Mini Split Air Conditioner Sizing Guide of 2023

A British thermal unit (BTU) is a measure of the heat content of fuels or energy sources. It is the quantity of heat required to raise the temperature of one pound of liquid water by 1 degree Fahrenheit at the temperature that water has its greatest density (approximately 39 degrees Fahrenheit). 

Used for the accurate and uniform measure for applications of heating and cooling applications, commonly expressed in tons in the HVAC industry. One ton represents 12000 BTU mini split, one and a half tons equals 18000, two tons equals 24000, and so on. This makes sizing and consumer expectations easily understood. 

Much is made of the problems encountered by homeowners if mistakes are made by either over-sizing or under-sizing AC and heating systems-particularly mini split systems, as they allow individual rooms to be addressed without all-house ducting. It Is important to remember that the sizing calculations involved must normally be quite a bit off to provoke these problems. If, for instance, we employ an 18000 or 24000 system in a small space, say 250 square feet (about twice the area of an apartment bedroom) under normal conditions, then we certainly may experience problems. By the same reasoning trying to heat 2000 square feet in Minnesota with a 12000, or one-ton system will provoke complication and failure. 

Proper sizing is important at the very outset of figuring out the right mini split for each zone in our home. Part of the appeal surrounding mini split systems is their versatility and variety in sizes and versions: from wall-mounted units to ceiling cassettes or floor-mounted units, as well as the selection of sizes, they offer easy solutions to any challenge homeowners may face. Today, homeowners can buy 9000, 12000, 18000, 24000 & 36000 air handlers to address the most complex systems, or simply heat a single space for commercial or residential projects. Below are the symptoms of mistaken sizing: 

• “Our 12k unit never stops running…” 
• Low airflow 
• “Our 1600 square foot home is never cooled enough by our 24k air handler…” 
• The home has temperature inconsistencies throughout 
• High energy bills 

• Short AC cycles 
• Your home is humid 
• High energy bills 
• Different temperatures in different rooms around the house 
• Poor air quality 
• AC is too loud 

Let’s move to the art and science of proper sizing and consider the various factors homeowners will need to take into account. 

Many homeowners are opting for ductless mini split systems to heat and cool new buildings and replace conventional HVAC equipment in existing homes. However, identifying the right size, type, and model of the mini split is key. 

We know that the square footage that a mini split can cool depends on the size, or number of BTUs of the mini split installed. The most common mini split installed is a 12000 BTU unit to one Ton which can heat or cool up to about 600 square feet (about twice the area of a parking space). Depending on the size of your home, you may need to install multiple mini split AC systems — perhaps of varying sizes — to adequately heat or cool the property. These multi-zone mini split systems are efficient and able to heat well below zero and cool at efficiency ratings as high as 31 SEER. Generally, mini split air handlers are offered in 9K, 12K, 18K, 24K, and 36K options. The most popular single-zone mini split is the 12K or one-ton unit. Used as a single zone for smaller spaces, it features often in workrooms, garages, offices, outbuildings, and additions. It is also a common part of multi-zone systems as well. 

It is worth noting here that there are 6000 btu mini split options available by various brands. Ideal for spaces of 300 feet or less, they offer an option for the “in-between” spaces that may be small or below ground, like basements, and therefore require power less than a normal space to cool or heat. Barring any meaningful conditions that affect heating or cooling in a given space (discussed below chart) here is a basic determinant scale of application: 

6000 BTU Mini Split = 200-300 sq ft 
9000 BTU Mini Split = 300-450 sq ft 
12000 BTU Mini Split = 500-600 sq ft 
18000 BTU Mini Split = 650-800 sq ft 
24000 BTU Mini Split = 850-1100 sq ft 
30000 BTU Mini Split = 1100-1450 sq ft 
36000 BTU Mini Split = 1500-1700 sq ft 
48000 BTU Mini Split = 1750-2100 sq ft 

The appropriate heating or cooling capacity for your space can be determined using our sizing calculator.

Over years of talking to over a hundred thousand homeowners and contractors’ certain themes emerge over time. The first, major consideration is Comfort vs Climate. As we have noted briefly, heating a hundred-year-old home for Christmas day in Grand Forks North Dakota, or cooling an attic in Memphis Tennessee in August are very different missions. The space in Memphis will require at least an extra half ton of dedicated power-say the difference between expecting a 12K unit to suffice when an 18K is needed, while the Fairbanks homeowner's 12K or 18K mini split may cool like a 24K in Memphis. 

So… will a 12K mini split cool my 600 sq ft basement in Minnesota? Will it heat it as well? What about an 18K mini split cooling my 850 sq ft upstairs bedroom in Louisiana? If I want to cool & heat a 420 sq ft glass fronted porch that was added last year to the east-facing side in Oklahoma…do I select a 9K, 12k, or an 18K model? 

Homeowners generally know more about their home and its peculiarities and challenges than anyone and understand the climate and tendencies of their locational factors better than a supposed expert from an armchair study.

