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Community and Q&A

Ductless heads in condo high-rises

johns3km | Posted in General Questions on

I noticed these units by me in West Roxbury decided to go with ductless minisplits which I applaud (and somewhat risky at their price-point). I saw 4 heads in the listing for an 1100sf unit, which seems crazy oversized. Does code require a unit in each bedroom? The units do have a fair amount of glass in the main areas but it’d be interesting to know the actual load of a unit surrounded by other heated units. They could probably get away with just running the dining room head by itself, but I imagine high operating costs in that scenario. Any thoughts or insights?


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  1. Expert Member
    Dana Dorsett | | #1

    Since when is a 4 story building a "high-rise"? :-)

    Code requires that each room be capable of being heated to 68F at the 99% outside design temp (about +10F in W. Roxbury).

    The heat load of an entire 1100' condo is no more than 10-12,000 BTU/hr (for a top floor corner unit), which is the output of just ONE 3/4 ton cold climate ductless head at +10F, less than 2 half-ton heads, so yes, it's crazy oversized. A single 3/4 ton mini-ducted Fujitsu with the ducts running in soffits below ceiling level would be more efficient than 4 half-ton heads on a 2.5 ton 4-zone multi-split, but the installation costs would be somewhat higher than the multi-split.

    Running just a single head at a high duty cycle is more efficient than running 4 heads that are all randomly cycling on/off at a low duty cycle.

    1. johns3km | | #2

      I hope for the new owner's sake they aren't in for a rude awakening with winter electric bills. I'm in Brookline a mile from here and they are proposing phasing out Natural Gas for all new construction in the coming years in favor of all electric- with exceptions for commercial spaces, restaurants/kitchens etc. Units like these unfortunately may give the wrong impression with low COPs, higher than expected energy bills, and worse comfort.

      1. Expert Member
        Dana Dorsett | | #3

        >"I hope for the new owner's sake they aren't in for a rude awakening with winter electric bills"

        At Boston's climate & utility rates a reasonably-sized cold climate mini-split isn't dramatically more expensive to heat with than a condensing gas furnace. Even if the nameplate HSPF is only 10 on the multi-split it'll still do better than 8 despite the oversizing factor, and the loads of condos are small.

        It's likely that the hot water energy use is as high as (or higher than) the heating & cooling energy use in a shiny-new code-min condo in a 4 story building, even at an HSPF of 8. I suspect it will be just fine for the condo owners (a function of ignorance + apathy), only the energy nerds would be questioning it.

        The options for right-sizing gas heating at the low loads of new code-min condos is limited. Micro-zoning the rooms with right-sized hydro-air coils running off a condensing water heater (that's also supplying the domestic hot water) would be about as good as it gets. But that's not going to air condition the place. The oversized ductless solution was probably considered a reasonable compromise on cost/efficiency, even if the minimum modulated output is a large fraction of the design load.

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