Mini-Split (Ductless) Installation

Ductless mini-splits are the fastest-growing segment of residential HVAC. They land on service calls that central systems cannot solve: no ductwork, a finished attic, a garage conversion, a sunroom, a historic home where ducts would be invasive. They are also the easiest system to install badly, because the skills that make a good central tech - brazing, air-handler static pressure, duct design - are not the skills that make a good mini-split installer.

Mini-splits are a flared-connection, torqued, leak-tested, deeply evacuated refrigeration system. Every installation mistake in the field falls into one of six categories: bad flares, wrong torque, no pressure test, shallow vacuum, crushed line set, and condensate failure. All six are avoidable in under 30 extra minutes at install.

System Components

A ductless mini-split is a split refrigeration system without ducts.

• Indoor unit (head) - wall-mounted, ceiling cassette, floor-mounted, or slim-duct. Contains the indoor coil, a small blower, a TXV or EEV, drain pan, condensate outlet, and control board. Wall mount is the most common in residential. Slim-duct hides in soffits and serves one or two rooms.
• Outdoor unit (condenser) - compressor, outdoor coil, outdoor fan, reversing valve (heat pump models), accumulator, and electronics. Single-zone or multi-zone (2-5 heads per outdoor).
• Line set - insulated pair of copper refrigerant lines. Usually 1/4 inch liquid and 3/8 or 1/2 inch suction on single-zone; varies on multi-zone.
• Control cable - 3, 4, or 5 conductor low-voltage cable between the heads and outdoor unit.
• Condensate drain - 5/8 inch corrugated tube, gravity-drained (preferred) or pumped if elevated.
• Remote / controller - handheld IR, wired wall controller, or Wi-Fi module.

Sizing Per Room

Mini-splits size per zone, not per whole house. A quick-ballpark rule of thumb:

• 9,000 BTU/hr (3/4 ton) - up to 400 sq ft typical room.
• 12,000 BTU/hr (1 ton) - 400-600 sq ft.
• 18,000 BTU/hr (1.5 ton) - 600-900 sq ft.
• 24,000 BTU/hr (2 ton) - 900-1200 sq ft.

This rule is only for quick sales conversations. A Manual J room-by-room calculation is the correct sizing approach in any permitted install, just like a central system. Oversized mini-splits short-cycle just as badly as oversized central systems.

Mounting the Indoor Head

Wall-mount is 95 percent of residential mini-split work.

1. Pick a wall section with a 24+ inch clear strip above the desired mount height. The head needs clearance above and to the sides for airflow.
2. Check the wall cavity for studs, plumbing, and electrical. Use a stud finder and an inspection camera if needed.
3. Mount the backplate level using the manufacturer's template. Secure to studs with lag screws, or into a drywall anchor (toggle bolt) if no stud is present on both sides of the template.
4. Mark and drill the exterior wall penetration. A 3 inch hole saw is standard. Drill with a slight downward pitch to the outside (about 5 degrees) so condensate drains naturally.
5. Install the wall sleeve if provided, or foam-seal the penetration after lines are pulled.
6. Route the refrigerant lines, control cable, and condensate drain through the penetration in a wrapped bundle. Most brands ship the bundle pre-assembled at the head; you extend it to the outdoor unit.
7. Hang the head on the backplate. Press firmly top to bottom until it clicks into position. It should lock flat against the wall with no rocking.

Flared Connections - Mini-Split Commandment Number One

Do not braze mini-split line sets. This is the single largest install error in the trade.

• Mini-splits ship with pre-flared ends at both the head and the outdoor unit.
• The field-side connections are made at both ends with a 45 degree SAE flare.
• Brazing mini-split connections voids the manufacturer warranty, traps flux at the flare, and often destroys the factory connection seal inside the coil.

Flare procedure:

1. Cut the line set square with a good tubing cutter. Leave plenty of length - you will trim on final fit-up.
2. Deburr the inside and outside. Any burr causes turbulence, pressure drop, and potential TXV contamination.
3. Slide the flare nut onto the tube BEFORE flaring. Every tech has forgotten this.
4. Flare with a listed 37 degree SAE flaring tool. Eccentric flaring blocks (like Yellow Jacket 60283 or Uniweld) produce a cleaner flare than fixed bar-type tools. Some brands (Mitsubishi, Daikin) recommend specific flaring tools that produce a slightly oversized flare for their oversized fittings.
5. Inspect the flare. It should be a mirror-finish even taper with no cracks, no tool marks, no oxidation. A poor flare is visible - looks scratched, dull, or asymmetric.
6. Torque with a calibrated torque wrench to the manufacturer spec. Typical flare torques:

Tube Size	Typical Mini-Split Torque (ft-lb)
1/4 inch	10-15 (124-180 in-lb)
3/8 inch	24-26 (286-310 in-lb)
1/2 inch	36-42 (434-506 in-lb)
5/8 inch	45-55 (540-660 in-lb)

Always check the installation manual for the specific unit - some brands (Mitsubishi, Fujitsu) spec differently. Stamped torque values are sometimes on the flare nuts themselves.

