How To Install a Ductless Mini-Split Air Conditioner

A ductless mini-split is a type of air conditioning system that consists of an indoor air handling unit and an outdoor condensing unit connected by refrigerant lines. These units are highly efficient, easy to install, and provide targeted cooling for specific areas of your home.

In this guide, we’ll walk you through the installation process, helping you understand the steps involved and when to call in a professional. This Old House plumbing and heating expert Richard Trethewey demonstrates the installation process in the video above.

How Ductless Mini-Splits Work

A ductless mini-split system circulates refrigerant between the indoor and outdoor units. The indoor unit, mounted on a wall, extracts heat and humidity from the room air, while the outdoor unit releases this heat outside. This process effectively cools and dehumidifies the indoor space without extensive ductwork.

Benefits of Ductless Mini-Split Systems

Ductless mini-splits offer numerous advantages for homeowners, which include the following:

• Easy installation: Mini-splits require only a small hole in the wall for the refrigerant lines, making them less invasive to install than traditional systems.
• Energy efficiency: These systems can be more efficient than central air conditioning, as they avoid energy losses associated with ductwork.
• Improved air quality: Without ducts to collect dust and allergens, mini-splits can provide cleaner air.
• Quiet operation: The indoor units are typically quiet, enhancing comfort in living spaces.
• Zoned cooling: You can control the temperature in individual rooms or areas, saving energy and improving comfort.

Preparing for Ductless Mini-Split System Installation

Before beginning the installation process, prepare and gather the necessary tools and materials.

Tools and Materials Needed

To install a ductless mini-split air conditioner, you’ll need the following tools and materials:

• Adjustable wrench
• Drill/driver
• Electrical cable
• Flaring tool
• Masonry screws
• Mounting bracket
• Pipe insulation
• Pressure gauge and nitrogen
• PVC pipe for condensate drainage
• Refrigerant lines
• Rotary hammer
• Torpedo level
• Vacuum pump
• Wire stripper

Choosing the Right Location

The indoor unit should be mounted on an interior wall away from direct sunlight and other heat sources. Check that there’s enough clearance around the unit for proper airflow. The outdoor unit should be placed on a level surface, preferably in a shaded area, with adequate space for air circulation and maintenance access.

Ductless Mini-Split Air Conditioner Installation Process

Installing a ductless mini-split air conditioner involves several steps, from mounting the indoor unit to connecting the refrigerant lines and electrical components. Here’s what to do.

Mounting the Indoor Unit

1. Turn off the electricity to the circuit.
2. Hold the mounting bracket to the wall and use a level to make sure it’s straight. Mark the screw hole locations for the hole for the refrigeration lines, condensate discharge line, and electrical cables.
3. Bore a hole through the wall using a rotary hammer and a 3-inch-diameter masonry coring bit. For a cleaner exterior finish, start from the inside and finish from the outside.
4. Securely attach the mounting bracket to the wall using masonry screws.
5. Carefully hang the indoor unit on the mounting bracket, and make sure it’s level and properly seated.

Installing the Outdoor Unit

1. Place the outdoor condensing unit on a level pad adjacent to the exterior wall where the indoor unit is mounted.
2. Check for adequate clearance around the unit for proper airflow and maintenance access.
3. If necessary, use vibration-dampening pads under the unit to reduce noise and vibration transfer.

Connecting Refrigerant Lines

1. Carefully route the refrigerant lines from the indoor unit through the hole in the wall to the outdoor unit.
2. Use a flaring tool to create proper connections on the copper tubing.
3. Connect the flared ends to the corresponding ports on the indoor and outdoor units, initially hand-tightening the nuts.
4. Use two adjustable wrenches to fully tighten the compression fittings, ensuring a secure and leak-free connection.
5. Wrap the refrigerant lines with pipe insulation to prevent energy loss and condensation.

Installing the Condensate Drain

1. Attach a PVC pipe to the condensate discharge line from the indoor unit.
2. Route the pipe through the wall to the exterior, ensuring a slight downward slope for proper drainage.
3. Secure the pipe to the exterior wall using appropriate fasteners.

Electrical Connections

Always exercise caution when working with electrical connections. Take these steps when installing a ductless mini-split:

• Connecting the thermostat: If your mini-split system includes a thermostat, mount it in an easy-to-access location in the room. Connect the thermostat wires to the designated terminals on the indoor unit according to the manufacturer’s instructions. Proper placement and connection are essential for efficient temperature control.
• Grounding the system: Connect the ground wire from the indoor and outdoor units to a suitable grounding point. Make sure the grounding system complies with local electrical codes and regulations. A licensed electrician can verify the grounding system’s integrity and effectiveness.
• Running electrical wires: Run a weather-tight conduit from your outdoor electrical box to the condenser unit. Feed electrical wires through the conduit and make the necessary connections at the indoor and outdoor units. Check that all connections are secure and insulated. Contact a licensed electrician for compliance with electrical codes if you’re unsure.

Ductless Mini-Split AC Testing and Finishing Touches

After completing the installation, test the system and make any necessary adjustments.

Pressure Testing

Here’s how to run a pressure test: 

1. Use a pressure gauge to test the system with nitrogen at 300 pounds per square inch.
2. Check for any leaks in the refrigerant lines or connections.
3. Once the pressure test is complete, vacuum out the nitrogen from the system.

Charging the System

Take the following steps to charge the system: 

1. Open the valves on the outdoor unit to release refrigerant into the system.
2. Use a manifold gauge to make sure the system is properly charged according to the manufacturer’s specifications.

Final Checks and Cleanup

Do the following final checks to make sure your system is running smoothly: 

1. Turn on the electricity and test the air conditioner for correct functioning.
2. Check for any unusual noises or vibrations during operation.
3. Install the decorative cover on the interior wall to conceal the refrigerant lines and electrical connections.
4. Clean up the work area and dispose of any packaging materials properly.