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Community and Q&A

Minisplit options

Dean Sandbo | Posted in General Questions on

Ok.. So Walk out basement application.
3 bedroom suites… each around 5k heating, 1.5 k cooling loads.
Lower living area.. 10.5k heating 6.2k cooling
workshop area.. 13k heating 3k cooling

total load in basement around 38k btu heating, 17k cooling

I really wanted to put one mini split head in each of the 3 bedrooms, one in the lower living area, and one in the shop.

6k for each bedroom
9k for lower living
12k for shop

one…mitsubishi mxz -5c42na for outdoor unit

indoor units…
3x msz-ge06na-8 one for each suite (7200 heat, 6000 cool)
1x msz-ge09a-8 for lower living (10,900 heat, 9000 cool)
1x msz-ge12na-8 for shop (14,400 heat, 12,000 cool)

I know that one head for each bedroom is more capacity than needed… but would love the individual control of each room… Is there any foreseeable problem with this plan with the limited details I have provided?

Thanks for your help.

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  1. D Dorsett | | #1

    How were the load numbers derived? A 38K heating load for a walk out basement is unusually high, unless it'a 5000 square foot basement built on permafrost.

  2. Dean Sandbo | | #2

    Yes it is a large 3600 sqaure foot basement. Manual J was done by a very reputable third party company who is well known on this forum..

  3. D Dorsett | | #3

    That's still more than 10 BTU/hr per square foot, which is on the extremely high side for a walk out, which implies little or no foundation insulation &/or a lot of glass(?).

    What's the 99% outside design temp?

  4. Dean Sandbo | | #4

    outside db 21degree f
    inside db 70 degrees
    design td 50 degrees.

    92 degrees
    75 degrees
    design td 17 td

    Yes, huge walls of glass on the back side facing a lake..
    r5 foam on top of slab..
    r10 rigid on below grade walls, zip r6 sheathing on walls out back with r13 in cavities.. big fairly efficient windows...

    Lets assume that the load calcs are correct, and it should be a good assumption based on the firm.
    What I need help with is I would really like to have a 6k head in each bedroom suite (they each are large) with around 5k heating and 1.5k cooling. If all of these are attached to one outdoor unit, what are the downsides of having a head in each room? I'm not concerned about the cost of the extra heads, it is more about the lower cooling loads, and discussions that I don't quite understand about how the outside unit modulates when only one unit calls for coolling, and it might be calling for less than the outside unit modulates down to.

  5. User avatar
    Dana Dorsett | | #5

    OK, let's assume the numbers are correct, but scrutinize the Manual-J carefully, and figure out where that 38,000 BTU/hr is really going. At a modest delta-T of 50F that seems even more insanely high. Must be a LOT of glass dominating the loss numbers.

    At a delta-T of 50F you have a heat load of 38,000 BTU/hr. A linear approximation would then be 38,000/50= 760 BTU/hr per degree-F. So at 71F indoors, 47F outdoors that's a 24F delta, for an approximate load of 18,240 BTU/hr @ +47F.

    According to the submittal pages and other documentation, the minimum modulated output of the MXZ-5C42NA is 7200 BTU/hr @ +47F, which is well below the 18K load at that temp, so it should have no problem staying in a modulation range.

    Capacity-wise, if you look up the capacities of a 6 + 6 + 6 + 9 + 12 configuration in the submittal it adds up to 44,601 BTU/hr.


    6962 BTU/hr per GE06 (not 7200 BTU/hr, which is the max output of a GE06 @ +47F, not +17F or +21F)

    10,292 BTU/hr for the GE09

    13,723 BTU/hr for the GE12

    You'll note that you would have a shortfall of 200 BTU/hr between the rated capacity and the calculated loads for the lower living are zone, and less than 10% margin on the shop zone. The shortfall zone is a problem, but not a huge problem, still it's an unnecessary problem. You'd be covered with a bit of margin if you bumped it up to a 6 + 6 + 6 + 12 + 15 configuration.

    In that configuration you would get:

    6107 BTU/hr per GE06 (covers your 5K BTU/hr load with 22% of over capacity, which is fine- nearly ideal in fact.)

    12,213 BTU/hr out of the GE12 (fully covers your 10,500 BTU/hr load with about 16% margin)

    15,267 BTU/hr out of the GE15 (fully covers your 13,000 BTU/hr load with 18% margin.)

    This is probably the better way to go, if we're assuming the load numbers are real, and the oversizing factors for all zones are comparable, which means they'll probably share the modulated compressor output a bit more efficiently. That also means you'd be covered even during those once rare, but increasingly frequent Polar Vortex type events when the daily highs might even be below the 99% outside design temp. You already have more than enough compressor to deliver that colder-than-cold design temp load, it's a matter of getting enough wall-coil head on it to deliver it. Bump up both of the bigger wall heads by one size and you're good.

  6. Dean Sandbo | | #6

    Thank you! I guess I am more concerned with the cooling load in the bedroom suites. 1500 btu for cooling in those suites with a 6k head. If I handled dehumidification separately with an ultra aire.... would there be any concern? I wish they made a 3k head, I think if they really want to go to whole house solutions, with incredible zoning, this would be an opportunity!

  7. D Dorsett | | #7

    With 1.5K of peak cooling load and a 6K head the head will be cycling on/off on a duty cycle most of the season, but as long as there's at least 6K of load for whole house the compressor will be cruising along in a modulating mode.

    Humidity may need to be handled separately in ANY low-load house, since there can often be latent loads with little to no sensible load. IIRC all of these Mitsubishi heads have a "dehumidify" mode, where you can set it to run at it's minimum modulation continuously, but at times of super-low sensible load that may provide too much sensible cooling. In dehumidify mode it does not respond to the air temperature.

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