How to Install a Mini-Split System: 10 Steps to Ductless Cooling

Choose the right size AC unit for your home. One that's too small won't cool your space properly, and one that's too large will cycle on and off often enough to damage the condenser.

Mini-splits are sized by British Thermal Unit (BTU) output. More BTUs mean a higher cooling capacity. To determine the BTUs you need, carefully measure the square footage of the area you want to cool. A rough rule of thumb is 20 BTUs per 1 square foot. For example, if you're cooling a 600-square-foot garage, you'll need a 12,000-BTU mini-split system.

Some DIY mini-splits come with a pre-charged refrigerant line, which streamlines installation. If you choose one without a pre-charged line, you’ll need to call a local HVAC contractor to complete your refrigerant line setup after installation.

Before you begin, ensure that you have an assistant to help you for the entire installation.

Mount your indoor evaporator unit on a wall that connects to your home's exterior. Choose a spot that's not exposed to direct sunlight or other heat sources, which impact your unit's ability to monitor indoor temperatures. Make sure that shelving or other obstructions won’t block the unit. 

Use a stud finder to locate and mark your stud layout. There should be no wires, pipes, ducts, or obstructions in the wall where your unit will go, as these can interfere with your mounting hardware and line set.

With your stud locations marked, place your bracket and use a level to ensure it's aligned. A unit that's not level won't drain properly. Drill pilot holes in the marked stud locations. If you don’t hit a stud, hammer hollow screw anchors into each hole. Use your drill to fasten the brackets in place with screws.

Your line set hole should slope slightly downward toward the outside so that condensate can drain via gravity. Drill a pilot hole in one of the lower corners of your mounting brackets according to the manufacturer's instructions for your unit. The hole should be long enough to reach your home's exterior.

Check outside to ensure no obstructions such as trim will complicate drilling the full-sized hole.

If the path for the hole is clear, use your hole saw to drill a 2- to 3-inch hole angled slightly downward for drainage. The hole should be long enough to exit the exterior. The exact hole width needed depends on the size of the line set, which includes refrigerant lines, control lines, and drainage tubing. Consult your manufacturer's instructions for the required measurements.

If the line set isn't already connected to the indoor unit, connect it per the manufacturer's instructions. Carefully unfold all lines from the back of the evaporator unit. Use electrical tape to keep the lines together.

With your assistant on the exterior side of the hole, feed the line set through. As it emerges, have your assistant carefully bend it downward toward where you plan to install the outdoor condenser unit. Feed the drain line through below the other lines. Place plastic trim around the edges of your hole to prevent rough edges from damaging your lines.

Lift the indoor unit and hang it on the mounting bracket. Secure the unit on the wall using the hardware that came with the unit. Wrap a foam sleeve around the line set within the hole in your wall and move back any insulation displaced during installation.

As long as the refrigerant lines can easily reach the condenser unit, you can either install the unit on a pad placed on the ground or mount it to a wall with wall brackets. Maintain plenty of clearance from bushes and other landscaping. Route your line set tight to the building exterior.

This step will vary by manufacturer and should only be done if your lines are already cut and pre-charged with refrigerant. If not, you must contact an HVAC pro to finish the job.

Connect the line set from the indoor unit to the outdoor unit per the manufacturer's instructions. You'll need an adjustable wrench and a torque wrench. Insulate and seal the hole in the outside wall and cover your line set with your plastic line set cover for a neater appearance.

Carefully inspect all refrigerant lines for kinks and ensure all electrical connections are secure. Turn on your dedicated circuit and test the function of your new AC.