Do not use pipe dope or thread sealant on flares. The flare is a metal-to-metal seal. A dab of POE oil (refrigerant oil) on the male cone before torquing improves seal and prevents seizing on the threads.

Outdoor Unit Placement and Mount

• Ground-mount on a composite pad or concrete, level, with 18-24 inch service clearance on the wiring side.
• Wall-mount bracket (metal, rated for the unit's weight) when space is tight or for snow-zone installs.
• Below-window locations work but expect window open / off cycle impact on the air handler.
• 2-3 ft minimum above grade in snow zones to clear winter snow drifts.
• Away from dryer vents, kitchen exhausts, and pool chemical storage.
• Secure against hurricane uplift in coastal zones with manufacturer-approved tie-downs.

Level is not optional. The compressor oils itself by gravity; an unlevel compressor oil-starves at one end of its rotation.

Wiring

Most single-zone mini-splits use a 3 or 4 conductor cable between indoor and outdoor:

• L1 (line).
• L2 (line) or neutral on 120V single-phase units.
• Ground.
• Communication conductor (on 4-wire). Some brands use an RS-485 serial communication between units; some multiplex DC on a control conductor.

Cable type must match the manufacturer spec - typically 14/4 stranded with a green ground, 600V insulation. Avoid solid conductors; the flex of thermal expansion will fatigue solid copper at the terminals.

Terminal assignments are printed inside the service cover. L1 to L1, L2 to L2, S (signal) to S at both ends. Reversing L1 and L2 on a single-phase unit does not always cause an error - it depends on the unit - but reversing signal and line will destroy the control board instantly.

Breaker sizing per the outdoor unit nameplate MOCP (maximum overcurrent protection). Outdoor disconnect within sight per NEC 440.14, fused or non-fused per the MOCP.

Leak Test with Nitrogen

Once flares are torqued at both ends, pressure test with dry nitrogen before opening the factory service valves.

1. Remove the caps on the service valves at the outdoor unit. Inside are two Schrader cores; the service valves themselves are closed (back-seated from factory).
2. Attach the manifold to the service port (which communicates with the line set side of the closed service valve).
3. Pressurize the line set and indoor head with dry nitrogen to 500 psig. Hold for 24 hours in optimal practice, 1 hour minimum in working-day practice.
4. Record the starting pressure and ambient temperature. Pressure varies with temperature - a 10 deg F swing in ambient changes pressure measurably.
5. After the hold, check for pressure drop. Any measurable drop beyond ambient-correction is a leak. Soap-bubble every flare and repair.
6. Release nitrogen slowly through the recovery port. Do not dump at the valve - the Schrader can frost and leak later.

Deep Vacuum

Skip the vacuum and your mini-split will fail within a season from moisture and non-condensables. The manufacturer spec is below 500 microns, hold 15 minutes with the pump isolated.

1. Connect a two-stage vacuum pump (5 CFM sized) and a micron gauge in-line on the system side.
2. Pump down. A clean line set with dry nitrogen residual should reach 500 microns in 15-45 minutes depending on line set length and humidity.
3. Isolate the pump (close a ball valve or the manifold low side). Watch the micron gauge.
4. If the micron reading holds below 1000 microns for 15 minutes, evacuation is complete.
5. If the reading rises past 1000 microns but plateaus, you have residual moisture - evacuate longer, or triple-evacuate with nitrogen breaks.
6. If the reading climbs steadily to atmospheric, you have a leak. Find it.

Release the Factory Pre-Charge

Most single-zone mini-splits ship pre-charged at the outdoor unit for up to 25 ft of line set. Longer runs require additional refrigerant per the manufacturer chart (typically 0.2-0.3 oz per extra foot of liquid line for single-zone 9000-24000 BTU units). Multi-zone units often require charge additions per head.

