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Community and Q&A

Sizing and Operating a Minisplit System

lazarus870 | Posted in Energy Efficiency and Durability on

Hi all, thank you for reading my post. 🙂

I’m just outside of Vancouver, BC.  I live in a uniquely shaped condo that’s just over 1,000 square feet.  It’s the top floor of a 3-story wood-frame building made in 2009, and it’s split level so the majority of the living area is downstairs (kitchen, bedroom, living room, one bathroom) with an upstairs loft area and a second bathroom.  The entire thing is pretty much open except the bathrooms, so the living room has a ceiling that slopes  well past 20 feet  on an angle.  Add a few more feet for the upstairs area.

Last summer was an insane time, weather-wise, and internal temperatures reached in excess of 110 degrees and the lack of air flow meant it took days to cool down.   I purchased a dual-hose portable rated for 14,000 BTU but I have read the actual output is actually lower.  Either way, it struggled to lower the temperature, and even when running full-bore for hours, the temperatures would climb into the mid 80’s at the very least on a milder day.  On a hotter day, much worse.  If I shut it off (thing is obscenely loud) it would start climbing in temperature immediately.  As in, if it was 86 degrees with the AC on, 5 minutes after the AC was switched off it would get to 87.   Underpowered, noisy, and probably lost a lot of efficiency by having the sun heat up the window right in front of the unit.  One day where it was 110 out I started running it from early in the morning and by 5 PM it was 97 degrees inside with the little portable not being able to do a damn thing about it.

Facing west, I get all the afternoon sun, and I have gigantic windows.  So in the summertime even 70 degrees and sunny can start pushing the temps into the low 80’s.

I have put a heat-rejecting film on the window and it seems to have helped but it’s not enough.  I finally got my condo board to reluctantly agree to a mini split installation.

So I have gotten a few quotes.  Again, 1000 sq ft, but facing west, scorching hot inside, lots of windows.  I got quotes for anywhere from an 18k unit and a 6k unit in the living room and bedroom respectively, to one company wanting to go up to 3.5 tons.  Two other contractors said 3 tons, and two contractors bid 2.5 tons.

I went with the company who installed the 3 ton Mitsubishi Mr Slim unit and they just have to come back to fix a couple of things.

But I had another local HVAC company tell me he thinks it’s way oversized and I should be more of a 2 ton unit.

My bedroom, for example, is about 150 sq ft and they put a 12k unit in there.  Sounds largely oversized, right?  Issue is my bedroom has a 20 foot 7 inch ceiling that goes to a second floor and the afternoon sun is relentless.

Don’t know much about the construction of the building but the listing for the building notes

  • wall insulation r14/r20
  • roof and ceiling insulation r40

Sorry for such a long post, lol.  I guess my question is, with all the info provided, does a 3 ton Mitsu unit sound oversized?  The ceiling height and “open-ness” of the layout throws a lot of estimators off, but I haven’t seen anybody do a proper manual J and I didn’t know they were supposed to.

Additionally, since it’s an “inverter” will it just ramp down?  The lowest BTU rating of the outdoor unit goes down to 12,600 per Mitsu spec, and the 12k unit goes down to like 1500 and the 24k unit can apparently ramp down to 8200 at minimum.  Combined it’s 9700 BTU minimum cooling if they both hit the bottom, but the outdoor unit has a minimum of 12,600 BTU.  Does that mean that the system, accounting for both units, can only ramp down to 12,600 as a minimum cooling load before the system kicks off?

Lastly, I know this may be inefficient, but living in a multi dwelling building, and living on the top floor, I am apprehensive to leave things running, lest the condensate pump fail (it has a safety shut off but that’s assuming it’s wired in right and that can fail), as I am at work for 12 hours a day and I don’t want to come home to a damaged floor from condensate leaking, and a big insurance claim.  That said, would there be any issue to shut this unit off when I leave for work, and turn it back on when I return?  I know it’ll have to be playing catchup and work harder to catch up to the weather, though.  I get that.  Sadly my portable AC, even when run from 9 AM, would be useless from 3 PM onwards and just start climbing in temp from the afternoon sun so I realized it was a waste to leave it on all day since it got easily overwhelmed by the afternoon sun.

Thank you all for reading this long, meandering post!

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  1. walta100 | | #1

    My guess is the unit is likely larger than necessary.

    I would not assume the unit was a condensate pump or safety cut off. Often, they drain via gravity and a sloped tube with the safety cut out being sold as optional equipment.

    At this point it is what it is they likely sized the unit using the local rule of thumb so it will be oversized and will work well enough to keep you happy. If you had asked before it was installed most of us would have recommended an accurate manual J calculation and that you get a separate compressor for each head.

    The problem with having an oversized system is that they run for a smaller percentage of the time and can’t remove enough moisture from the air given the shorter run times. This forces you to set thermostat lower in order to remove enough moisture to be comfortable. You end up with a cold and clammy feeling house. Setting your unit to its “dry” mode can help if this seems to be a problem.


  2. lazarus870 | | #2


    Thank you for your reply. If it ends up being too powerful, could I just switch off one unit and rely on the larger 24k unit to kind of act like a single-zone cooling situation unless I need the bedroom one to assist? Like run the living room head, open the door to the bedroom and rely on one 24k unit to cool everything, effectively bringing it from a 3-ton 2-zone to a 2-ton 1 zone?

    I live in the Pacific Northwest, which is generally known for its milder weather, but I am inland so I get a few more degrees and our summers lately have been sweltering hot, especially that last one where people died.

  3. walta100 | | #3

    One of the best reasons to avoid the multi headed minis is because of their much more limited ability to run at slower speeds. General a multi can slow to 30% and the singles to 10% but since you could turn off one of the singles and run the other at 10% its lowest output would be 1/6 as much.


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