How To Install a Ductless Heat Pump

A ductless heat pump, also known as a mini-split system, provides year-round heating and cooling in spaces on your property that don’t have existing ductwork, such as detached garages or home additions. It is a compact, efficient, and flexible solution that provides climate control and energy-efficient comfort in any weather.

In this guide, This Old House plumbing and heating expert Richard Trethewey and host Kevin O’Connor install a ductless heat pump in a 700-square-foot space above a garage. You’ll need a licensed and experienced professional for this project.

What are Ductless Heat Pumps?

Ductless heat pumps are becoming increasingly popular for their energy efficiency and adaptability. These systems have two main parts: an outdoor condenser unit and an indoor air handler. They transfer heat between the indoor and outdoor units using refrigerant, providing heating and cooling capabilities.

In cooling mode, it extracts heat from the building and expels it outside. In heating mode, the process reverses, extracting heat from the outdoor air and bringing it inside. The technology allows the system to provide efficient heating even in cold climates. Some models run at temperatures as low as 5 degrees Fahrenheit. 

Preparing for Ductless Heat Pump Installation

Before installing a ductless heat pump, assess your space to determine the appropriate system size. 

Calculating Heat Load

First, calculate the space’s heat load, or how much heat is lost and gained during different times of the year. This calculation takes into account several factors:

• Square footage of the area
• Local climate conditions
• Insulation levels in walls, ceilings, and floors
• Number and type of windows
• Heat-generating appliances and occupants

For the 700-square-foot space in the video, Trethewey determined a heat loss of 18,000 BTUs per hour in winter and a heat gain of 16,000 BTUs in summer. 

Choosing the Right System

Based on the heat load calculation, select a ductless heat pump that can handle both your heating and cooling needs. Consider factors that impact the performance and efficiency of the system, such as:

• BTU capacity
• Energy efficiency ratings (SEER and HSPF)
• Inverter technology for variable speed operation
• Noise levels
• Brand reputation and warranty

Ductless Heat Pump Installation

You might be tempted to tackle the installation yourself, but professional installation is typically required because of the complexity of the system and the need for specialized tools and knowledge. Expect to spend at least $4,350 for a professional installation.*

*Cost information is based on 2024 reports from Angi.

Indoor Unit Preparation

Here’s how to prep to install the indoor component of your ductless heat pump:

1. Select an appropriate location on an exterior wall for the indoor air handler that is free from obstructions.
2. Mount the bracket for the indoor unit securely to the wall, aligning it correctly.
3. Use a hole saw to create a 2 5/8-inch diameter hole through the exterior wall for the line set. Angle it slightly downward to allow for drainage.
4. Insert the manufacturer-provided plastic sleeve through the wall hole to protect the line set and wiring.

Outdoor Unit Preparation

Next, prep the outside of your house for the other components:

1. Choose a location for the outdoor condenser unit, considering accessibility and noise. Make sure it has ample airflow and is away from direct sunlight.
2. Install a sturdy mounting bracket on the exterior wall to elevate the unit above potential snow levels and prevent issues with flooding.
3. Run the electrical wiring from the outside through the sleeve and make sure it is securely attached to the inside unit. 

Attaching the Inside Unit

Now that both spaces of your space are ready and the electrical wiring is connected, start by attaching the inside unit:

1. Tape the refrigerant lines, condensate drain, and electrical wiring together to keep them secure and manageable.
2. Lift the air handler up to its mounting bracket and feed the bundled refrigerant lines and cables through the sleeve down to the metal bracket outdoors.
3. Hook the air handler onto its wall-mounted bracket and secure it with screws.

Attaching the Outside Unit

Make sure the bundled lines run all the way down to the metal bracket, then follow these steps:

1. Snap the cover onto the sleeve to conceal all the lines and cables.
2. Connect the copper refrigerant lines to the outdoor unit, using the appropriate fittings to prevent leaks. Be sure to use a flaring tool for these connections.
3. Have a licensed electrician run a dedicated circuit from the main electrical panel to the outdoor unit, including a service disconnect switch near the outdoor unit for safety and convenience. Check that your electrical work is installed in compliance with local codes.
4. Connect the electrical cable to the condenser.
5. Lift the condenser onto the mounting bracket and bolt it in place.

Ductless Heat Pumps: System Controls and Operation

Modern ductless heat pumps offer many types of control options to suit different preferences and integrate seamlessly with smart home systems.

Remote Control Options

Most ductless systems have a wireless remote control that allows users to adjust the temperature, fan speed, and airflow direction. You can wall mount the remote or place it on a table for mobility and convenience.

Smart Thermostat Integration

Many ductless heat pumps can be integrated with smart home systems. In the video, Trethewey shows how a wireless clock thermostat can control the system. A gateway device connected to a local router enables Wi-Fi or Bluetooth control through smartphones, tablets, or computers. Some smart thermostats offer advanced features such as scheduling, energy usage monitoring, and remote control from outside of the home.

Taking Care of Your Ductless Heat Pump

Establish a regular cleaning and maintenance schedule to get the best use out of your new system.

Routine Maintenance Tasks

We recommend these regular maintenance tasks to keep your system in shape:

• Clean or replace air filters monthly for proper airflow and efficiency.
• To prevent blockages and overheating, keep the outdoor unit free from debris and vegetation.
• Inspect and clean the condensate drain line periodically to prevent clogs and water damage.
• Schedule professional maintenance annually to have a licensed technician inspect, clean, and repair the system if needed.

Professional Service

Have a professional HVAC technician service the system annually. They can:

• Check refrigerant levels and pressure to ensure the system is operating efficiently.
• Inspect electrical connections for safety and proper operation.
• Clean coils and components to maintain heat transfer efficiency.
• Address any issues and repair more complex problems.