Guide to Installing a Wall-Mounted Mini-Split | Ask This Old House

The Evolution of Home Comfort: Installing Modern Mini Split Air Conditioners

Air conditioning has long been the savior of hot summer days, turning sweltering interiors into refreshing havens of cool air. Traditionally, people have relied on ubiquitous window air conditioners to achieve this comfort, but these systems come with their own set of challenges. From seasonal installations to potential security issues, window air conditioners have inherent drawbacks. In contrast, the new trend in air conditioning technology is moving towards the sleek and efficient mini split systems. This comprehensive article dives into the detailed process of installing a mini split air conditioning system, using an actual installation project as a case study.

The Basics of Air Conditioning

Understanding how air conditioning works is essential to appreciating the efficiency of modern systems. All air conditioners operate based on a simple physical principle: heat always travels to cold in any direction. An air conditioner capitalizes on this by using two key components: the evaporator and the condenser.

  • Evaporator: This component is filled with extremely cold refrigerant. Warm room air is drawn in by an internal fan and blown across the cold evaporator coil. As a result, heat from the room air is absorbed by the cold refrigerant.

  • Condenser: After absorbing heat from the room, the refrigerant travels to the condenser via a compressor. The condenser increases the refrigerant’s temperature, causing it to become superheated. Another fan then expels this heat to the outside environment.

The beauty of this system lies in its efficiency: warm air is cooled and recirculated inside, while the absorbed heat is transferred outside.

Window Air Conditioners Versus Mini Split Systems

Window Air Conditioners

Window air conditioners are a common sight in many homes and are known for their ease of installation and affordability. They combine the evaporator and condenser within a single unit that mounts in a window frame. However, they come with several disadvantages:

  1. Installation Hassles: Installing a window air conditioner can be labor-intensive, particularly as seasons change. Units must often be removed during the winter and reinstalled in the summer.

  2. Aesthetic and Security Concerns: These units can detract from a home’s appearance and pose security risks, as they can be easily removed or tampered with from the outside.

  3. Limited Placement Options: Given their design, placement options for window air conditioners are restricted to specific window frames, which can impact their efficiency and effectiveness in cooling a space.

Mini Split Systems

The emergence of mini split systems represents a significant progression in air conditioning technology. Unlike window units, mini split systems separate the evaporator and condenser, with the evaporator mounted indoors and the condenser installed outdoors. Connecting these two units requires only a small hole in the wall for the refrigerant line, offering several benefits:

  1. Versatility in Placement: The indoor unit can be installed almost anywhere in a room, allowing for optimal air distribution. This freedom also means minimal visible impacts on a home’s exterior.

  2. Quiet and Efficient Operation: Mini split systems are advanced and quieter than traditional window units, providing a more comfortable and less intrusive home environment.

  3. Zoned Cooling: These systems offer the ability to cool individual rooms or zones selectively, thereby saving energy and increasing efficiency.

Case Study: Installing a Mini Split System

Let’s examine a real-world scenario of installing a mini split system in a two-family house, focusing on the first floor. This project involved numerous logistical and technical considerations to ensure both practicality and efficiency.

Site Assessment and Planning

The initial step in the installation process involved a comprehensive assessment of the site:

  • Outdoor Unit Placement: Urban environments present considerable challenges for the placement of the outdoor condenser unit. In our case study, narrow sidewalks and parking restrictions limited our options. Ultimately, we decided to mount the condenser on the wall above people’s heads at the back of the building, ensuring that it wouldn’t obstruct pedestrian traffic or parking.

  • Indoor Unit Placement: For optimal performance and aesthetics, the indoor unit was planned to be installed centrally in the dining room. This positioning allowed the refrigerant lines to be conveniently routed through a master closet and down into the basement, concealing the lines and preserving the home’s interior design.

Installation and Professional Involvement

The complexity of installing a mini split system makes it a task best left to professionals. Our project enlisted the expertise of a local HVAC contractor, Joel, and his apprentice, Jorge. Here’s a step-by-step breakdown of the installation process:

1. Equipment and Tools

The key components and tools for the project included:

  • Outdoor Condensing Unit: Featuring an inverter that allows for heating even in cold climates.
  • Wall Bracket: To mount the condenser on the exterior wall.
  • Refrigerant Line Set: Copper lines insulated to carry refrigerant between the indoor and outdoor units.
  • Condensate Drain and Electrical Wiring: For disposal of water from dehumidification and powering the units, respectively.
  • Indoor Evaporator Unit: To be mounted on the designated interior wall.

