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Ductless Mini-Split Decision

JWWine | Posted in General Questions on

Hi, a friend of mine is an HVAC contractor and he’s suggested a 19k BTU ductless mini-split system to replace the gas furnace that currently heats my 1000 sq foot home. I live in California Climate Zone 5. The house has just 2 beds and 1 bath. While it has no insulation now, I’m going to blow cellulose into the attic and condition the crawlspace with closed-cell foam. 

Will a 19k BTU Fujitsu outside along with a 15k unit in my living room (316 sq ft) and 9k unit in one bedroom (162 sq ft.) make the house comfortable? It never gets below 40F here and rarely above 90F.

I had a load calculation done before embarking on the insulation job, and that number came out to 34k. There are single pane windows that I don’t have immediate plans of replacing.

Many thanks!

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  1. Paul Wiedefeld | | #1

    Two huge problems with this plan: way too big and way too expensive. You have a furnace already, so you have ductwork. Use the existing ductwork for a ducted heat pump! It can be a ducted minisplit or just a plain old heat pump. Also, there's no way your heat loss is that high in a climate like that, so adding that much capacity will be very uncomfortable, inefficient and expensive. I suspect a ducted 12,000-15,000 btu heat pump will be sufficient. Use this method since you have an existing furnace:

  2. JWWine | | #2

    Hi I do t have ductwork. There’s a wall-mounted furnace in the living room.

    1. Paul Wiedefeld | | #5

      Check on how much heat that existing furnace can output and how much is actually being output. 34,000 seems very high.

  3. Eric Habegger | | #3

    I’m offering this info as a point of reference rather than advise. I live in California and imagine my climate is not far different from yours. I also have a two bedroom one bath home with 1100 sq feet. So far it seems like a pretty good comp (unless you live high in the Sierras). The differences are that I have double glazed windows and insulation in attic, walls, and crawl spaces. I have also decreased the leakiness to 2.25 ACH 50. I’m existing comfortably with a single18k heat pump.

    This is a long winded way of saying your calculations seem mostly correct for your proposed insulation improvements. However it still seems like a lot of excess capacity for such a small house in a moderate climate. Excess in terms of what could be done, and usually is, in other parts of the country. I’d think long and hard about any improvements you might consider doing additionally to what you are proposing to improvements For example:

    1 getting a contractor to pump cellulose into your walls
    2 replacing your singe pane windows with double pane windows.
    3 tightening up the house. This includes sealing all holes in the attic/ceiling division with canned foam or’ll never have a better opportunity than before you put cellulose up there. Replacing windows will also help in sealing the house if done conscientiously,

    The point is that while this will certainly be more expensive part of that cost will be offset by lower future electric bills and possibly a decrease in the number of heat pumps. Remember, my house is about your size and only uses one 18k unit.

    1. JWWine | | #4

      Hi Eric,
      Thanks so much for your thoughtful reply. I don't plan to pump anything into the walls because the house is 116 years old and I'm concerned about damaging the plaster and lath. The plan is to do as much as possible viz insulation, though, including air sealing the crawlspace and the attic. At some point maybe I'll remove the old single pane windows and replace with energy efficient ones, but for now I'm going for the biggest bang for the buck--that is, attic and crawlspace. You're right for sure about air getting through the old windows.

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