Does anyone have the ductless split AC systems offered by Mitsubishi, etc.? Is installing them a DIY job? Can you share your comments about them, in general?

Edit: Inspired by DA01 comment, I'd like to add:

From what I see in NE US, they don't seem that popular. Few installers deal with them and all the quotes were higher than conventional systems. Why would this be?

Also if they are so popular why doesn't anyone comment on them here?

  • 1
    I don't have them but have been keeping an eye on them. They seems very popular. I've read that short of charging the system, it can be a DIY install as it's mainly just running flexible copper via small holes in the walls.
    – DA01
    Apr 20, 2011 at 15:04
  • Hm, popular? Yes, in Europe and Japan but in the US?
    – Peter Q
    Apr 26, 2011 at 17:21
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    Well, popular is relative, but in MN I'm seeing more of them. Maybe it's our humid summers combined with lots of hydronic heating systems. I see a lot of them in small businesses around here.
    – DA01
    Apr 26, 2011 at 19:32
  • So you think this can be done as DIY? I'm actually OK with someone taking care of the installation but they are asking for too much money.
    – Peter Q
    Apr 27, 2011 at 3:33
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    I don't know a whole lot, but from a little bit of reading and an episode of This Old House, it seems doable. Pour a slab for the condenser, mount the interior portion on a while, drill some holes, pull copper and wiring. The only potentially tricky part appears to be charging the system.
    – DA01
    Apr 27, 2011 at 13:59

2 Answers 2


Mitsubishi Mr. Slim is the most common example of such a system. I have seen them in several places out west, but not so much in the eastern US. They are very popular in Japan, and other Pacific countries (notice the band names of most of the systems), but they are just starting to become common in the US.

As I understand it, these systems became popular in urban settings because of high-rise buildings. It is easier to route coolant lines down from the rooftop instead of massive central air ducts.

If you are retrofitting a house that doesn't have existing ductwork, these mini-split systems can be a preferable option. As any HVAC contactor can tell you, designing ducts for proper airflow can be a difficult task, and it can be almost impossible to get right in existing construction.

If you get one of the multi-zone ductless systems, you have a thermostat at each indoor unit, which can allow for more flexibility in setting different rooms to different temperatures.

They generally have very high efficiency ratings. In many cases, they have a higher SEER/EER than an equivalent centralized AC system.

The drawbacks to these systems are:

  1. You have the indoor unit sticking out from the wall.

  2. Since they are less popular, it can be harder to find a competant service company.

  3. Fewer companies selling/servicing mean it can be more expensive to get work done.

  4. The residental systems I have seen can support at most 3 indoor units. So, if you want good airflow in every room of your house, you're not going to get it.

  5. The residential systems I have seen max out at ~3.5 tons of total cooling. So, a single system may be insufficient for a big (or poorly insulated) house.

I have a relative who converted his screened-in patio into a sun room. He then added a Mr. Slim to provide A/C. It's quiet and keeps the room cool. And since it is independent from the main house HVAC, the whole house doesn't need the A/C running constantly. This seems to be a really good fit.

At my office, we have one of these units installed in our server room at the office to provide cooling. We've had several problems, but I think the unit was undersized for the application. It's also a residental unit that wasn't made to cool a server room 24/7/365. It's quiet though.

I don't have any first hand experience with the heat pump version of the unit, so I am not sure how well they do in a heating scenario.


We've installed dozens of these and similar systems. I even have one in my own home.

You can up to 4 heads, all of various sizes according to the rooms demands.Efficiencies vary greatly between brands and models.

DIY??? No. Unless you have a vacuum pump ($800), micron gauge ($150), nitrogen tank ($400) and purging gauges ($75), charging gauges ($125) and refrigerant tank for top up ($250 - $800), along with pipe benders, flaring tools, etc. These items will cost you far more than the ductless system itself.

Now, you can of course just connect everything together without any of these items, but your efficiency will drop by a great deal, you will not know if you have a leak, the air and moisture left in the system WILL cause a compressor failure ($1700 repair), your TXV will likely freeze resulting on no cooling anyways, and you will void your warranty.

You can install the mounting brackets, drill the holes for the refrigerant lines, etc. yourself, but not rest of the work.

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