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The outdoor condenser and the indoor air handler of a ductless mini-split system are connected by a line set. They transport the refrigerant in and out of the indoor and outdoor units in the heating and cooling process. Because of this, it is essential that ductless line sets are selected and installed correctly. If not, your mini-split will not function in the right way.

Keep reading to learn everything you need to know about the importance of a line set for your mini-split system.

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What Is A Ductless Line Set?

image of a ductless line set

A line set is made up of two copper tubes connecting the condenser to an indoor air handler. The line sets include one larger tube and one smaller tube. The liquid or discharge line is in the smaller tube. Its purpose is to transport the dense liquid refrigerant into the indoor unit to absorb heat. Conversely, the suction line is in the larger tube. This line moves the gaseous refrigerant to the condenser and allows heat to release.

These two lines provide you with conditioned air by constantly circulating the liquid and gaseous refrigerant. Depending on whether you need heating or cooling, the direction of the refrigerant is changed. If you need heat, the system reverses the flow of refrigerant.


Single-Zone And Multi-Zone Ductless Setups

You will only need to buy a single line set if your home has a single-zone unit. But, if you have multi-zone systems, you will have to get one line set for each indoor unit. Make sure you think about details such as the total line length, distance, and diameter of each set. If not carefully thought through, your system might not function in the correct way. There is no need to do this alone, a professional HVAC contractor can determine your needs for ductless installation.

Branch Box

Some multi-zone units need a branch box to bring refrigerant from the condenser to the indoor units. Because of this, it will need its own line set to hook up to the outdoor unit. Not all branch-box combinations are supported because they are manufacturer-specific. To make sure the correct branch box is selected, choose your HVAC installation company wisely.

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Port Adapter

Oftentimes, your HVAC contractor may recommend a port adaptor. This allows there to be a connection when the diameter line does not fit the standard fitting on your mini-split. Some people think these adapters can restrict the refrigerant flow. However, their design allows the refrigerant to flow properly. A port adapter makes it easy for your HVAC contractor to work on your ductless installation. They allow an HVAC contractor to use a correctly-sized line without any complications.

The installation company you choose should rigidly follow the manufacturer’s recommendations about the sizing of refrigerant piping connections. Installing a ductless mini-split system should be done by a knowledgeable HVAC contractor.

Line Set Accessories

mitsubishi heating and cooling system

Below are a handful of accessories that help make your mini-split installation experience easier:

Rectorseal NoKink

The first recommended accessory is the Rectorseal NoKink. It is an adjustable connection between the refrigerant line and the mini-split evaporator. NoKink technology and a three-inch copper stub to allow for smooth installation. It helps to prevent the refrigerant line from kinking which will save time and labor costs. It will also make sure that your unit is running efficiently.

Ball Valve

A ball valve is used to allow you to suspend refrigerant flow without removing the seal cap. This makes servicing refrigerant lines super easy. Ball valves also feature a Schrader valve to speed up charging. This accessory is a necessity for multi-zone systems. An HVAC contractor can service the system without needing to remove the refrigerant out of the lines.

Line Set Covers

ductless hvac system line set cover

To hide and protect the copper line set of your mini-split, get a line set cover. If you don’t have one, your line set will be exposed to elements such as direct sunlight and rain. Your line set can deteriorate quickly if it is unprotected from UV rays. A ductless line set cover is a barrier against these harmful elements and weather conditions.

Line set covers can also easily match your home because they can be painted. If you value your home’s aesthetic, this is a great bonus.

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Line Set Sizing

Line sets come in various sizes. The sizing depends on factors such as the number of evaporators, the distance between units, the mini-split make, and model, and the condenser size. The size of the line set is determined by the manufacturer. Therefore, it is suggested to choose a ductless installation company with plenty of experience to know what sizing is needed for the mini-split you purchase.


image of a ductless mini-split system installation

The size of your mini-split corresponds to the size of your line set. The larger the diameter line set needs, the higher the British thermal unit (BTU) or tonnage will be. Line sets sizes for mini-splits are displayed with numbers. The diameter of the liquid line is indicated by the first number, and the diameter of the suction line is indicated by the second number. For example, a large line set has a 3/8″ liquid line and a 3/4″ suction line. A smaller line set has a 1/4″ liquid line and a 1/2″ suction line.


The distance is the maximum line length needed for your mini-split. It’s vital for the HVAC contractor to determine this length since it impacts the location of your outdoor and indoor units. When they measure the distance, make sure the corners and bends in your house are included. Also, consider the wall thickness and the vertical rise to get the most accurate measurement.

You will inevitably encounter an issue with your mini-split if you choose an inexperienced ductless installer. It’s possible they could place the indoor unit too far from the outdoor unit. In this case, your unit will not properly operate because the maximum length required was exceeded.

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The refrigerant in the line set is also affected by the distance between the outdoor and indoor units. A new mini-split already comes charged with the required refrigerant. Usually, the factory charge will be satisfactory for the majority of installations. But, if your unit needs a longer line set, the HVAC contractor may need to add more refrigerant to make sure your mini-split operates at maximum capacity. It is important that the contractor you hire has the experience to know how much additional coolant is needed if this applies to your home.



Like any other purchase, you will want to make sure that your HVAC contractor offers you high-quality line sets. Also, check to see if the line set went through annealing. Annealing is a heat-treating process that prevents the copper from breaking or kinking. Additionally, find out if the line set has at least a five-year manufacturer warranty.

To make for easier installation, high-quality line sets have flared ends. So, your HVAC contractor will not have to braze it. Also, a line set should come with insulation that is at least half an inch thick. It should be UL listed for weather resistance and rated for fire and smoke. These elements will make sure that your line set is safe to use.

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Incorrectly Sized Line Sets

If you have an improperly sized line set, then you are destined to run into some problems. Below are some of the issues you will encounter if the line sets are too big or too small:


The system will not have enough operating pressure if the suction line in your line set is too small. Because of this, your mini-split system’s capacity and efficiency will be greatly reduced. This will also negatively impact your system’s lifespan.

A liquid line that is too small will result in an improper amount of refrigerant in both the indoor and outdoor units. It will reduce the operating pressure as well. If this occurs, the system won’t be able to function to its maximum capacity.



A suction-line that is too big is just as bad as one that is too small. An oversized line results in lower refrigerant pressure than what is necessary. The liquid may not be able to be transport back to the compressor as effectively. There won’t be enough refrigerant to help the compressor release any heat build-up.

An oversized liquid line will be able to hold a more considerable amount of refrigerant. But it will make the liquid quite diluted. Due to the lack of refrigerant, the compressor will end up damaged.

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For your mini-split system to work at optimal performance and operate at maximum efficiency, it’s critical to have the right supporting components. A properly sized line set, along with correct installation is essential to your unit’s functionality. Always choose to work with an HVAC company that closely follows the manufacturer’s guidelines to ensure correct installation.

A licensed HVAC contractor can help you with everything from purchasing the right unit and line sets to testing the mini-split. With professional assistance, your new system will greatly improve the comfort of your home.

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If you want to discover what a ductless HVAC system can do for you, call R.F. Ohl. Our NATE-certified technicians will help you determine the type of HVAC system that works best for your needs.

R.F. Ohl is one of the region’s leading full-service HVAC companies. We offer a wide range of HVAC services, from installations and tune-ups to repairs and consultations. We can help you find different approaches and solutions to ensure that our services meet your requirements and budget. Call us today for a free, in-home estimate.

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