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Do mini-split systems need to balance capacity inside and out?

Helena True | Posted in General Questions on

In a nutshell: I’ve done the load calcs for a two zone building (large studio, small office).  I need 24k BTUs overall in cooling (Bahamas) — that’s 4k in the office and 20k in the studio. We’re using ceiling cassettes for the indoor delivery, but of course the lowest capacity cassette is  7k.  Can I do this:
INSIDE:
7k  ceiling cassette Office
2 @ 12k ceiling cassettes Studio

OUTSIDE:
24,000 BTU compressor/heat pump

Will that work?   My other option is to bump up to a 28,000 BTU outside and provide a 9k & 12K combo of cassettes in the studio. 
Thanks gurus.

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Replies

  1. Jeff Wasilko | | #1

    Check mylink for the spec sheets for the units you are considering.

    Generally, Mitsubishi allows indoor capacity to be 130% of outdoor capacity. So it looks like your config might be supported.

    The spec sheets list all supported combinations.

    1. Helena True | | #3

      Thanks! Fast, helpful, awesome.

  2. Expert Member
    Akos | | #2

    To expand on what Jeff said. Most manufacturers do allow for a size mismatch between indoor and outdoor units. With multi splits you have to get the sizing pretty close to your load otherwise efficiency will suffer, so make sure that the 24k load you calculate is accurate.

    The one benefit in your case of over sizing the indoor heads is that it reduces their effective capacity. You would have to check tables but the 7k indoor head paired in the combination you suggest drops it closer to 5k, which is a much better match to your load.

    Depending on the type of work you are doing in the studio, especially if it is dusty, a ducted unit might better there. You can then install a larger intake filter on it which are much easier to clean than blower wheels of the wall mounts.

  3. Helena True | | #4

    @akos Thanks!! Potentially excellent news. When you say "check the tables" what do you mean? I'm not a mechanical engineer (I'm a former structural engineer, all around sustainability focussed designer on small projects that can't budget for a whole team but still want a better building, sigh). Re load calcs I did my best using manual J via coolcalc.com and checking that against a few less detailed free interfaces. I also used loadcalc.net for the sizing recommendation. So I'm feeling fairly good about the load accuracy in a relative way. I'm lining up with the greenie rules of thumb and at half the load of the contractors initial recommendation. It's only 1500 SF total, new building with external window shading (Bahamian shutters), quality windows and french doors, block walls, fully vented r19 cathedral ceiling. The studio is for movement (but only a few people) and also has 4 quality ceiling fans, all lights are led. Originally I was thinking duct to the studio, but then I learned about the ceiling cassettes which I think will simply a lot.
    Question: can you comment on the noise factor of cassettes and how they compare to ducted delivery? Thanks!!

    1. Expert Member
      Akos | | #6

      Most manufacturers provide engineering tables for their units. This should include indoor unit and cooling capacity. For example, see:

      http://meus1.mylinkdrive.com/item/MXZ-3C24NA2.html
      http://meus1.mylinkdrive.com/files/M-Series_MXZ_Operational_Performance.pdf

      All modern celing/wall/floor mount units are incredibly quiet. Ducted units with some inlet and exhaust ducting are also very quiet. The choice between the two mostly comes down to looks, if you don't want to see the indoor units, go for ducted.

      Ducted units are also more work, sometimes worth it if you can reduce the zone count. For example, if you could run everything on a single 24k ducted unit, vs your 3 zone multi, the part cost is about 1/2. If your local labour is expensive, the cost save is typically not worth it.

      1. Helena True | | #7

        Thanks!

  4. Expert Member
    Dana Dorsett | | #5

    >"I need 24k BTUs overall in cooling
    ---------
    >"It's only 1500 SF total,..."

    Unless you have really lousy attic/roof insulation or a lot of west facing glass you don't need 24,000 BTU/hr of cooling. The real load is probably less than 1.5 tons.

    https://www.greenbuildingadvisor.com/article/manual-j-load-calculations-vs-rules-of-thumb

    LoadCalc is only Manual-J-ish, and even when using fairly aggressive assumptions on R values & windows & air tightness it tends to oversize by 30-35% or more. CoolCalc is a fully registered Manual-J tool, but most newbies aren't really aggressive enough.

    >".. comment on the noise factor of cassettes and how they compare to ducted delivery..."

    Most indoor cassettes are sub-whisper-quiet when operating at low speed, and much quieter than the typical 2x+ oversized single speed high static pressure air handler type central air. Ducted delivery using RIGHT SIZED and MODULATING equipment with CORRECTLY SIZED & SEALED ducts can be even quieter than wall cassettes. But you're somewhat at the mercy of the duct designers & duct installers. Leaky ducts can do a lot of hiss & whistle even at low cfm.

  5. Helena True | | #8

    Thanks for your input. Indeed there's significant W glass (due to view desires). The project is in the bahamas and I am not, so while I've specified a good envelope I'm cognizant that there could be significant leakage at the wall openings etc. and we may not get a true R19 roof. Thanks for the article link, which I'd seen and used as a benchmark of where I don't want to be. I ran as accurate a coolcalc as I possible and then used those results in the manual S taking the allowed reduction of cooling minum as 90% of total load Again, thanks for taking the time to weigh in.

  6. Helena True | | #9

    Thanks to all that helped on this thread.

    I wanted to share an online supplier resource for anyone coming across this thread. I'm NOT ordering from them (not my dept.) or endorsing -- but because its the only site I found that makes it easy and explicit that the indoor unit capacity can definitely be more than outdoor capacity (cooling load) its worth a look for those tackle similar issues.
    https://www.totalhomesupply.com/
    However --the built in load calc tool on the site is definitely to be ignored.

    Update is I'm holding the line against the dudes that want to "compromise" and size it at 30K. My client is worried that when they have an occasional party it will be hot. The answer is yes it will be hot if there are 50 people dancing! The best solution to that design consideration is an auxiliary system.

    1. Expert Member
      Akos | | #10

      If you are looking for something that can handle a lot of variable load, the best option is to go with individual outdoor units for each indoor unit. The part cost between two one to one mini split and equivalent sized 2 zone multi split is very little.

      Unlike a multi split, one to one units have a much wider modulation range and you can oversize them a fair bit an not loose efficiency. About the only issue you might run into is that dehumidification will suffer at low load at which point you'll have to switch the unit to dehumidify mode.

      The one benefit of over sized one-to-one installs is the units tend to be much more efficient at partial load. For example a mitsubishi PUZ-A18NKA and PUZ-A30NH have similar minimum capacity but the PUZ-A30NH would be more efficient while cooling at 18kBTU.

      The larger unit is a hyper heat so it will be more expensive but seems to have better performance than the non-hyper heat version.

  7. Helena True | | #11

    Thanks @akos for your thoughts. The difference between everyday use and occasional parties (tbd) is so large that what I am recommending is a separate auxiliary system for party-mode. No one can handle any even modest extra expense at this point -- so we're just going to run a dummy lead line through the chase to make a possible additional 1 zone w/ cassette doable down the road.

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