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Why is turndown so different for Mitsubishi wall unit vs ceiling cassette?

kg72nh | Posted in Energy Efficiency and Durability on

I’ve been reading through the forums here, and learned a lot. (Fortunately in time, that is, before I went and bought a 36k BTU Mitsubishi multisplit for the 2nd floor of my house, which has a design heat load of more like 15k, as it turns out). I’m in zone 5 (NH 03861, design temp 9F).

So anyway, I kinda put the upstairs on hold for the time being, as I’m considering options (I was hoping 3 one-way ceiling cassettes for the 3 bedrooms, that doesn’t seem like a feasible option, and something ducted would involve the unconditioned attic 🙁 ).

Instead, I figured I might start with a single unit downstairs, and that might in fact be enough to get the whole house through fall and spring, with our propane furnace providing heat upstairs as necessary during the coldest months.

I would prefer to go with the 9k or 12k BTU ceiling cassette downstairs, which does 12k/15k heat down to 5F (http://mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10006\M_SUBMITTAL_MLZ-KP09NA2_SUZ-KA09NAHZ_en.pdf, http://mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10006\M_SUBMITTAL_MLZ-KP12NA2_SUZ-KA12NAHZ_en.pdf). But after reading about short cycling, what concerns me is that min capacity at 47F is about 8k for both of these. According to my calculations the downstairs load is 8k at about 25F outside. So the unit would cycle when it’s above 25F, I suppose? How bad is that in real life? (Of course, if there’s no active heat upstairs and the heat rises as it naturally does, the actual load at 25F could be quite a bit higher. But at 25F outside, I kinda doubt enough heat rises naturally to keep upstairs comfortable, and anyway, the longer term plan is to have some kind of heat pump upstairs, too).

Something like  MSZ-FS09NA & MUZ-FS09NA (http://mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10006\M_SUBMITTAL_MSZ-FS09NA_MUZ-FS09NA_en.pdf) would essentially give me similar amount of heating for less money, higher efficiency, a simpler install, and much better turndown to 1600 BTU/h. However,  we also really would like to avoid having a wall unit in the living room.

So here’s the main thing: It doesn’t make sense to me that the more recent one-way ceiling cassettes are so much behind what the (older technology) wall units can do, in particular when it comes to turndown. In fact, I suspect it’s not so much the wall unit vs ceiling cassette, but that they pair with different outdoor units. But that’s still kinda the same puzzle — why does the older outdoor unit have so much more turndown, while it looks like the new SUZ ones have only about 50% turndown? Something just doesn’t seem quite right here, and I wonder whether there’s some other explanation? (e.g., have the rating system / performance tests changed?)

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