What it is

A single-zone ductless mini-split is a small, high-efficiency heat pump (or AC) that conditions one space with no ductwork. An indoor "head" mounts on the wall (or ceiling/floor) and blows directly into the room; a small inverter-driven condenser sits outside; a slim line set and a control cable connect them through a 3-inch hole in the wall. Most are heat pumps, so they heat and cool.

The thing that makes a mini-split different from a conventional split isn't the refrigeration cycle — it's the inverter. Instead of a compressor that's slammed fully on or off by a contactor, the inverter varies the compressor speed continuously to match the load. That's why they're so efficient and quiet, and it's also why they're diagnosed differently: there's no contactor and no run capacitor on the compressor, and the boards talk to each other over a communication line and report faults as codes.

How it's built

Indoor head:

  • A small cross-flow (barrel) blower and an evaporator coil.
  • A control board with thermistors (room air, coil temp) and the receiver for the handheld remote.
  • A condensate drain — usually gravity through the line-set sleeve, sometimes a small condensate pump for the head.
  • A vane/louver actuator for airflow direction.

Outdoor unit:

  • An inverter-driven compressor (variable speed, often DC) run by a power/inverter board — no contactor, no compressor run cap.
  • The outdoor coil and a variable-speed (often DC) condenser fan.
  • A reversing valve (on heat-pump models), an electronic expansion valve (EEV) that meters refrigerant electronically, and the main control/power board.
  • A base-pan heater on cold-climate units to keep defrost melt from freezing in the pan.

The link:

  • A flared line set (small liquid + larger suction), insulated.
  • A communication cable carrying power and serial data between the two units. Most mini-splits are powered at the outdoor unit and feed the indoor head over this cable — so the indoor head usually has no separate power feed.

How it's wired and plumbed

Power: typically one 240V (or 120V on small units) feed lands at the outdoor unit; the indoor head is powered from the outdoor unit through the interconnecting cable. Communication: the boards exchange data over a serial line in that same cable — this is how the head tells the compressor how hard to run and how faults get reported. Refrigerant: flared connections at both ends (not brazed in most residential mini-splits), with the metering done by the EEV. The factory ships the outdoor unit with a charge for a base line-set length; longer runs need added refrigerant per the install table.

In the field — commissioning

Mini-splits live and die on the install:

  1. Mount and run the line set with smooth bends (no kinks — small soft tubing crimps easily).
  2. Make the flares correctly. A clean, properly sized 45° flare, deburred tube, right flare-nut torque. Bad flares are the number-one mini-split leak. Torque to spec — over-torquing splits the flare, under-torquing leaks.
  3. Pressure test and evacuate. Pressure-test with nitrogen, then pull a deep vacuum and do a decay test — 500 microns and holding is the target. Mini-splits are unforgiving of moisture and non-condensables because the EEV ports and capillaries are tiny.
  4. Add charge for line-set length beyond the factory-included length, per the table on the unit.
  5. Open the service valves, power up, and let it run; verify operation in both modes.

Normal values & targets

  • Evacuation: 500 microns with a passing decay test — same as any quality install, but mini-splits really punish a sloppy vacuum.
  • Charge: factory charge plus the per-foot addition for line set over the base length; weigh it in.
  • Voltage: 208–240V (or 120V small units) at the outdoor unit.
  • Compressor: no fixed RLA to clamp against the way a single-speed unit has — current varies with inverter speed. Use the manufacturer's service data and fault codes instead of a simple amp check.
  • Superheat/subcool: less directly useful than on a fixed-speed system because the EEV is constantly trimming; rely on weigh-in and manufacturer service mode for charge verification.

Common faults & what they mean

  • Leak at a flare. The classic. Loss of charge shows as weak capacity and can throw low-pressure/EEV faults. Reflare and re-torque, don't just tighten harder.
  • Fault code on the head's display or LED blink pattern. Mini-splits self-diagnose. Get the code, look it up for that brand, and it points you at the subsystem (communication, sensor, EEV, inverter, etc.). Don't guess — read the code.
  • Communication error. Bad interconnect wiring, wrong terminal, a nicked communication wire, or moisture in a connection. The head and condenser can't talk, so nothing runs.
  • Inverter/IPM board failure. The power module that drives the compressor can fail, especially after a surge. Reports as an inverter or compressor-drive fault. This is a board, not a capacitor.
  • Indoor coil freezing / weak airflow. Dirty head filters or blower wheel (mini-split barrel wheels load up with dust and biofilm), low charge, or a dirty coil.
  • Condensate overflow from the head. Plugged drain line or a slimed-up drain pan — mini-split heads are notorious for biofilm clogging the small drain.
  • Won't heat in cold weather / weak heat. Low charge, defrost issue, or a model not rated for that low ambient.

Tech tips & gotchas

  • Flares and vacuum are 90% of mini-split reliability. Most callbacks trace to a bad flare or a lazy evacuation. Take the time on both — torque the flares, pull a real 500-micron vacuum with a decay test.
  • Read the fault code first. These systems tell you what's wrong if you let them. Each brand has its own code list and a service mode — use it instead of guessing at a contactor and cap that aren't even there.
  • There's no compressor contactor or run capacitor. Don't show up expecting to swap a cap. The compressor is driven by an inverter board. Different parts, different diagnosis.
  • Clean the head. The barrel blower and drain pan load up with dust and slime far faster than a ducted system. A deep cleaning fixes a lot of "weak/smelly/dripping" complaints.
  • Add charge for line length. Forgetting the per-foot addition on a long run leaves it undercharged from day one and chasing low-capacity complaints.

Safety / code notes

  • Recover refrigerant per EPA Section 608 before opening the system; never vent. Flared joints still hold a charge.
  • Outdoor-unit power and disconnect per NEC §440; a disconnecting means within sight is required. Since the head is powered from the condenser, killing the outdoor disconnect de-energizes both — verify dead before opening either.
  • Condensate must drain to an approved point per the mechanical/plumbing code; a head condensate pump needs a float safety where overflow would cause damage.
  • Line-set penetrations through the building envelope should be sealed; follow flare-connection torque and listing — improper flares are both a leak and a refrigerant-loss issue.