Ductless Mini-Split Sizing Guide

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If you are considering a ductless mini-split installation, selecting the right-sized mini-split unit for your home is one of the most crucial steps. Mini-split systems are one of the most efficient HVAC systems on the market, thanks to inverter technology. This technology allows mini-splits to generate only the necessary amount of cooling or heating to maintain your temperature specifications. Even if mini-splits are equipped with this type of technology, it is still essential that the unit is sized correctly for your home. In this article, we share a ductless mini-split sizing guide to help you out.

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Why Is Proper Ductless Mini-Split Sizing Crucial?

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A properly-sized mini-split heat pump is essential for its performance. A system that is correctly sized means that it can efficiently and effectively cool or heat your home. Therefore, you must install a system that has a BTU rating that matches your needs and specifications.

British Thermal Unit

British Thermal Unit, or BTU, is the measurement of thermal energy. It measures the heat needed to raise the temperature of one pound of water by one degree Fahrenheit. Therefore, one BTU equals the power to cool or heat one pound of water by one degree Fahrenheit.

In general, the larger the room, the higher the BTU rating your unit will need. A mini-split can have a rating of as low as 6,000 BTUs to as high as 56,000 BTU. When you are choosing a mini-split based on the BTU rating specifications of your home, bear in mind that the units’ heating capacity should be a tad higher than its cooling capacity.

It is also essential to take note that your system should not be too small or too big for the living space you want to heat or cool. Otherwise, there would be unwanted repercussions that will affect your everyday comfort, heating and cooling costs, and your mini-split system’s overall functionality.

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What Happens With An Undersized Mini-Split?

image of a couple feeling cold due to undersized ductless mini-split

A mini-split that is too small will only lead to an inefficient and uncomfortable home. Your unit will continuously run, but it will not be able to condition the space it needs to effectively. You end up wasting energy as you won’t even fully feel the unit’s cooling or heating power. This inadequate cooling or heating will cause hot or cold spots as the desired temperature won’t reach all sections of the zone.

A ductless mini-split that is too small also tends to work harder and longer to make up for its inadequacy. This will cause your unit to age and wear out faster than it should. In turn, this increases the chances of a mini-split breakdown. Not only that but the longer your mini-split works, the higher your utility bills will get as well.

Bigger Doesn’t Always Mean Better

A bigger mini-split does not necessarily mean that it is a better choice. Firstly, you will spend more when you buy a mini-split that is too large. This basically throws out any energy savings you will be making.

Second, a mini-split that is too large will lead to short cycling. Short cycling is when your system repeatedly turns on and off in a short period. A short cycling unit will lead to faster wear and tear, as well. As a result, your system is likely to experience malfunctions often, and this can lead to premature failure as well.

Lastly, your mini-split also functions as a dehumidifier. Unfortunately, an oversized mini-split that short cycles cannot remove humidity properly. Your home will feel cool, but humid and damp as a result.

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How To Size Your Mini-Split

ductless heating and cooling system

When selecting a mini-split system for your home, your sizing consideration should include the size of the room for a single-zone application or each room for multi-zone configurations. Your outdoor compressor may have a different but complementary size from each indoor unit installed in various zones throughout your home. Individual rooms in your home most likely have different sizes. Therefore, the BTU rating of each indoor air handler will also vary.

Keep in mind that it is always best to have a professional help you determine the correct mini-split size you need. An expert technician may conduct the following steps to get a general estimate of the right mini-split sizing.

  1. Determine the square foot of the rooms. A professional will measure the length by the width and multiply them. For example, if the room is 30 feet long by 40 feet wide, then it would be 30 x 40 = 1,200 square feet.
  2.  To get the approximate BTU rating, the technician will multiply the square footage of the room by 25. So if you have 1,200 square feet and you multiply that to 25, you get 30,000 BTUs.

The calculation above works for rooms that have a rectangular shape. Other standard room shapes include triangular and circular. A professional can help you regardless of what room shape you have. Therefore, it is always a good idea to contact your local trusted technician to help you determine the ductless mini-split sizing appropriate for all the rooms in your home.

Below is an approximate BTU sizing chart for mini-split systems:

Ductless BTU Sizing Chart

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Factors That Affect The Ductless Sizing

Keep in mind that although room size can significantly affect the BTU capacity you need, it is not the only factor at play. The environmental conditions and other external factors also play a huge role in determining the proper size of the mini-split you need.


The climate outside can affect the temperature indoors. Depending on whether you live on hotter or colder weather, your mini-split can be sized differently because of it. For instance, if you live in a place that has temperatures that frequently exceed 90 degrees Fahrenheit, then the BTU rating of the system you will purchase should increase by at least 30 percent.

Age And Insulation

The age of your home is a factor because newer homes typically have better insulation than older ones. This is due to the revised building codes. If you live in an older home that has poor insulation, then it requires more power to cool or heat. The BTU rating needs to be increased by at least 30 percent so that you can be comfortable. Whereas, if you have a new home with good insulation, then it requires fewer BTUs.

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Ceiling Height

High ceilings affect the room size as they make the rooms larger. The standard ceiling height of most homes is eight feet. If your ceilings are higher than that, then it is recommended that you increase the BTU capacity of the mini-split you will purchase by at least 20 percent.

Other Factors

A few of the other factors that affect the BTU load calculation of your home include:

  • Windows And Doors: The number of windows and doors in the zone you are placing your mini-split in affects the BTU load as these are not insulated. Therefore, the cracks and small gaps in your doors and windows can leak air.
  • Direct Sunlight: Direct sunlight heats the room. Determine how much direct sunlight a room gets as it can affect the size of the mini-split the room needs.
  • Kitchen: If the room you want to put the mini-split unit in is a kitchen or a room close to the kitchen, then you might need a bigger sized mini-split so that you can adequately cool it. This is because kitchens are open spaces and have appliances that are sources of heat like ovens and stoves.
  • Number Of Occupants: People generate heat and can affect the size of the mini-split you need. Therefore, you need to take note of the usual number of occupants in the room you are putting the equipment in.
  • Lightbulbs: Lightbulbs also generate heat, and some types of bulbs create more heat than others. Therefore, the size of the unit is affected if a room has a lot of lightbulbs.
  • Floor Type: A floor that is carpeted is more insulated than one that is just hardwood. This could, therefore, affect the mini-split size that the room requires.
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Finding the correct size of the mini-split your living space needs is done via a process called load calculation. This calculation determines the right amount of warm or cold air that is required to ensure that the area is at a constant and comfortable temperature.

There are a lot of load calculators available on the internet. However, these can lead to miscalculations. If you want a more precise sizing calculation, it is best to consult a professional HVAC contractor. Your qualified local ductless technician is guaranteed to give you a complete and professional sizing for your space and help you get the exact product your home needs.


Call Townsend Energy For All Of Your Home Heating And Cooling Needs

te logoIf you are ready to switch to a ductless HVAC system or have more questions, call Townsend Energy today. Our experienced and certified technicians will discuss your needs and requirements to help you find the best ductless HVAC system for you.

Townsend Energy is a full-service HVAC company that services residential and commercial properties throughout New England. We offer a range of heating and cooling services, including HVAC installations, repairs, tune-ups, and more. Our experienced, NATE certified professionals can find the best home heating and cooling solutions for your home while working with your budget. We aim to improve your home’s level of energy efficiency, comfort, and indoor air quality. We back our work by a warranty to ensure your satisfaction. Also, we provide free, in-home estimates. Call Townsend Energy today.

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