To release the charge:

1. Confirm vacuum is broken. The system must be at or near atmospheric before opening service valves.
2. Remove the allen-head cap on each service valve.
3. Insert the correct-size allen wrench (typically 4 mm or 5 mm depending on the unit).
4. Open each valve counter-clockwise until fully back-seated. Factory charge floods the line set and indoor coil.
5. Replace the service caps with the o-ring intact. Hand-tight plus a snug quarter turn with a wrench. These caps are the final refrigerant seal, not decorative.

Commissioning

With power on and the breaker closed:

1. Verify voltage L1-L2-GND matches the nameplate.
2. Power on the head via the remote or wall controller.
3. Command cooling mode and setpoint 5-10 deg F below ambient.
4. Outdoor unit should start compressor after a short protection delay (30-60 seconds on most brands).
5. Indoor fan runs immediately on low, ramps up as the coil cools.
6. After 15 minutes of runtime, read suction pressure, suction temp, and compute superheat or subcool per the unit (TXV-equipped, so subcool). Target per manufacturer chart.
7. Record commissioning numbers. Log the serial numbers of both units for warranty registration.
8. Install the Wi-Fi module (if included) and walk the customer through the app setup.

Multi-Zone Quirks

Multi-zone mini-splits drive 2-5 indoor heads from one outdoor unit. Nuances:

• Each head has its own internal TXV or EEV. Refrigerant distribution is balanced by the outdoor electronics and the line set length matching.
• Most brands specify a minimum line set length (often 10 ft per head) to avoid liquid slug-back. A short line set starves the refrigerant distributor.
• Line sets from the outdoor to a branch box (on Y-branch multi-zones) must match the brand's branch box kit - aftermarket tees are not interchangeable.
• Communication wiring is almost always serial (Mitsubishi M-NET, Daikin Link, Fujitsu Network) and must match polarity. Getting S1/S2 reversed can destroy control boards.
• Charge the line set extension per each head's required adder from the manufacturer chart - multi-zone adders are larger than single-zone.

Common Install Mistakes

• Brazing instead of flaring. Voids warranty, contaminates the coil, almost always leaks.
• No torque wrench on flares. Under-torqued leaks cold. Over-torqued cracks the flare and leaks hot.
• Skipping the nitrogen pressure test. The leak will find itself in the customer's living room.
• Shallow vacuum. "500 microns for 5 minutes" is not the same as "500 microns held after isolation for 15 minutes."
• Condensate pump installed without an anti-backflow trap. Water siphons into the drain pan when the pump shuts off, floods the head, water damages the wall.
• Condensate line run with a mid-line sag. Gravity drains need a continuous 1/4 inch per foot slope from head to outside termination. A 6 inch sag in a 15 ft run traps water, overflows the indoor pan, and ruins drywall. Pour a cup of water through the pan at commissioning and watch it leave - gravity on paper is not gravity in a wall cavity.
• Kinked line set at the wall penetration. The outside of the sleeve is where kinks hide. Inspect every bend after fish-through.
• Pre-charge not released. Unit runs, compressor starts, but no refrigerant flow - customer calls it "blowing warm." The service valves are still closed.
• Wall penetration unsealed. Rain drives into the wall cavity for years until drywall molds out.

Day 1 Checklist

• [ ] Read the installation manual for the specific brand. Mitsubishi, Daikin, Fujitsu, LG, Carrier, and Bosch all have slightly different flare, torque, charge, and wiring specs.
• [ ] Only flare, never braze, at the head and outdoor service ports.
• [ ] Use a calibrated torque wrench on every flare. Hand-tight plus a guess is how mini-splits leak.
• [ ] Pressure test with dry nitrogen to 500 psig, hold 1 hour minimum, 24 hours optimal.
• [ ] Pull vacuum below 500 microns and confirm hold after pump isolation.
• [ ] Verify the service valves are open (back-seated) before commissioning. A closed valve stops all refrigerant flow.
• [ ] Wire L1/L2/ground to L1/L2/ground. Never crossed, never forced, and never to the signal terminals.
• [ ] Seal the wall penetration with non-hardening putty or appropriate foam. No open holes behind the cover.

Expert Tips

• "Flare or die. Never braze the field connections." Mini-splits are warrantied as flare systems.
• "Torque wrench, every joint, every install." The flare is the whole system's seal.
• "Deep vacuum and hold. 500 microns and 15 minutes, isolated." The manifold is not a vacuum gauge.
• "Line set length is a charge question." Check the adder chart on every install over 25 ft.
• "Multi-zone eats amateurs. Read the manual twice." Communication wiring and branch box details trip more techs than the refrigeration does.
• "Condensate fails silently, then loudly." Test the float switch before you leave, or buy a flooring job next month.