can see how that might be a bit overkill.

    Honestly I had assumed that a 15k unit was just have lower specs overall than the 18k unit, so I'm still bit confused about why the 15k unit puts out more heating than the 18k unit, but I think I'm going to choose the 15RLS3.

    thank you very much for your input.

  5. User avatar
    Dana Dorsett | | #5

    The decision to call one mini-split a 1.25 ton instead of a 1.5 ton is a marketing decision. Sizing the model names by the nominal cooling capacity isn't a very good indication of it's heating capacity, particularly at lower temperatures. The max temperature difference between indoor & outdoor temperatures in cooling mode is typically 30F or less (75F indoors, 105F oudoors), whereas in heating mode it's twice that, or more. Cold climate mini-splits are designed with slighly larger coils on both the indoor & outdoor components than cooling-only minisplits, and use different valving and control algorithms to extend capacity at lower temperatures than those designed for more temperate climates.

    Fujitsu's model naming conventions for the RLS series does not reflect either it's maximum cooling capacity, but rather it's "nominal" capacity at which efficiency is tested, which results in a higher SEER & HSPF numbers due to the "oversized" coils for the load under the standardized testing conditions. It is in fact a slightly "bigger" mini-split than some other 15K rated mini-splits. With the 15RLS3-H, 15,000 BTU/hr is the heating output at -15F which is well below the +17F HSPF test condition, and 15,000 BTU/hr is the nominal modulation rate at which it is tested for cooling efficiency. But it puts out more than 18,000 BTU/hr in cooling mode when running it full-out as well as in heating mode at +17F, and thus could have been legitimately been marketed as a 1.5 tonner, but would have required efficiency testing it at higher modulation rates, which would have cut into the efficiency numbers.

    I suspect their marketing goal here was to sell a 1.25 tonner that beat Mitsubishi's 1.25 ton FH15NA cold climate mini-splits on both capacity and efficiency. It most respects it's more comparable the Mitsubishi's 1.5 tonner, the FH18NA.

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