How to Size a Mini-Split System Air Conditioner

Written by meg jernigan
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How to Size a Mini-Split System Air Conditioner
LG Three Zone mini-split air conditioner (LG Electronics)

Mini-split air conditioners are a sensible choice for many home cooling applications because they combine many of the benefits of a central air system with those of a window air conditioner. The bulky, noisy compressor and condenser components are located outside and connected by refrigerant and wiring lines to a wall-mounted evaporator and fan. Since most mini-split compressor-condenser units can supply more than one indoor air-handling device, they're an ideal choice for zone cooling---cooling individual rooms at different rates. Follow this guideline to estimate the cooling capacity you'll need when sizing a mini-split system.

Skill level:
Moderately Easy

Things you need

  • Tape measure

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  1. 1

    Calculate the square footage of the room to be cooled by multiplying its width by its length. A 16-foot-by-14-foot room would measure 224 square feet.

  2. 2

    Multiply the square footage by 25 to estimate the needed cooling capacity in British Thermal Units per hour (BTU). The 16-by-14-foot room would require 5,600 BTU.

  3. 3

    Increase the cooling capacity by 30 per cent if the room has little or no insulation and by 20 per cent if it's in a sunny location. So a sunny, uninsulated room would require an additional 50 per cent of the initial BTU rating. Increase the capacity by 50 per cent if the room is a kitchen, regardless of whether it is uninsulated or exposed to strong sunlight.

  4. 4

    Add the required BTU values for each room if you're installing a zone system. Use the sum for all rooms when selecting the air conditioner.

  5. 5

    Select a mini-split system that supplies at least the BTU capacity that you've calculated. Units that are up to 30 per cent more powerful will provide efficient, comfortable cooling, but undersized air conditioners may struggle to meet the demand when it's very hot or humid.

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