Mini Split Sizing
All mini split air conditioning systems feature specific ratings and numbers such as BTUs and tonnage. These values refer to the system’s capacity or size. Every system is designed to operate in rooms of a specific size, and choosing the right size for your needs is critical to ensure your air conditioning system works as intended.
Follow this complete mini split sizing guide and make the best purchasing decisions when shopping for a mini split air conditioner or heat pump.
Sizing Your Mini Split Air Conditioner: What’s the Right BTU Rating to Effectively Cool or Heat Your Room?
When buying a ductless mini split system, ensuring your system has the right size for your house is critical. Remember that size, in this respect, means capacity or BTU rating. While there are many other data points and elements to consider before deciding what to purchase, making sure you have chosen a properly sized unit should be on top of your checklist.
Careful consideration of your mini split’s sizing will significantly impact the comfort level you’ll be getting from your unit. It will also influence its cost-effectiveness, affecting your long-term energy bill expenses. In other words, choosing the right size, capacity, or BTU rating for your mini split system is one of the most critical decisions you can make.
Here is a breakdown of every element relevant to air conditioning sizing. Learn everything you need to know to choose the right size and capacity for your home’s dimensions and environment.
How the Size of an Air Conditioner Affects Its Cooling Capacity
When it comes to mini split air conditioners, the term “size” refers to the capacity of the unit to heat or cool a specific space or room.
The HVAC industry uses two specific units of measurement to calculate an air conditioner’s size: British Thermal Units (BTU ) and tonnage.
- British Thermal Units: The BTU is a unit of heat. One BTU is equivalent to the heat needed to increase the temperature of one pound of water by 1°F. One BTU is also equivalent to approximately 1,055 Joule.
- Tonnage: In HVAC contexts, tonnage refers to the ton of refrigeration, a unit of measurement of heat transfer. One ton of refrigeration equals the amount of heat needed to melt or freeze one short ton (2,000 lbs) of pure ice in 24 hours.
In HVAC contexts, the BTU value of an air conditioner specifically refers to the amount of BTUs it can remove (or add, if it is a heat pump) per hour. Consequently, some manufacturers may prefer to use the term “BTU per hour” or “BTUh.”
One ton of refrigeration is approximately equivalent to 12,000 BTU per hour. Therefore, a system rated at 1.5 tons has a capacity of about 18,000 BTU, whereas a 4-ton system has a capacity of 48,000 tons. There are also systems with less than one ton, such as 0.75-ton mini splits (9,000 BTU) and even 0.5-ton (6,000 BTU) models.
The higher the BTU or tonnage rating, the more powerful the mini split system is, as it has a higher heat transfer rate per hour.
All Ductless Mini Splits Are Designed with a Specific Size in Mind
Air conditioners, including ductless mini split systems, are built in different sizes to fit inside rooms and zones of different dimensions.
Each system is designed explicitly to operate in a specific range of room size. The indoor and outdoor units have varying sizes or rated capacities in ductless mini splits.
Knowing the size of each unit will guide you to install a correctly sized system properly. When installing a ductless AC solution to your home, your sizing consideration should factor in the size of the room for single-zone application or each of the individual rooms for a multi-zone configuration.
In other words, your outdoor unit may have a different but enough size to complement the size of each indoor air handler installed in different rooms inside the house. Because it’s more likely that each room in your home may have different sizes, the size or rated capacity of individual air handlers may also vary.
Example: A tri-zone mini split air conditioner with a total capacity of 30,000 BTU may feature indoor units of three different sizes, such as in a 9-9-12 system. In this configuration, two of the indoor units have a capacity of 9,000 BTU, while the third is for up to 12,000 BTU. This configuration is ideal for controlling the temperature in one larger room and two smaller rooms.
With Air Conditioners, Bigger Size Doesn’t Always Mean Better
Contrary to popular belief, bigger doesn’t always mean better when it comes to air conditioners. Purchasing the system with the highest capacity you can afford is unwise. Having a higher BTU or tonnage rating than you need for your space creates more problems than it solves.
The principle to remember is to choose an air conditioner size that is proportionate to the amount of space you want to control. So, when choosing a ductless mini split, it is essential to ensure that its BTU rating is suitable to the size of the room where you will install the system.
As a general rule, the larger the space you have, the higher the system’s capacity should be. The key is to ensure that your chosen mini split system is adequately sized for the specific space you need to heat or cool.
Why Is Proper Sizing for Your Mini Split Unit Important?
To better understand why your mini split system must be properly sized for the room that you will heat or cool, it is critical to know what happens when a unit of the wrong size is installed in a living space.
There are two scenarios to consider: using an oversized unit (too many BTUs) and using an undersized unit (too few BTUs).
Effects of using an oversized air conditioner
If your air conditioner’s size is too large for your space, it is oversized. Although there is no hard limit on what defines an oversized system, you can determine whether your current air conditioner is oversized by checking for the most common symptoms.
