Tips to Mini-Split Installation Success

Updated: Dec. 12, 2023

A mini-split system can add year-round comfort to any home, and they can be installed just about anywhere. Check out these installation best practice tips.

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Man Installing a Mini Split | Construction Pro Tips

Mini Split AC Installation

Simply put, a “Split System” is a heating and air conditioning system, which has two main components, the indoor unit and outdoor unit. The indoor unit absorbs heat energy in the cooling mode, and the outdoor unit rejects the very heat absorbed by the indoor unit. The cycle is repeated until the set temperature is met. Although the indoor and outdoor units are located in physically different “split” locations, they are connected and operate as, one system, continuously circulating refrigerant liquid and vapor by means of interconnecting, dehydrated copper refrigerant lines, commonly referred to as a “Line Set”.

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Two people looking at a computer screen | Construction Pro Tips

What are the advantages?

Split systems range from as small as 9,000 BTU’s per hour, comparable to say, a small window AC unit, for individual room use, up to 4 and 5 ton capacities for “Room By Room” (also known as “Zoning”) and whole house conditioning. With near universal design, mini-splits are “heat pumps” which provide both heating and cooling for a home. Here are key advantages when compared to traditional split systems:

  • Efficiency- Mini-splits are highly engineered systems which offer attractive operating efficiencies, in many instances, eligible for utility company rebates.
  • They heat and cool providing year-round comfort.
  • Solar friendly- Due to very low power requirements, mini-splits are an ideal complement to “Net Zero Energy” homes, or any home using renewable power.
  • Carbon footprint- Mini-splits are 100% electric, with zero operating emissions of any type.
  • Zoning- Most mini-split manufacturers offer systems capable of room-by-room “zoning”, to account for peak demands of a room based upon construction type, exposure orientation and usage.
  • Variable capacity- Mini-splits are typically DC inverter driven. That is, they have a variable speed compressor, which modulates in proportion to instantaneous demand. Unlike conventional cooling systems, which operate with an ON-OFF strategy. Would you use an ON-OFF switch as the accelerator in your vehicle?
  • Location, Location, Location!- Mini-split outdoor units are compact; they can be located far away from noise sensitive areas.
  • They’re perfect for additions and remodels where access to current HVAC system is difficult to access.
  • SSSHHHHH! – Modern mini-split systems are remarkably quiet. 
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A screen showing the calculation of a mini-split load | Construction Pro Tips

Determine the Heating and Cooling Requirements

Too often, in the world of HVAC equipment, larger is interpreted as more effective and provides a higher degree of comfort. The truth of the matter is, oversized equipment can result in a shortened equipment lifespan with erratic temperature and humidity control in the conditioned space. A comprehensive load calculation is the foundation of all mini-split systems success.

A “Rule of Thumb” approach to equipment selection can jeopardize not only your customer’s investment in the equipment but their good opinion of you as a professional contractor. If load calculations are not within your realm of expertise, many equipment manufacturers will provide their own design software, or you may consider numerous industry approved software programs, which will help you correctly determine the required heating and cooling capacities. You can also reach out to independent companies, which will perform a comprehensive load calculation for you. In fact, many municipalities now require a bona fide load calculation to ensure the contractor has done their due diligence in determining a correctly sized mini-split system.

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Reading the instructions | Construction Pro Tips

Installation Manuals are Mandatory!

While most mini split systems have similar operating characteristics, it is absolutely IMPERATIVE that the installation instructions specific to the piece of equipment you’re installing, are obtained prior to installation and read through thoroughly. Often, an installation manual is as much of a legal document as it is a technical reference, thus all cautions and warnings must be in here to for your personal safety.

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Man installing a mini-split | Construction Pro Tips

How To Install a Mini Split

Stated clearances to the front, sides, and rear MUST BE ADHERED TO (not a suggestion) in order to provide adequate airflow through the outdoor unit. A homeowner may want a unit placed in an area that does not meet the manufacturers minimum clearances. In this case, spending time to be creative in outdoor unit placement to ensure clearances are met, and the keeping the homeowner happy, is a wise investment. A poorly located outdoor unit can and will inhibit airflow, reduce system capacity, may result in nuisance shut down and even lead to premature equipment failure. If your customer’s wish is to conceal the outdoor unit, they can certainly do so, with shrubs or even an equipment screen, gate, etc. However, the minimum unit clearances must never be compromised.

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People in a room who look very sad | Construction Pro Tips

Indoor unit Location

Before an indoor unit is permanently installed, clarify and verify with the homeowner, exactly where the [indoor] unit is to be located. Too often, a location the contractor has in mind may vary from what the homeowner had expected, which can result in a less-than-harmonious situation. That said, you’ll want to refer to the manufacturers design literature to ensure that the air distribution will properly cover the room. Although, literally whisper quiet, units which are located too closely to where the customer is going to be most often may result in objectionable noise. And avoid installing the unit so it blows air directly on the customer or occupant of the room.

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Condensate piping | Construction Pro Tips

Condensate Piping is Critical

Most wall-mounted units require a gravity type drain, where other unit types (cassette, ceiling or floor mount) may incorporate a factory installed condensate pump or provide the option for a field supplied condensate pump. Figure out where the condensate drain will go BEFORE installing the indoor unit.

