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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.
The secret behind modern ductless heat pumps’ cold-weather performance is a component called the reversing valve, which changes the direction of refrigerant flow between seasons. As Richard Trethewey demonstrated on the show, it’s “just a simple little gate” that directs hot refrigerant to the indoor coil for heating or to the outdoor coil for cooling. Paired with inverter technology — which allows the compressor to modulate its output rather than simply cycling on and off — today’s cold-climate models can extract usable heat from outdoor air at temperatures as low as −15 degrees F or even −22 degrees F, depending on the unit.
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
Why Inverter Technology Matters: A variable-speed, or inverter-driven, compressor operates at only the speed necessary for current conditions, reducing energy use and preventing the system from cycling on and off frequently. According to TOH Magazine, this helps maintain consistent temperatures, improves dehumidification during cooling season, and increases indoor air quality because the air filter is engaged for longer periods of time. An inverter is a good idea in any climate but is most needed in areas that experience big swings in temperature between seasons.
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:
- Select an appropriate location on an exterior wall for the indoor air handler that is free from obstructions.
- Mount the bracket for the indoor unit securely to the wall, aligning it correctly.
- 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.
- Insert the manufacturer-provided plastic sleeve through the wall hole to protect the line set and wiring.
Pro Tip: Richard Trethewey, TOH plumbing and heating expert, points out that you’re not limited to exterior walls: “We’re installing this indoor air handler on an outside wall. We actually could put it on any interior wall as long as we can get those refrigerant line sets to outside.”
Outdoor Unit Preparation
Next, prep the outside of your house for the other components:
- Choose a location for the outdoor condenser unit, considering accessibility and noise. Make sure it has ample airflow and is away from direct sunlight.
- Install a sturdy mounting bracket on the exterior wall to elevate the unit above potential snow levels and prevent issues with flooding.
- Run the electrical wiring from the outside through the sleeve and make sure it is securely attached to the inside unit.
Clearance Requirements: According to TOH Magazine’s mini-split installation guide, the outdoor condenser needs at least 4 inches of clearance behind it and 20 inches of clearance above and in front of it for proper airflow. The unit can be mounted on brackets fastened to the wall or bolted to concrete blocks resting on the ground. If mounting on a bracket, use blocking behind it to protect the siding.
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:
- Tape the refrigerant lines, condensate drain, and electrical wiring together to keep them secure and manageable.
- 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.
- 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:
- Snap the cover onto the sleeve to conceal all the lines and cables.
- 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.
- 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.
- Connect the electrical cable to the condenser.
- Lift the condenser onto the mounting bracket and bolt it in place.
On one Ask This Old House project, the crew opted to wall-mount the condenser on angle brackets rather than set it on a concrete pad — a smart move in snow-prone climates. As the team explained, a ground-level pad risks burying the unit in drifting snow. They fastened the bracket to the building with lag screws and used blocking behind it to protect the vinyl siding, then carefully lifted the condenser into place with two people — one on a ladder.
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.
Pro Tip: Richard Trethewey, TOH plumbing and heating expert, emphasizes that maintenance is essential for these increasingly popular systems: “77% of the world’s air conditioning market is done by mini splits. They’re quiet. They’re efficient. They can heat even on the coldest day. They can cool perfectly. But sooner or later, these units need to be maintained.”
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.
