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help with Manual J for ductless splits

Kieran973 | Posted in General Questions on

Hi,

I’m trying to do a room-to-room Manual J using Cool Calc so I can size a ductless split system at my house. The inputs that I’m having trouble with are walls and hot water pipes. The house is wood framed, it was built in 1925, and it has a hydronic radiator system (baseboard radiators on the first floor; upright cast iron radiators on the second floor). But for example, Cool Calc wants to know if the walls have cavity insulation or board insulation, and whether the walls are 2×4 or 2×6, but I have no idea. Similarly, Cool Calc asks for the following about the hot water pipes: type of metal, length, thickness, insulation, ambient temp, and hot water temp. I don’t know the answer to most of these questions. As a WAG, I put: steel, 50 feet length, 2 inch thickness, not insulated, ambient temp 40F, and hot water temp 180F (mainly because these two were suggested as default temps). But again, I don’t know how accurate this is. Besides tearing up walls all over the house, is there any way to arrive at a more informed guess about some of these inputs? Thanks.

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Replies

  1. Jonny_H | | #1

    For the walls, based on being built in 1925, it's almost certainly 2x4 construction. You can also get an idea of this by looking at wall thickness around windows and doors and such -- a 2x4 stud wall will be something like (3.5 - 4 inches stud) + (0.5 - 1 inch interior plaster) + (probably around 1" exterior sheathing) + (siding). The insulation part is pretty important for heat loss calculations, so if you don't know if the house was ever insulated or not, you're probably going to need to punch a couple holes in exterior walls to see what's there -- probably either "nothing" or "blown in cellulose" unless it was more extensively remodeled in the past.

    Note that if you have good data on fuel use for the past winter, you can also get a good approximation of your whole-house load by following the method in this article: https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new This won't help with a room-by-room, but at least gets you in the ballpark, and you may be able to iterate on the coolcalc model to get the whole-house data to line up with your actual measured data.

    For the hot water pipes, is that looking for the pipes serving the hydronic system, or the pipes providing domestic hot water?

  2. Kieran973 | | #2

    Thanks for the tip on the walls. Agreed on the importance of knowing the wall insulation: when I put R-11 (the minimum allowed by Cool Calc), the heat load is 35K BTU, but when I put it none, it jumps to 46K BTU.
    I did try out Dana Dorsett's method from that link and I wrote a long post about it:
    https://www.greenbuildingadvisor.com/question/help-figuring-out-heating-load-for-mini-split-system
    The short version of the post is that based on mid Feb to mid March gas usage, the total heating load in the house would be 79K BTU (well, actually 56.3K but I multiplied that by the recommended 1.4 ASHRAE oversizing factor). But this includes gas for the stove, and more importantly, it was during a month that we had absolutely zero insulation in the attic. We just had the attic insulated to R-49 (and the basement rim joists insulated to R-15). So assuming that stove/oven usage is 10% of the monthly gas usage total, and assuming that the added insulation would reduce heating energy needed by 30% (two assumptions that are more guesses than anything), then our whole house heating load would be 49.7K BTU.

    Good question about the pipes. I don't know. Cool Calc just says "hot water pipes." It could be for the hydronic system, or for the domestic hot water. Either way, most of the pipes are in the walls, so I'm not sure how I'm supposed to determine their length or insulation. I also don't know what is meant by "ambient temperature" for the pipe inputs. Ambient temperature where? In the house?

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