Gravity drains typically require a pitch in the range of ¼ inch per foot, without any dips in the piping. Condensate traps may be required in gravity type drains, depending upon the location of the drain (before or after the blower) as well when prescribed by local code. If you are using a factory or field installed condensate pump, always reference the lift capability to ensure the drain height is not exceeded.

If a pump is needed, try to locate it somewhere where it wont’ be heard at night. You could explain that the pump “hums”, because it doesn’t know the words, but that might not be well received if the customer is repeatedly woken up at 2:30 in the morning. These types of non-textbook details must be thought out before a unit is installed.

One frequent source of callbacks is condensate leakage. Since the temperature of the condensation can often be at or below the dew point temperature of the air. Condensate lines should be insulated to prevent “sweating” during the cooling season, and “heat tape” consideration should be made in the winter to prevent freezing, if applicable.

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Correct thermostat placement

If a thermostat (aka “remote”) is to be mounted on the wall for sensing the room temperature locally, it should be mounted on an inside wall, approximately 5 to 5 ½’ feet above the floor. Keep it away from any source of heat gain, such as direct sunlight, a coffee pot, or in the path of the supply air.

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Power supply

New electrical wiring, please!

It may be that there is existing power wiring between the circuit breaker panel and the previous outdoor unit location. Even if the existing wiring meets the electrical requirements for the proposed Mini-Split outdoor unit, it’s still a good practice to propose new wiring for both indoor and outdoor units. Why? It’s common for Mini-Split manufacturers to combined power wiring with communications wire within a single cable. If there are hidden, corroded or faulty wiring splices within walls, junction boxes, etc., this can wreak havoc with digital communication between the indoor and outdoor unit and result in the system shutting down or “locking out”. If this happens, your perfectly executed install will not impress your uncomfortable (and probably crabby) customer.

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Line Set

Install a new line-set

In some cases there may be an existing line-set (the copper line that connect the indoor unit(s) to the outdoor). Just like the wiring, it’s tempting to reuse the line-set, especially in a situation where Home Owner’s Association by-laws may forbid exposed lines. However, reuse of ANY existing refrigerant lines is highly discouraged, and in many cases, prohibited by Mini-Split manufacturers. The existing line-set will likely be too large or too small, and the designated line size MUST BE (again, not a suggestion) used.

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Copper pipe

Line-set lengths

Most Mini-Split manufacturers have both minimum and maximum length requirements. Most outdoor units are shipped with a refrigerant charge, where an additional refrigerant charge may not be needed at all; a function of the refrigerant line lengths. Refrigerant lines that are too long will compromise a system’s performance. Lines that are too short may result in premature compressor failure! When referring to the manufacturers literature on line lengths, it can be thought of as a one-way measurement of actual pipe length, flare nut to flare nut, between the indoor unit and outdoor unit.

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Avoid leaks

Avoid leaks!

Mini-Split Systems require a critical refrigerant charge, therefore cannot be “topped off” as conventional split systems can. The investment of installation efforts to ensure a leak free system will avoid costly call backs! If a Mini-Split System leaks refrigerant, the entire refrigerant charge must be recovered, and weighed back in using a digital scale. (This of course, after the leak has been fixed) The majority of leaks can be traced back to less than ideal flare connections made during installation. Therefore, a maximum effort during installation to ensure a leak free system will contribute to operating efficiency and system longevity-just what your customer paid for. In order to properly pressure test the refrigerant lines, use of nitrogen is required along with a regulator capable of introducing up to 600 pounds of pressure, PSIG. Most mini-split manufacturers will require a pressure test in the 500 to 600 PSIG range, up to a 24-hour duration. Here again, the manufacturers Installation Manual will provide the prescribed pressure test details, so don’t bypass this critical step!

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HVAC tools

The right tools for the job

Mini-Split system installation falls within the direct category of air conditioning installation. Although much easier to install than conventional systems, proper installation of Mini-Split Systems DOES require specialized tools such as, but not limited to:

  • Nitrogen tank and regulator
  • Digital micron gauge
  • Digital scale
  • Quality flare block
  • Torque wrenches (for flare nut connections)
  • Vacuum pump
  • Refrigerant manifold gauges
  • Refrigerant recovery tank
  • Refrigerant recovery unit
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Victor Gomez

Meet the Expert

In summary, application and installation of Mini-Split Systems like Fujitsu General is most often made up with significantly less effort compared to traditional split systems. In fact, as we mentioned, Mini-Split installation is a direct transfer of skills you already have. However, Mini-Split installation also requires much more attention to detail and avoidance of rules of thumb. It would not be a stretch to even stay that application and installation of Mini-Split Systems requires an entirely different mindset than conventional split systems, a new discipline, but not a difficult one by any means. To learn more, professionals can visit or call (888) 888-3424.

Victor Gomez

Victor Gomez is the Vice President of Technical Support and Aftermarket for Fujitsu General America. Victor is an Army Vet, who grew up in New York. He has a degree in Electrical Engineering and a passion for all things HVAC. Victor has been with Fujitsu for over 10 years and is responsible for tech services, training, warranty and IT.