2. Mounting the Indoor Unit

Joel began by securing the mounting bracket for the indoor unit on the designated wall. This step involved ensuring precise alignment and marking locations for various connections, including:

  • Electrical Connections: Power and control wiring.
  • Refrigerant Lines: For the flow of refrigerant.
  • Condensate Drain: To channel the water produced during dehumidification.

Once the bracket was in place, holes were drilled to route these connections through the wall into the master closet and down to the basement.

3. Routing Lines

The next stage involved running the electrical wiring, refrigerant lines, and condensate drain through the pre-drilled holes. Careful consideration was given to routing these lines without causing damage or compromising the structural integrity of the building. Once routed through the closet and into the basement, the lines were neatly secured along the basement joists.

4. Installing the Outdoor Unit

The exterior installation commenced with the mounting of the condenser unit. Here’s a breakdown:

  • Mounting the Bracket: The wall bracket was secured to the exterior wall with appropriate support. Blocking was used behind the bracket to protect the vinyl siding from damage.

  • Placing the Unit: The condenser was then carefully mounted onto the bracket, ensuring a level installation for optimal operation.

5. Connecting the Systems

With both the indoor and outdoor units mounted, the final step involved connecting the refrigerant lines, electrical wiring, and condensate drain. Each of these connections is crucial for the system’s performance and safety:

  • Refrigerant Lines: High and low-pressure copper lines were connected to transport the refrigerant between the indoor evaporator and the outdoor condenser.

  • Electrical Connections: Ensuring watertight connections to prevent any electrical hazards. This involved setting up a 220-volt line and installing a service switch and surge protector for added safety.

  • Condensate Drain: Properly pitched to ensure gravity-driven drainage of condensate without requiring a pump.

System Testing and Commissioning

The final stage before commissioning the system involved thorough testing to ensure the integrity and performance of the installation. Two critical tests were performed:

1. Nitrogen Pressure Test

To verify the system for leaks and contamination, a charge of nitrogen was introduced into the lines. This step pressurizes the system, and any drop in pressure would indicate a leak that needs to be addressed.

2. Vacuum Test

A vacuum pump was employed to evacuate all air and impurities from the refrigerant lines. This step is vital to maintain the purity of the refrigerant and ensure efficient operation. The vacuum process continued until all nitrogen and potential contaminants were removed.

Final Touches and Operation

After the successful completion of these tests, the system was charged with refrigerant. The indoor unit, blending seamlessly into the room’s decor, was ready for operation. The mini split system offered versatile control options:

  • Remote Control: A handheld remote for adjusting temperature, fan speed, and operational modes.
  • Wi-Fi Control: Integration with a smartphone app for remote monitoring and adjustment, including smart diagnostics for troubleshooting.

Benefits and Cost Considerations

This mini split air conditioning system provided numerous benefits:

  • Enhanced Comfort: With precise temperature control and quiet operation, the indoor environment was significantly improved.
  • Energy Efficiency: The ability to cool specific zones selectively helped in reducing overall energy consumption.
  • Aesthetic Appeal: Concealed refrigerant lines and a sleek indoor unit maintained the home’s aesthetic integrity.

However, it is important to consider the costs involved. Installation costs can vary based on the complexity of the project, the size and number of units, and the specific electrical demands. While an experienced crew completed this project in a day, DIY installation is not recommended due to the technical expertise required.


Air conditioning technology has come a long way from the bulky, cumbersome window units of the past. Modern mini split systems offer a sophisticated, efficient alternative that goes beyond mere cooling. By allowing flexible placement, reducing noise, and offering zoned climate control, these systems represent the future of home comfort. While the installation process can be intricate, the result is a long-term, efficient solution that significantly enhances living conditions.

In our case study, the thoughtful planning and expert execution of the mini split system installation underscore the benefits of embracing new technology for home climate control. With the right team and proper planning, homeowners can enjoy the seamless, quiet, and efficient air conditioning that mini split systems provide, making them a worthy investment for modern living.

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