Sizing a Ductless Air Conditioner

January 8, 2009 by Shawn Mack  
Filed under Featured, Technical Guide

Properly sizing your mini split air conditioner or other ductless system is a relatively easy task. I get asked daily by consumers what size air conditioner is right for me? Whenever you purchase a ductless air conditioner make sure to get a unit that can handle the area that needs to be cooled with a few variables thrown into the equation. The worst thing that could happen is that you purchase the equipment and everything gets installed properly, but the unit doesn’t live up to expectations.

Remember first that ductless air conditioning equipment can only cool or heat areas that can be reached by the expelled air. Like a fan, if you close doors to rooms surrounding the indoor evaporator, those rooms will not feel the effect of the circulated air. Most ductless mini split systems will have fan settings from relatively low to high speeds. A higher speed, of course, will help the room air circulation.

For larger areas it is a good idea to check the evaporator fan’s Cubic Feet per Minute (CFM) output in the specifications. The higher the CFM, the better the circulation.

Let me start with the basics of ductless sizing for the best case scenario. For every 12,000 btu., an average of 600 square feet can be cooled or heated. If your air conditioner produces 18,000 btu.’s the coverage improves to 900 square feet.

For me, best case scenario means a square room with 8 foot ceilings, no windows in a climate of 72 degrees. Now let’s look at some of the variable that reduce the amount of square feet covered by a ductless air conditioner

6 Variables that Reduce an Air Conditioners Useful Area:

1. Climate: If you live in an area of the world that has seasonal temperatures in excess of 90° F. Increase 30%
2. Windows: If you have south facing windows in your designated area, or if you have large windows. Increase 20%
3. Ceiling Height: High ceilings increase a room’s size reducing the air conditioners effectiveness. Increase 20%
4. Appliances: Heavy kitchen appliances and electronics increase the heat created in an area. Increase 20%
5. People: The more people in the area the more heat produced. Increase 20%
6. Insulation: Poorly insulated homes are much more difficult to cool or heat. Increase 30%

Taking into consideration the above variables we can form a good estimation on the size of air conditioner needed to do the job. We know that 12,000 btu.’s can cool 600 square feet or a room approximately 25’ x 25’. If any of the 6 variables from above are a factor we will not be able to maintain a comfortable environment for your area. So we need to move up to a higher btu. ductless air conditioner.

If our area is a 16’ x 16’ room (256 sq. ft.) in Orlando, Florida (Increase 30%) with south facing windows (Increase 20%) you would need an air conditioner that can cool a 384 sq. ft. area.

There are only a few basic sizes produced in the ductless air conditioning industry, so it makes it easier to find the unit that will work best for you.

Standard Sizes of Ductless Air Conditioners:

1. 9,000 btu. Coverage area 450 sq. ft.
2. 12,000 btu. Coverage area 600 sq. ft.
3. 18,000 btu. Coverage area 900 sq. ft.
4. 24,000 btu. Coverage area 1,200 sq. ft.

If you come close to the maximum square feet numbers listed above after adding in your variables make sure to bump up to the next higher btu. system. I have never seen a problem of over-sizing a ductless air conditioner. The thermostat is always in the area being cooled, so if you have a larger system expelling more cool air, the system will run less often.

These numbers have worked great for me in the past, and I hope they help you find the right size air conditioner in the future.

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37 Responses to “Sizing a Ductless Air Conditioner”
  1. Klimaanlæg Says:

    nice post, thanks for sharing!

  2. Sadie Says:

    How much would you decrease for foam insulation?

  3. Jeremy Says:

    You don’t want the system running for less time, the system will not take the moisture out of the air which is a primary concern for ac, for reducing mold growth

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    thank you for posting.

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  20. Bill Says:

    You mention that over sizing is not a problem. Does this include the usual humidity removal concerns ? Thanks, Bill

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  24. Mortimer452 Says:

    I have to disagree on the over-sizing comment - there are definitely problems with having a unit over-sized for an area.

    Humidity removal is an important aspect of any AC system, although most ductless units have a “dehumidifier only” setting so you could just use that if the temp is where you want it. But who wants to mess with switching settings between “dry” and “cool” all the time?

    Another issue is short-cycling. This is when the AC system turns on and off a lot, rather than running constantly. Short-cycling causes way, way lower efficiency and unneeded wear and tear on your compressor. It’s like city vs highway driving - your car gets better mpg and highway miles cause less wear & tear on the car.

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  26. Carrie Says:

    Agree with the concerns about over-sizing. If a person does a little research, there is a lot of evidence that over-sizing the system will not only make your install cost higher, but also decrease energy efficiency over time. The humidity level of your region should be considered. Also, a machine that is oversized, will be turning off too often.

  27. James Says:

    What if you just turn the larger unit on low. You may not need the extra cooling but what about the extra heating you may need it then as well. I am for over sizing because under sizing just will not fly at all.

  28. James Says:

    Further note if you have a fast car you can still drive the speed limit. Meaning the larger unit does not have to be placed on high cool all the time. You just have to find what speed works properly.

  29. Dave G Says:

    My tall north facing windows are a problem during summer afternoons when the sun is in the north. Why do you only add capacity for south facing windows?

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