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Minisplit sizing and location

Fabromsil | Posted in General Questions on

Bear with me while I first give you some context about my house.  

My 960 square foot home (2 bed, 1 bath) can get pretty warm during the summer.  It is a 95-year-old tudor style home with lots of original wood casement windows (which I plan to keep). 

There is R38 insulation in the attic and none in the walls.  The house is only 4 miles away from the ocean and it’s regularly pretty cool (West Los Angeles), but there can be 20-30 days per summer when the rear of the house (facing south) can get very hot.  (The kitchen and dining room are at the rear of the house — see attached floor plan.)

I plan to install swing doors in the two doorways that lead to the dining room and the kitchen (where the house gets the hottest).  As a result, I would like to install two mini-splits.  One in the kitchen (perhaps near the back door, facing length of kitchen) and a second one perhaps in the living room. I’m concerned over a few things:

(1)  Whether a 9k btu in the kitchen and a 12k btu in the living room would be sufficient for the respective spaces they will cool? Or would it be overkill?  (Note that I would try to avoid having both units on at the same time, because my plan is to zone, using the swing doors.)

(2)  What is the proper location for the unit in the living room?  So far, I’ve concluded that I should install the unit on an interior wall (on wall shared with linen and bedroom closet), and would run the AC lines up through attic and out to outdoor unit installed on flat roof over kitchen and bedroom.  (The reason I don’t want to install the unit on an exterior wall is because that wall faces the street and I don’t want unsightly AC conduit lines.) The water line would likely run down wall of linen closet and into basement, to a floor drain. Problem with this location is that it would blow cold air in direction away from bedrooms (one of which can get pretty hot during the summer).  Am I setting myself up for having to install 3 headers? (Perhaps a third in the rear bedroom, which is the one that gets the warmest?)  For such a small house, 3 headers seems excessive.  

(3)  Finally, if I install more than one header in the house (two seems likely), does it make more sense to buy a multi-zone unit (i.e. connected to one outdoor unit)?  I would install the outdoor unit on the flat roof that sits above kitchen and rear bedroom. 

Thanks for any input.

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Replies

  1. Fabromsil | | #1

    Another option would be install the living room air handler on the wall near the front door. See attached. The water line would run down the closet wall to basement. Meanwhile, a/c lines would run up to attic and out to flat roof, where they would be connected to outdoor unit.

  2. Expert Member
    AKOS TOTH | | #2

    When it comes to cooling, you need to get airflow into all the rooms. Without that, the rooms will never be comfortable.

    Your best bet is a ducted unit either in the closet in the back bedroom or in the basement. You need to do a heat load calc (try coolcalc or loadcalc) to figure out the proper sizing.

    Multi splits are bad idea for your application as the load on both the outdoor unit and the heads needs to be well matched. Even the smallest 3 zone unit is more than the load for your house, efficiency and comfort would be bad. Even the smallest head is typically 2x the load of a bedroom, that much oversizing is bad for comfort.

    1. Fabromsil | | #3

      Thanks for responding. A ducted unit is so much more expensive. The man-hours to do the ducting is off the charts for such a small house. Am I wrong about this?

      If I do ducted, I can’t zone — unless I pay significantly more. Maybe I’m wrong about that too.

      Also, I had the understanding that a mini-split would only put out as much heating/cooling to deal with the room. Doesn’t that mean that a unit that is “too large” (in terms of BTUs) wouldn’t overwhelm the space because it can “read the room”?

      1. Expert Member
        AKOS TOTH | | #4

        The simple one is a single head in the main space. This should do a decent job of keeping the living/dining area cool. The kitchen is a bit too tucked away, most likely it will be warm.

        There is no easy answer here. If you want the bedrooms cooled, you need to get airflow there. Keeping the doors open mostly works, depending on the solar gain, the bedroom will be within a couple of F of the main space. As soon as you close the door, the bedroom will get hot.

        Ducted units are not a lot, if you have the space for a bit larger duct work, a low static unit is maybe a couple of hundred dollars over a wall mount.

