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Mini split sizing/placement

ChristiGS | Posted in Energy Efficiency and Durability on

We’re considering installing a multi-zone mini split in a 915sf home. The house is 100+ years old, mostly plaster walls, good insulation in the attic and in the rear walls of the house (which have been replaced)– the rest, we aren’t sure. We are in zone 4. 

The big front room has 9 ft ceilings, the back bedroom has 7 foot ceilings.
The dining room has a 9 foot ceiling– this room adjoins the kitchen (no separation), which has a 7 foot ceiling.
Kitchen and back bedroom receive afternoon sun.

At the far end of the large living area is a second bedroom that closes with a sliding wall (marked with a red dashed line). The wall is only closed at night, otherwise, we leave it open as one space with the living room.

We were considering a 4 head system, but in DIY kits, they only come in 36k BTU. That seems oversized from what I’m reading. The most appropriately sized option seems to be a 24 or 27k with 3 heads.

Here are my questions: 
1) Could we put a 12k BTU unit over the window at the far end of the large room (as part of the bedroom with the sliding wall)– would it be capable of conditioning the living space during the day? Sf is about 350 for both spaces. The sliding wall (bedroom) portion accounts for 130 of that. 

2) Are we better off to do a 9k in the living room, and a 9k in the “bedroom” portion? It seems that the air flow would be improved, but I’m wondering if 18k is oversized.

3) Is a 9k unit critically oversized for the 100sf+ bedroom in the back? It seems it should be more like a 7k, but they’re not available that small in DIY models. 

4) Do you think the kitchen will be adequately cooled if a 12k head is placed over the front window in the dining room? Sf of the dining/kitchen is around 215. Ceiling drops from 9ft to 7ft between the dining room and kitchen. 

Thanks for considering!! 

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  1. Expert Member
    Dana Dorsett | | #1

    In order to get this right it pays to do a room by room load calculation. A 4 head.3 ton system is almost always a mistake in a house that size. The odds are that a 9K head (or even a 6K head) is going to be ridiculously oversized for it's room/zone load for bedrooms, and the minimum modulation of the compressor in a 3 ton system is likely to be more than half your 99% design load (but maybe not if it's uninsulated and has all single-pane windows.)

    I've seen homes that size in zone 4C adequately heated with a single 12K head (with a few cold spots in doored-off rooms.) A single 12K-18K ducted minipsplit with a cold-climate type compressor would likely cover your load more efficiently and comfortably than ridiculously oversized multi-split. Putting it in perspective, a cold climate type 3 ton multisplit has enough output to heat my ~2400' (+1600' insulated basement) 1920s bungalow down to about 0F or a bit lower.

    Freebie online Manual-J-ish calculators such as or tend to overshoot reality by 25% or more, but is WAY better than a WAG. Some other random online calculators produce INSANE overestimates- don't assume they're really worth looking at.

    Are the exterior walls insulated? What type of siding? Is there a basement or crawlspace under this house?

  2. ChristiGS | | #2

    Thanks so much for the speedy reply. This is helpful. Before we decided to go the DIY route, we had 3 installers all size our needs differently. It left us a bit stymied.

    The exterior walls that we've rebuilt (along the rear of the house) have good insulation. The old walls (front and sides), we aren't sure about. The house stays remarkably snug with just 4 space heaters, so I'm guessing there may be something (though it has a harder time holding cool air in the Summer).

    The house has 3 layers of siding that have accumulated over the years: wood siding, asphalt siding, and steel siding from the 50's, which we have replaced with aluminum siding as needed. The house has a full unfinished basement. We're in Zone 4A.

    I'm curious if not having a dedicated head in bedrooms will be a problem in terms of building codes or appraisals. (In order to count a space as an "official" bedroom, I had thought (perhaps mistakenly!) that it had to have its own adjustable source of climate control-- like a vent or a window unit, or in this case, a head.) Any thoughts there?

    Thanks again. Will get to work on the load calc.

    1. Jon_Lawrence | | #4


      For a quick estimation of your heat load, you can add of the watts of your space heaters (if electric) and multiply by 3.412 to get BTU’s. If they are kerosene, the listed output will be in BTU’s.

      I am not a fan of wall mounts in bedrooms because they are almost always oversized and they are not silent, especially when going through a defrost cycle. That noise in a family room with a TV on is not an issue, but in a 100sf bedroom with no other background noise, can be a problem for some. Better to use a ducted unit in my opinion.

  3. joshdurston | | #3

    These numbers will result in oversizing in many cases.

  4. Expert Member
    Dana Dorsett | | #5

    >"These numbers will result in oversizing in many cases."

    Make that " nearly all cases for houses with glass in the windows, and insulation in the walls.", and LUDICROUS oversizing if you're talking merely room size rather than whole-house size.

    >"The house has a full unfinished basement. We're in Zone 4A"

    With a full unfinished basement to work with it's usually easy to run with a single ducted system. If the ducts are designed such that the duct velocity on all trunks & branches are < 400 feet per minute, with a big oversized 4-6" deep pleated filter it'll be easy to use just about any low-static mini-duct cassette. With low velocity they're very quiet (quieter than your refrigerator), with very little wind-chill or draft.

    Whether ducted or ductless, insulating and air sealing the basement walls to the current IRC code minimum (for zone 4A) R10 continuous insulation (2" of closed cell foam is easiest) makes a significant improvement in bare-foot comfort, and lowers the total heat load by 15-25% in most single story homes. If it's poured concrete or reasonably flat CMU or cinder block or brick it's often DIY-able to insulate on the cheap using RECLAIMED 2" roofing foam, with only a small amount of spray foam for air sealing.

    There isn't any substitute for running the load calculations first though. It may be counterintuitive, but as a rule smaller=better, as long as it's still big enough to cover the peak loads, and very often oversizing leads to both poor comfort and poor efficiency.

    >"I'm curious if not having a dedicated head in bedrooms will be a problem in terms of building codes or appraisals. (In order to count a space as an "official" bedroom, I had thought (perhaps mistakenly!) that it had to have its own adjustable source of climate control-- like a vent or a window unit, or in this case, a head.) Any thoughts there?"

    Bedrooms have to be automatically heated to meet code, but it doesn't have to be an independently controlled micro-zone. (If that were true the vast majority of homes in the US would be out of compliance.) If you run the room by room load calculations and decide to use a single ductless head, you may need to install (but you don't necessarily have to use it) a sufficiently sized electric baseboard sized for the heat load in each bedroom.

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