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Minisplits or Central Air in Bronx NYC

JAL55 | Posted in Energy Efficiency and Durability on

Hi all,

My wife and I just bought a house in the Bronx NYC. There is no central A/C. We are considering installing 6 minisplits but a recent contractor recommended central air. We have quotations for Fujitsu and Mitsubishi minisplits. There is a big difference in price, which would you recommend?
Thanks in advance.

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Replies

  1. Matt_McLagan | | #1

    I would consider what efficiency improvements could be done to this house first to improve comfort and save money. Then, get a smaller sized & less expensive HVAC system to suit it.
    6 heads seems like a lot.

  2. walta100 | | #2

    Sound like a bait and switch to me.
    You call asking for a mini split they give you a bid for the mini with a head in every room with 4 times the tonnage needed to cool your home. When you are shocked at the price quoted they switch you to a standard AC unit with half the tonnage for a much lower price.

    I think you should consider finding another contractor.

    Walta

  3. Expert Member
    Dana Dorsett | | #3

    There is almost no combination of 6 mini-split head that would be even remotely right-sized for a normal sized house.

    With the "ductless head in every room" approach most of them will be grossly oversized for the peak loads and will only cycle on/off, never (or rarely) modulating. With a multi-zone compressor big enough to serve that many heads even the compressor will be oversized for the peak load, and will be short-cycling itself into low efficiency with all the heads cycling.

    Take a big step back and hire a third party like an engineer (not an HVAC contractor) to calculate the heating and cooling loads of very room. If there isn't more than 3-4000 BTU/hr of heating or cooling load (roughly half the max output of a half ton ductless head) it should NOT get a dedicated head.

    There are many ways to incorporated ducted mini-split cassettes to split the output between several low-load rooms, and that's often a better solution than 1-2 stage central air from both a comfort & efficiency point of view.

    Central air needs to be right-sized per the Manual-J calculated loads too, not just sized to some idiot's rule of thumb, and to avoid a cascade of cold air coming out of those registers in winter (or condensing moisture in the ducts) the ducts need to be somewhere inside the thermal boundary of the house, not in a hot/cold attic above the attic insulation. Most rule-of-thumb contractors in NY/NE use "a ton per 750 feet of conditioned space" come up with a number and round up from there to the nearest ton, which is often oversized to the point where the latent cooling (moisture control) is really lousy- it can cool the place in a hurry, but leaves it clammy, not comfortable. If stooping to a rule of thumb approach a ton per 1200' or a ton per 1000' is usually closer to reality for homes under 2000 square feet. A ton per 1500' or more is usually closer for bigger homes, but there is a lot of variance (in both directions) from those ratios.

    This graphic plots the square feet per ton of a few dozen homes (mostly in the southeastern US) based on Manual-J cooling load calculations performed by Energy Vanguard, a company in Decatur GA:

    https://www.energyvanguard.com/sites/default/files/styles/panopoly_image_original/public/square-feet-per-ton-air-conditioner-sizing.png?itok=vsJxOobH

    Note that only the WORST performing homes are in the commonly used 750 square feet per ton common rule of thumb range, and those were likely to be in locations more humid (bigger latent cooling loads) than the Bronx.

    Undersizing the AC by a bit is almost always more comfortable than oversizing it by 2x. Undersized AC will run very long cycles, long enough to dry the air out. So even if it's not quite keeping up with the temperature for the hottest couple of hours in a day, it's still pretty comfortable because it's considerably less sticky. For most people 35% relative humidity @ 80F is going to be more comfortable than 60% relative humidity at 75F.

    So, six half ton heads is usually worth 3 tons of cooling if all tied to a single multi-split, but could be as high as 4 tons (or more) if all on separate compressors. At 1000' per ton that would be a 3000-4000' house. At 1200' per ton it would be a 3600-4800 ' house. How big is your house?

    To get six zones on a single multi-split compressor usually takes 4 tonner, with a minimum modulated output of more than 1 ton, running a crazy short duty cycle if serving just one half-ton head.

  4. JAL55 | | #4

    Thanks so much for all your responses. Currently I am thinking two minisplits to cool the kitchen, dining room and living room on the ground floor with three 7000 BTU units in each of the three bedrooms on the second floor and one additional unit for a finished attic. Do you think this sounds reasonable? I was also thinking it may be worthwhile to investigate if I could get solar panels installed to offset the running costs?
    Thanks again

  5. Expert Member
    BILL WICHERS | | #5

    The cost to put up solar panels would probably be better spent doing some insulating and air sealing work. On older homes, as many in urban areas are, air sealing and insulation can result in BIG savings for minimal cost. Go for the low hanging fruit first, you can always add solar panels later.

    Bill

  6. lookloan | | #6

    To Dana Dorsett - Where are you located? You sound more knowledgable than most - (no offense to others)

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