Here are the most common factors that we hear most of our customers grappling with. As always it is important to realize that common sense is the first rule of thumb, closely followed by experience in space, if any. Homeowners generally know more about their home and its peculiarities and challenges than anyone and understand the climate and tendencies of their locational factors better than a supposed expert from an armchair study. 

Mitigating factors: 

• Comfort vs Climate - Take into account what your climate challenge is, to either heating or cooling in your locale. Humidity and high heat throughout Summer, or Dry, Southwestern conditions will require an uptick in tonnage for cooling. Think about your past comfort challenges if you have lived in the climate previously or talk to other homeowners and HVAC pros about their experience. As noted, and adjustment as simple as an extra half-ton-or a step up by an entire ton may be required. 
• Location in Home & Usage - In humid locations consider the challenge of attics, closed-off workrooms, and exposed porches and sunrooms. Ditto the challenge of heating ground floor spaces with north-facing widows in New Hampshire. Make a suitable adjustment. Often the answer is as simple as using a 24K mini split instead of the 18K that looks good on paper. 
• High ceilings, loads of windows, high winds and north facing exposure - High ceiling challenge both heating and cooling requirements. After figuring in climate challenges homeowners will want to address the added BTU needs. Again, 1 24K system may be required to fill in where an18K would be commonly recommended, based on square footage alone-but the homeowner knows. The same is true for projects in high wind areas along the coasts or out West, say in Wyoming. If my sunporch is all windows and faces north, I need to adjust my 9K for a 12K …or even an18K air handler may be called for.  
• Age & insulation - The older a home, the draftier the rooms” seems like a safe bet. The more north-facing window casings the bigger the heat challenge for mini split systems. These are the commonsense factors in mini split sizing that we talked about earlier. How your home or project space is insulated, thickness, grade-and how long ago all matter. Homeowners are wise to talk to neighbors about their experience and consult an HVAC pro if they are new to the home and area. It is not unusual to hear that a well-insulated 700 square foot attic in Montana is cozy in December with only a 12K air handler or that an older home’s living room of 400 square feet -with less efficient insulation-in Mobile Alabama needed an 18K mini split to take care of the August heat instead of a 12K unit. 

There are also multi-zone mini split systems that can connect two, three, four-up to 9 zone systems indoor zones to promote more whole-house and commercial climate control. As you might imagine, all the mini split sizes available-from 9k & 12K all the way up to 48K air handlers, can be involved in today’s homes. These mini splits offer more total BTUs of cooling power and cover more space throughout the home, with outdoor units sized up to five tons. All zones connect back to the outdoor condenser via wires and refrigerant lines — not ductwork. Multi-zone mini split systems that plan to run more than four zones can have units installed on different levels of the property, utilizing “branch boxes” inside the home that allow just two-line sets to run into the home, with a junction then allowing lines to flow to each space. This makes them flexible and versatile when it comes to installation. 

How many rooms your mini split system can cool depends on whether you install a single-zone or a multi-zone model. Single-zone mini splits only have one indoor zone, which usually means they cool just one room. Multi-zone mini splits can connect to four indoor zones, offering simultaneous multi-room cooling. Another factor to consider is how many BTUs the mini split is rated for. In an “open plan” home that is well insulated we have seen 1200 square foot, two-bedroom homes in Ohio served by a single 36K mini split system. We know that a single 24K system can heat and cool an 1100 square foot cabin in the U.P. All the variations and adjustments in sizing, in the end, are about two major calculations: Your personal preferences for comfort and utilizing the proper mini split system for the job. The versatility allowed by the availability of ceiling cassettes, floor-mounted units, wall-mounted systems…even ducted ceiling cassettes means that by using Mini split systems for heating and cooling, there is an answer to every challenge homeowner face. 

One big advantage of mini split systems is that they offer customized cooling for each zone, using a handheld remote, something that comes in handy when a multi-zone system is installed. 

Say you like it cool in the master bedroom, but your teenager likes a more moderate temperature. The zone in the master bedroom can be set to low, and the zone in the teen's room can be set to a higher temperature. This differs from the one-temperature-fits-all approach of conventional HVAC systems. 

Properly sized mini split systems allow greater personalized control over every area of your home or office. The energy efficiency, savings, comfort, and ease of installation and operation are all based on choice and preference. Today’s homeowner is living in a golden age of HVAC.  
 
Remember, we talk to thousands of homeowners and HVAC pros every week. We know the challenges and costs our customers face, from the smallest workroom to the largest multi-level modern home, and our selection of brands and models offers the most value for the best mini split solutions in the marketplace. 

For all the sizing and application knowledge you need to choose the right mini split system for your home or office, contact HVACDirect.com today and experience the direct difference in HVAC-Expert sales staff as well as technical support, award-winning customer service for the best selection of mini split solutions direct to the public at wholesale prices.  
 
Online or on the phone, find quality mini split solutions with expert sales and technical support staff available to you. Call or click and talk to a sizing and performance expert today. The solution is in the name — HVACDirect.com.