14.1 Mini-Split Architecture and Inverter-Driven Compressors

14.1 Mini-Split Architecture and Inverter-Driven Compressors

The Mini-Splits sheet of the HVAC Excellence Competency and Task List contains 156 task lines — larger than Gas Heat or Electrical. It covers everything from thermodynamic fundamentals through installation, evacuation, charging, and component replacement, and it explicitly requires the technician to "differentiate between a single-speed, multispeed, and inverter-controlled compressor" and to describe "variable speed compressor sequence of operation."

Ductless equipment is now a mainstream part of the trade, and its service model differs enough from conventional split systems that treating it as "a small heat pump" produces expensive mistakes.

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1. What a Mini-Split Is

A ductless mini-split consists of:

• An outdoor unit containing the compressor, condenser coil, condenser fan, inverter drive board, reversing valve (on heat pumps), and usually the electronic expansion valve.
• One or more indoor units — wall cassette, ceiling cassette, floor console, or a small ducted air handler — each with a coil, a variable-speed blower, a filter, and a control board.
• A line set of insulated liquid and suction tubing, usually with flare connections at both ends.
• An interconnecting cable carrying power and a low-voltage communication signal.
• A condensate drain, gravity or pumped.

Terminology: a mini-split has one indoor unit; a multi-split has several indoor units on one outdoor unit with fixed refrigerant piping; VRF (Variable Refrigerant Flow), sometimes called VRV, scales the concept to dozens of indoor units on branch piping with sophisticated controls (Section 14.2).

Why they are used: no duct losses (which in a leaky attic system can be 20–30% of capacity, Section 10.5), individual zone control, easy retrofit into buildings with no duct chases, very low sound levels, and high seasonal efficiency from inverter modulation.

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2. Single-Speed, Multi-Speed, and Inverter Compressors

How an inverter drive works

1. Rectifier: incoming single-phase AC (208/230 V) is converted to DC by a diode bridge.
2. DC bus with capacitors: smooths the DC and stores energy. The bus capacitors hold a lethal charge after power is removed — this is the single most important safety fact about inverter equipment.
3. Power factor correction / reactor: shapes the current waveform to reduce harmonics.
4. IGBT inverter section: insulated-gate bipolar transistors switch the DC bus on and off thousands of times per second in a pulse-width-modulated pattern, synthesizing a variable-frequency, variable-voltage AC output.
5. Motor: the compressor is typically a brushless DC / permanent-magnet synchronous motor whose speed follows the synthesized frequency directly.

Because the drive controls frequency, the compressor runs anywhere from roughly 15–20 Hz to 100–120 Hz, giving perhaps 20% to 120% of nominal capacity.

Why modulation matters more than peak efficiency

• Long run cycles dehumidify. A modulating system runs continuously at low speed, keeping the indoor coil below the entering-air dew point for hours instead of minutes (Section 11.2).
• No cycling losses. Every start-up wastes energy re-establishing pressure differential and re-cooling the coil.
• Stable temperature. The system trims capacity rather than overshooting.
• Boost capacity in cold weather. Running above nominal frequency lets a cold-climate heat pump hold rated capacity at low outdoor temperature (Section 9.5).
• Soft start. The drive ramps the compressor up rather than applying line voltage across a stalled rotor, so inrush current is a fraction of conventional LRA — which is why mini-splits can often be added to a service that could not accept a conventional condenser.

Sequence of operation

On a call for cooling, the indoor board sends a demand signal over the communication line. The outdoor board starts the compressor at a low frequency, monitors indoor coil temperature, suction pressure or temperature, and the difference between room temperature and setpoint, then ramps frequency up or down to close the gap. The electronic expansion valve modulates simultaneously to hold target superheat, and both indoor and outdoor fans vary speed. Nothing in the sequence resembles a contactor pulling in on a thermostat call.

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3. Electrical and Communication Wiring

The convention on most single-zone systems: line voltage lands at the outdoor unit, which then feeds the indoor unit through the interconnecting cable. There is no separate disconnect or breaker at the indoor unit. On many multi-zone systems the arrangement is reversed or the indoor units are separately powered — read the wiring diagram for the specific model; guessing here destroys boards.

The interconnecting cable typically has 3 or 4 conductors:

• L (line), N (neutral), G (ground) for power to the indoor unit.
• S (signal) for serial communication, often sharing the neutral as a reference.

Rules that boards do not survive being broken:

• Terminal numbers must match end to end. Terminal 1 to terminal 1, 2 to 2, 3 to 3. Crossing conductors between numbered terminals is the single most common cause of a destroyed control board on a new installation.
• Use the specified cable type and gauge, stranded or solid as required. Some manufacturers explicitly prohibit stranded wire under their clamp terminals without ferrules.
• Do not run the interconnecting cable in the same conduit as unrelated line-voltage circuits. Induced noise corrupts the serial signal and produces intermittent communication faults.
• Do not splice the communication conductor. If the run must be extended, use the manufacturer's approved method.
• Torque the terminal screws and pull-test each conductor. A loose signal conductor gives an intermittent E-code that looks like a board failure.

Fault codes are the primary diagnostic. Every manufacturer publishes an error-code table, displayed by blinking LEDs on the boards, by a code on the indoor display, or through a service tool. Read the code before removing anything — mini-split diagnosis without the code table is guesswork.

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4. Indoor Unit Types

Condensate is the most common callback. A wall cassette drains by gravity through the wall sleeve; the drain hose must slope continuously downward, must not be kinked inside the sleeve, and must not be routed above the level of the drain pan at any point. Ceiling cassettes use a small integral pump with a float switch that shuts the unit down on a high level — a nuisance-trip source when the reservoir accumulates biofilm.

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5. Service Constraints Unique to Inverter Equipment

These appear repeatedly on the exam because they invert conventional practice.

Three-phase-style rotation problems do not apply, because the drive sets the phase sequence electronically.

Grounding is not optional. Inverter drives generate high-frequency leakage current, and an ungrounded or poorly grounded unit produces communication faults, nuisance GFCI trips, and a genuine shock hazard at the cabinet.