        I have installed both wall mount and ducted units, with a unfinished basement, I would guess it would take about the same amount of time to install a ducted unit with ducts as it would to install a 3 head multi-split. If you want to simplify, looks like you can run most the ducts in the corner of closets.

        The smallest head on a multi split is either 6k or 9k btu. For a larger bedroom load is around 2k. Multisplits don't modulate all that well, an indoor head usually needs to run close to rated max output, so the bedroom will not be comfortable.

        I'm living with a 3 zone multi with a 9k head in my bedroom, it took a lot of mods to get the airflow low and directed away the bed to be semi comfortable. The efficiency is also terrible in this configuration (my wife's studio with a standard mini split that is larger and un-insulated uses 1/2 the power as it takes to cool the bedroom only). I'm very seriously contemplating ripping the head out and replacing it with a ducted unit.

        1. johns3km | | #6

          What mods did you do may I ask? I have a Fujitsu multi as well with a 7k in the bedroom. I tend to keep it on low vs auto however when the refrigerant calls it is a bit noisier this way. It's certainly way oversized.

          1. Expert Member
            AKOS TOTH | | #9

            I blocked off the inlet to the unit to reduce the flow by about 1/2, hard to measure exactly but I get around 100cfm out now.

            I also modified the outlet guide vane so that it blows air up towards the ceiling not down at the bed.

            There is no way around the noise issues, I can still hear the refrigerant calls. I do find it a bit better if other heads are running though.

        2. Fabromsil | | #8

          I think I’m now pretty convinced that if I do multi split, I won’t do any in the bedrooms. The front bedroom stays pretty cool because it faces north. But it also faces the street, which is why I don’t sleep in that one. Meanwhile, the rear bedroom (where I sleep) gets a lot of sun in the summer. While I wouldn’t now consider installing a unit there, I do think it would benefit from a unit installed in the living room. (I don’t sleep with my bedroom door closed.).

          Also, I should have mentioned that I work from home. The breakfast/dining room next to the kitchen serves as my office. Rather than having a main unit running in the living area and trying to cool the breakfast room (i.e. office), I figured closing the two doors (that I plan to install to separate back of house from front) and cooling that back area with a unit installed at the far end of the kitchen. This is why I want to avoid still ending up with a “warm” kitchen area.

          I’m also now thinking that a 9k and a 6k are the way to go (rather than a 12k and a 9k). These systems apparently put out a lot of cooling.

          I reached out to an HVAC specialist. He plans to visit my house sometime soon. I will share with him everything you said. If a ducted system would make more sense (given the large attic space and the basement clearance) and it’s within my budget, that may be where I end up.

          Thank you!

          1. Expert Member
            AKOS TOTH | | #10

            Running the online load calcs is quick. Don't guess at this, you can end up with a system that uses a lot of power.

            The smallest 2 zone Mitsubishi unit is 20kBTU, which is probably the heat load for your entire house, not just those two areas. The min on those is 5700btu, which means the unit will likely be cycling most of the time. Fujitsu 2 zone is about the same specs.

            If you do go with dual wall mount setup, better to go with individual outdoor units for each head. These have much better modulation, even with a fair bit of over-sizing on the head, they'll run much more efficient. The part cost on a two zone multi split and dual single zone is about the same.

            All hvac guys that I've ever talked to ALWAYS want to install a head in each room, I don't blame them as both Mitsubishi and Fujitsu flyers show this. This is a really bad idea, push back hard on it. Better to find an installer that will discuss the ducted option and provide a reasonable cost for the install.

        3. Fabromsil | | #11

          Thank you Akos! You’ve given me some really useful information. So glad I delayed any install and posted my questions on this site first. Nothing worse than ending up with the wrong solution.

  3. paula_builds | | #5

    Is there a way you can provide some exterior shade to alleviate some solar gain? Just a thought.

    1. Fabromsil | | #7

      That is a real challenge. My house sits on a slope, with the rear of the house sitting higher than my neighbors. Absolutely no trees either.

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