When using an oversized unit, you may notice that your mini split air conditioner will quickly cool the zones inside your home. However, if it cools an area too quickly, your system risks running into a process called short-cycling.
When a system short-cycles, it will turn itself on, distribute cool air until the target temperature is met, turn itself back off, and then repeat the cycle continuously. This constant on-off cycle occurs because the system cools your home too quickly and has trouble maintaining the temperature you set in the thermostat.
Repeated on-off cycles also prevent your air conditioner from removing ambient humidity, resulting in significant discomfort. It is also very costly; an air conditioner constantly turning itself on and off consumes more energy than a properly sized one.
In short, while using an oversized system may result in faster short-term cooling, it is also a system that fails to regulate humidity properly and costs more to operate than it should.
Oversized systems are exceedingly wasteful to run in the long term, as the repeated short-cycling causes premature wear and tear to your system, resulting in significant maintenance and repair costs.
Effects of using an undersized air conditioner
If your air conditioner’s capacity is too small for the size of your living space, it is undersized.
Like oversized systems, there is no hard limit or definition, but there are symptoms to tell whether your system is undersized.
One of the most obvious signs of an undersized system is the runtime. An undersized mini split air conditioner is typically forced to run constantly at full blast because it doesn’t have enough capacity to cool down a room efficiently. The immediate result is an AC that runs all the time yet doesn’t provide adequate cooling.
Another effect of insufficiently-sized air conditioners is energy consumption. Undersized units have poor energy efficiency because they run at 100% for far more extended periods than a properly sized unit.
The constant runtime at maximum power causes premature wear and tear, as with the constant short-cycling of an oversized unit. Consequently, you risk experiencing costly parts breakage.
In short, using an undersized system offers no benefits. The cooling is insufficient, the energy consumption is higher than normal, and the failure rate increases.
Your Mini Split System Sizing Guide
Now that you understand the effects of improperly sized air conditioners, you may be wondering how to calculate the right unit size for your needs. To determine the ideal BTU for your living space, you’ll need to know your room’s exact dimensions and environment.
According to the EPA, the basic rule to determine your basic BTU requirements is to add the square footage of every room covered by your system’s indoor units and multiply the result by 20. This principle is the 20 BTU per square foot rule.
However, knowing your total square feet isn’t sufficient. A given room’s BTU requirements may change depending on your environment, local climate, each room’s exposure to sunlight, the ceiling height, and the number of regular occupants. Free BTU calculators can help you determine the climate control needs in each room or zone.
Remember that even the most accurate BTU calculations are still just estimates. Once you have the BTU needs for each room, add them together; the result is unlikely to match the BTU capacity of any given system.
For example, if the BTU calculator has estimated you need a total of 21,961 BTU to cover all of your zones adequately, you will not find a 21,961 BTU system on the market. The best practice is to take the number you’ve obtained and select a mini split system with the closest capacity to your calculated needs while still being above your estimated numbers.
For instance, you may find many 20,000 BTU and 24,000 BTU products. Here, the best solution is to select the 24,000 BTU system. A 24,000 BTU system in a living space with a nominal 21,961 BTU requirement is neither undersized or oversized and should provide adequate climate control without ill effects. However, although a 20,000 BTU system has the closest number to your target, it is almost 2,000 BTU under your needs and risks being undersized.
Consider expert help
If you need more accurate sizing estimations, you can always ask for the help of an HVAC expert. HVAC technicians can calculate your cooling or heating requirements as accurately as possible and determine the right size of the mini split air conditioner for your home. They can also provide you with expert advice regarding the best make and model for your needs and help you with the installation process.
Sizing Means Load Calculation
Sizing an air conditioner or a mini split system is conducted through what the industry calls load calculation. Calculating a system’s load means determining the correct amount of cool or warm air needed to establish a comfortable temperature or maintain the desired temperature inside the room.
When HVAC experts conduct a load calculation for a specific space, they will provide you with an estimated number of BTU or tons needed to cool or heat the room. The data they use to calculate the load is the same data used to determine the right BTU rating for a given zone.
When sizing for a multi-zone application, you want to select each air handler in a size that can handle the square footage for each room. Once you have done that, you'll want to ensure the system is built with a condenser that can handle the entire load.
You may have noticed that the total capacity of many multi-zone products doesn’t match the total capacity of each indoor unit. A typical example is a 36,000 BTU quad-zone system with a 9-9-12-12 configuration. Although the total capacity of all four indoor units appears to be 42,000 BTU when added together, the outdoor condenser can only produce a maximum of 36,000 BTU.
However, this discrepancy is normal. Unlike central air conditioning, mini split systems are not designed to run at 100% capacity all the time. Even if you switch on all four of your indoor units, properly sized units in each room should ensure none of them run at 100% capacity, providing you with adequate cooling without expending excessive amounts of energy.