The Ultimate Guide for Sizing a Mini-split System

A mini-split can be a great solution for a small space or basement addition. A mini-split is a ductless air conditioner designed to handle heating, cooling, and humidity management in single rooms, small apartments, or other spaces where ductwork isn’t practical. Like a traditional system, a mini-split system includes an outdoor condenser unit and an indoor air handler. But because mini-splits are ductless, installation can be extremely simple, often requiring little more than a small hole through the exterior wall to accommodate a connection between the two units.

They also offer a great deal of flexibility. Lennox mini-split air handlers are available in wall, ceiling and floor-mount configurations, and our high-efficiency condenser units come in a range of capacities. Though they are designed for ductless applications, a single mini-split condenser can be combined with multiple air handlers to condition multiple spaces.

While a traditional system is ideal where existing ductwork is present or you need to cool a large, multi-room space, mini-splits excel at providing high-efficiency heating and cooling to smaller spaces such as add-on rooms, out-buildings, or basements.

Why Proper Mini-split Sizing Matters

Once you’ve determined that a mini-split is right for your space, proper sizing is the next step. An improperly sized unit, whether too small or too large, can have negative effects on performance, reducing your system’s efficiency – and your comfort.

Too small

Installing an undersized system can cause the unit to work harder and longer than necessary, compromising its efficiency and putting unnecessary wear and tear on components. This makes it harder for the unit to maintain a consistent temperature within the space, resulting in higher electric bills. 

Too large

While you would think an oversized system will have an easier time cooling a space, buying too big can also result in inefficiencies that can cost money and reduce performance. Installing a mini-split that’s too large will cause the unit to short-cycle. Short-cycling is when the unit starts and stops too frequently. This is inefficient for the cooling cycle and can result in temperature fluctuations within the space. Frequent cycling can also raise energy costs and cause humidification concerns. 

Long-term, an improperly sized mini-split is likely to need more frequent maintenance and down the road, may even break down earlier than a properly sized unit.

Calculating the Right Size for Your Space

The capacity or size of an AC unit determines how much space it can cool within a given time. This is measured in BTUs (British Thermal Units). BTUs measure the amount of heat required to raise 1 pound of water by 1 degree Fahrenheit. In the HVAC industry, this is commonly expressed in tons; with one ton representing 12,000 BTUs.

When determining the right size mini-split for your space there are a few key things to consider.

Square Footage Just like a traditional HVAC system, the first thing you should consider is square footage. Carefully measure the living area or areas you’ll be cooling with your mini-split to determine the total square footage.

Ceiling Height Ceiling height matters the same way square footage matters. The higher ceilings of your she-shed getaway will place very different demands on your mini-split than a low-ceilinged basement space.  

Insulation Type and Quality How well your space is insulated can affect the performance and requirements of your mini-split. Both the quality/amount and the type of insulation you have should be taken into account when sizing your system. If you don’t have this information readily available, you can reach out to an HVAC professional to help you get an assessment.

Climate Depending on where you live, the weather can place additional demands on an HVAC system – mini-splits are no different. Whether you experience longer, colder winters or live in a relatively mild climate, the environment your system will be operating is an important consideration.

In addition to these key factors, you’ll want to also think about the space’s purpose. Will it be a center for family or group activity? Does it contain heat generating appliances?

Before deciding on the proper size mini-split that’s right for you, we strongly recommend consulting with a local HVAC professional to make sure you cover all of the bases. There may be specific characteristics to your space that require a technician to assess before selecting your system.   

Exploring Lennox Mini-split Options

Lennox offers a wide variety of mini-split configurations in a range of sizes to meet your needs.

Mini-split Outdoor Units

• • Mini-Split Heat Pumps (heating and cooling)

Mini-split Indoor Air Handlers

• • Wall Mounted Mini-Splits
  • Ceiling/Floor Mounted Mini-Splits (wall-adjacent)
  • Ceiling Cassette Mini-Splits (mounted in ceiling)
  • Ducted Mini-Splits (for multi-space ducted applications)

We also carry a variety of control options including smart programmable thermostats, non-programmable thermostats, and remote controls. Compatible systems can also be controlled from anywhere using the SmartThings app on your mobile device.  

Plus, with energy-efficiency ratings up to 25 SEER2 and 11 HSPF2, a properly sized and installed mini-split can save you hundreds of dollars a year over other systems.

As with all of our equipment, Lennox takes into account more than just performance. All of our mini-split equipment is engineered for whisper quiet operation and our indoor air handlers are designed with clean lines that blend into any space.

Our mini-splits also employ Lennox’ air dispersal technology, producing a gentle flow of air that maintains optimal temperatures without any uncomfortable blasts of air on startup – which can be noticeable in smaller spaces.

Give your local Lennox dealer a call today and let one of our skilled pros help you properly size and install your mini-split system.

This has been reviewed by Justin Huntington, Director of Product Management – Indoor Cooling, for accuracy. Justin brings 12 years of expertise in the HVAC space.