How Ductless Split Installation Works (2024)

The installation process of a ductless mini-split heat pump involves mounting the indoor and outdoor units, connecting the refrigerant lines and electrical wiring, and ensuring proper drainage. The indoor unit is typically mounted high on an interior wall, while the outdoor unit is placed on a secure pad or bracket.

The HVAC technician will also drill a small hole through the wall to accommodate the refrigerant lines and wiring. Once they connect everything, they will pressurize the system, check for leaks, and vacuum out the air before charging the unit with refrigerant. Unlike a ducted system (central air and heating system), you don’t need to install ductwork.

Here are some precautions that the technicians will need to take when doing the ductless split installation.

How to Install Ductless Mini-Split Heat Pumps

Here is how the process of installing a ductless mini-split air conditioning system generally works. Specific instructions vary for different models. Always consult your unit’s instruction manual for the correct steps.

Choosing the Right Location

Proper placement of the indoor and outdoor units is crucial for efficient operation. For best performance, mount the indoor unit high on an exterior wall, away from direct sunlight and obstructions. Then, find a good shaded spot for your outdoor unit on the other side of the exterior wall where the wall-mounted unit is. Make sure it has enough clearance for ventilation of your ductless air conditioning system (about 3–feet all around).

Prepare the Mounting Bracket for the Indoor Unit

Securely attach the indoor unit’s mounting plate to the wall, ensuring it is level. Drill a 2-3 inch hole through the wall (through the drywall and all the way outside) for the refrigerant lines and condensate drain.

Place the Outdoor Unit on a Stable Pad

Install the outdoor unit on a cement pad/base just outside your home. Secure it to the pad with bolts provided with your system.

Connect the Refrigerant Lines

Run the copper refrigerant lines through the wall hole, being careful not to kink or damage them. Use a flaring tool to connect the lines to the indoor and outdoor units. Insulate the lines to prevent energy loss.

Install the Condensate Drain

Attach the condensate drain hose to the indoor unit and run it to a suitable drain location. Ensure the hose has a continuous downward slope to prevent water from backing up.

Wire the System

Connect the power cable and communication wires between the indoor and outdoor units. Follow the manufacturer’s wiring diagram and local electrical codes. Consider hiring an electrician is this is out of your comfort zone.

Pressure Test and Vacuum Air

Perform a pressure test to check for leaks, then use a vacuum pump to remove air and moisture from the system. This step is critical for proper operation, optimal energy efficiency, and to prevent damage to the compressor.

Tips for Outdoor Unit Installation 

The outdoor unit of your ductless mini-split system is also known as the condensing unit, where heat is rejected to the atmosphere. The cooling capacity of the unit will drop as the condensing temperature increases; hence, maximum transfer of heat to the surroundings will ensure better capacity.

• VENTILATION — The outdoor unit should be installed in a location that is well-ventilated. Remove any obstacles that block the discharge air or intake air to the unit.
• SHORT CIRCUIT — Check that the hot air that is discharged from the condenser is not drawn in by the unit. This is called a short circuit in the hot discharge air. When this happens, the unit will not operate efficiently as heat is not rejected properly. Check the air intake and air discharge clearance distance from the manual provided. 
• ENVIRONMENT — If you live near an oil refinery plant, hot spring, or coastal area, the environment may contain a large amount of oil, sulfide gas, or salt, which could damage the unit. In these cases, ask the installer if there are any modifications they should make to the unit for it to last longer in these harsh environments.
• SUNLIGHT — For better efficiency, use an awning to prevent exposure of the unit to direct sunlight. Alternatively, you can place it on the north side of your home so that it is in the shade all day.

Tips for Indoor Unit Installation

• WATER — Ensure no water enters the refrigerant when connecting the refrigerant piping between the indoor and outdoor units. Don’t leave the pipe exposed, and extra care has to be taken during rainy days.
• DUST — Ensure not to expose the copper AC piping to the atmosphere as this will cause dust to enter the pipes. Make sure the caps are in place, as dust in the refrigerant circuit can cause clogging in the capillary tube.
• PIPING LENGTH — Make sure the length of the piping between the two units is within the recommended length. If the piping is too long, the compressor oil and refrigerant will not be able to circulate properly hence causing the cooling capacity to drop. Not enough oil in the compressor will damage it. The best configuration is placing the unit back to back thus having minimal length. Maximum piping length may range from 15m to 40m.
• PIPING ELEVATION — If the height between both units is too high, the compressor oil will not be able to return easily to the compressor. This will degrade the performance of the compressor and reduce the cooling or heating capacity of the system. Follow the recommended elevation. Maximum elevation may range from 8m to 15m.
• PIPING SIZE — The sizes of the pipes in the specifications have been designed for optimum performance. Wrong sizes may cause the compressor to break down. Typical liquid pipe sizes are 1/4″ and 3/8″, whereas suction pipe sizes include 5/8″ and 3/4″.
• PIPES INSULATION — Make sure that the pipes are insulated to minimize the loss of energy to the atmosphere, which in turn will cause a drop in cooling capacity.
• LIQUID and GAS PIPES — Make sure there is no wrong connection between these two pipes. If the connection is wrong, the evaporator will freeze during cooling, causing water to drip. The capacity will also drop, and you will hear a noisy refrigerant sound.
• BENDS — During installation, try to reduce the number of piping bends as the system’s resistance increases with the number of bends. It will disturb the refrigerant flow and cause the capacity to drop. Maximum number of bends should be 10.
• CRUSHED PIPES — Use proper tools to bend the copper pipes. Dented pipes will restrict the flow of refrigerant. This will cause the copper pipes to freeze.
• REFRIGERANT SHORTAGE — Make sure the amount of refrigerant is enough. Lack of refrigerant will cause the compressor to be damaged due to overheating and drop in capacity. The indoor coil may also freeze or blow warm indoor air.
• REFRIGERANT SURPLUS — Too much refrigerant is also known as overcharging. When this happens, the liquid compression will damage the compressor valve plate of the mini-split unit.
• POWER CONNECTIONS — When connecting the power supply to the terminal, make sure the connection is firm and not loose. Loose connection will cause arching causing heat to be generated. As a result, the lead wires may burn. At the same time, the compressor running ampere will go up as the rated voltage is not obtainable. The compressor will go into overload causing the air conditioner to stop operating.
• LEAKAGE TEST — During maintenance and when checking for leakage of refrigerant, do not use oxygen or other flammable gas. These gasses may cause an explosion. HVAC pros use nitrogen or refrigerant when doing the leakage test.

Related: Split Air Conditioner Control Troubleshooting

Final Thoughts

Compared to installing a central HVAC system with ductwork, ductless mini-split installation is much easier. However, I always recommend hiring a licensed HVAC contractor to ensure your system is properly sized, installed, and commissioned.

That said, some savvy homeowners can complete a DIY installation of a ductless heat pump — many manufacturers offer DIY kits. Most can complete the installation in less than a day with the help of one other person. Still, a lot can go wrong if you’re inexperienced.