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Manual J help for upstairs only

eldarchik | Posted in General Questions on

Hey Everyone again.

I was fortunate enough to get some good guidance on preparing my second floor (hats off to Dana and Martin). Now that all the insulation is in, need your help calculating Manual J loads specifically for cooling since we already have hot water heat upstairs. Looking into getting a ductless mini split unit to cool.

I have drawn out to my best ability and marked everything that I thought needed. Based on my guestimates, I’m zeroing in on 6k BTU or half a ton unit to be able to cool both the bedroom and the bathroom (when door is open to the bathroom).

Dana earlier advised on using a single head unit for better efficiency, so I’m thinking of the Mitsubishi 6k BTU 33.1 Seer Ductless H2i Hyper heat pump system. There is one with pan and without, not really sure what the pan is for. Unit would be placed to blow away from the bathroom door, and more towards the stairs. Will 6k be enough to cool inside bathroom with door open or should I go slightly higher?

Here is the original image in case the forum shrunk it and you can’t see details: https://imgur.com/a/Nkam91z

Thanks again,

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Replies

  1. K T | | #1

    Hi I happened to see this, I have no experience with this kind of thing but recently discovered the free loadcalc tool online and entered your dimensions for kicks. See attached. Not sure if I got all the math right, essentially converted your linear dimensions to area by multiplying by 8 feet. Couldn't quite get the assumptions lined up (R11 w R5 board = R16, right?) but imagine it's close. For a minisplit just assumed duct system was in conditioned space. Others can advise better. You can play around with it at http://www.loadcalc.net/load.php

  2. Expert Member
    Peter Yost | | #2

    Nice K T -

    I played around a bit with the calculator you found and seemed in the right ballpark to me; others with more load calculations experience care to comment on the loadcalc online free tool?

    Peter

    1. Expert Member
      Dana Dorsett | | #6

      >"...care to comment on the loadcalc online free tool?"

      Regarding the heating loads with that tool, I've found that it comes closer to a properly done Manual-J using Wrightsoft only the ventilation & infiltration assumptions are zeroed out. In at least one instance it overshot by about 35-40%, even with aggressive assumptions on ventilation & infiltration on a room with a large amount of glass area. But it delivers pretty reasonable heating load numbers for most rooms/houses. Of the online freebie tools it's the most consistently accurate on heating loads I've found to date.

      I've never done the comparison using that tool on cooling loads. There isn't a lot of flexibility on the inputs for shading factors, so I can believe it would dramatically overstate the loads in some instances, but would likely be pretty close in others.

      >"Will 6k be enough to cool inside bathroom with door open or should I go slightly higher?"

      Looking at both the ~10K heating & ~6K cooling load numbers it looks like the FH06 would cover both if the 99% outside design temperature is +15F or higher but not during a cold snap if the other heating system was down. The FH06 delivers a maximum of 10,700 BTU/hr @ +17F, 8700 BTU/hr @ +5F. It has the cooling load covered quite well. For about $200 more in hardware cost the FH09 would cover that, delivering 10,900 @ +5F, and it modulates to the same minimums as the FH06.

      The CFM numbers on the FH06 head are identical to those of the FH09 (same internal blower, different coil), so it's ability to cool the bathroom would be pretty much the same with either.

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

      https://nonul.mylinkdrive.com/files/MSZ-FH09NA_MUZ-FH09NA_Submittal.pdf

      The FH09 isn't a necessity for covering the loads, but wouldn't be extreme overkill either, despite having 2x the cooling capacity of the calculated load, with little (or negative) hit in as-used efficiency hit. Whether it's worth the upcharge to have a full heating system backup in place is up to you, but it's probably worth getting quotes for both.

      The west facing window in the bathroom delivers a late-in-the day bump in solar gain. But if the door has been open all day and the mini-split has been keeping up with the load that PM boost in cooling load isn't likely to be a problem.

  3. Trevor Lambert | | #3

    This is of no practical use, but for the sake of curiosity, here is a comparison between my actual house, as modeled using PHPP (but excluding any solar gains for heating load) and my house dimensions and details using the best available options for insulation and glazing provided by loadcalc (all of which are worse than my actual house). I'm guessing this would represent a somewhat-better-than-code, but I don't know.

    Actual house, PHPP: 11,500BTU/h
    theoretical house, loadcalc: 34,000BTU/h

  4. eldarchik | | #4

    I wonder if I can get away with the 6K BTU and not go with 7KBTU as I heard good things of this Mitsubishi unit.

  5. Trevor Lambert | | #5

    It's standard practice to oversize the equipment a bit, so going under is maybe not a good idea. However, that is assuming you trust the loadcalc. Maybe try a different tool and see if you get the same results.

    1. Expert Member
      Dana Dorsett | | #7

      I would assume the loadcalc numbers to be slightly overstated for typical houses with standard construction. There's no way I'd use it to estimate the performance of a high-R house. The max-cool of the FH06 is 9000 BTU/hr, so it's already 1.5x oversized in capacity for the calculated cooling load, and will surely cover it.

      See my comments in response #6 regarding upsizing to also be able to use it for heating under more extreme conditions.

  6. eldarchik | | #8

    That is a good point Dana, I think it would be wise to spend extra $200 and have that safety net. Another question, for additional $65 there is one with base pan heater, do I need this?

    1. Expert Member
      Dana Dorsett | | #9

      >"... for additional $65 there is one with base pan heater, do I need this?"

      In most US zone 4 locations and warmer, not at all. At the cold edge of zone 5 or colder it may be a requirement to be warranteed for damage related to ice build up in the pan.

  7. eldarchik | | #10

    Thank you Dana, going with the FH09 without pan as per your recommendation. Any particular vendor I should be looking at that is reliable and has a good standing with GBA community?

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