Air-source heat pump outdoor unit beside a house in the snow
8 components covered

Field manual · Air Conditioner

Fujitsu Halcyon RLS3 / RLS3H wall-mount mini-splits (ASU9/12/15RLS3, AOU9/12/15RLS3): quick sheet

Covers: Fujitsu General Halcyon single-zone wall-mounted inverter heat pumps, introduced for the 2015 season: indoor ASU9RLS3, ASU12RLS3, ASU15RLS3 paired with outdoor AOU9RLS3, AOU12RLS3, AOU15RLS3, and the low-ambient H versions ASU9RLS3H/AOU9RLS3H, ASU12RLS3H/AOU12RLS3H, ASU15RLS3H/AOU15RLS3H. Fujitsu calls the H versions Extra Low Temperature Heating (XLTH); 'Hyper-Heat' is not a Fujitsu term (it is used by another brand), so search both. Sold in the US as 9RLS3, 12RLS3, 15RLS3 and 9RLS3H/12RLS3H/15RLS3H systems. Related but not covered: older RLS2/RLS2H, RLFW/RLFCD, multi-zone and ducted models, and the AirStage commercial line, whose code lists differ. Search terms: RLS3, RLS3H, 12RLS3H, ASU12RLS3, AOU12RLS3, Halcyon wall mount.

Single sourceBrands: Fujitsu8 components

The RLS3 is a high-efficiency inverter wall-mount heat pump (9,000, 12,000 and 15,000 Btu/h nominal). The RLS3H adds cold-climate parts, including a base pan heater, and a lower heating limit. The outdoor unit takes the 208/230 V supply and powers the indoor unit through a 3-wire interconnect that also carries the serial communication signal, so many 'no operation' calls are wiring faults. Faults are reported by blinking the indoor unit's OPERATION and TIMER lamps; the code list depends on the model, so always confirm against the RLS3 service literature.

Safety first

  • The indoor unit is powered from the outdoor unit: open the outdoor disconnect before working on either unit.
  • Inverter boards hold a DC bus charge for minutes after power-off; verify 0 V before touching.
  • All refrigerant work (charging, recovery, flare or brazing repair, leak search with pressure, compressor or valve replacement) is blocked: refer to an EPA 608-certified technician.
  • Do not splice the interconnect cable; replace it with a continuous run of the type Fujitsu specifies.

How it works

The indoor controller reads room and coil thermistors and sends a capacity request to the outdoor unit over the serial line. The outdoor inverter board varies compressor speed to match load, which gives the wide modulation range these systems are known for.

Power and signal share the interconnect: two conductors carry line voltage to the indoor unit and the third carries the serial signal referenced to them. The signal conductor is not a neutral, and voltage readings on it fluctuate; a steady reading is not a meaningful pass/fail test.

In heating, the outdoor coil becomes the evaporator and ices up; the outdoor board runs reverse-cycle defrost. On RLS3H models a base pan heater (about 150 W per installer analysis of Fujitsu data) runs as needed in freezing weather so defrost water does not refreeze in the pan.

ESP motion sensing (where fitted) relaxes the setpoint when the room is empty: about 4°F up in cooling or 8°F down in heating after 20 minutes, returning when occupancy is detected (single source).

Components & tests

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Official manufacturer documentation

Always confirm values against the tech sheet for the exact model.

OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work. © 2026 OnCall Tech. Original content; not a reproduction of any manufacturer document.Generic part photos (not of this brand): Air-source heat pump outdoor unit beside a house in the snow by PeterEastern, CC BY-SA 4.0

Diagnostic guides for this appliance

Guide · Air Conditioner

PTAC not cooling (fan runs, air not cold)

Most PTAC no-cool calls are airflow (filter, coils), a wrong control setting or a failed compressor start component. Rule those out before suspecting the sealed system.

6 steps ~60 minmoderate
Guide · Air Conditioner

PTAC not heating (electric heat or heat pump)

PTACs heat with electric strip heat, a heat pump, or both. Heat pump models switch to strip heat in cold weather. Check settings, then the heater element and its limits, then the reversing valve.

5 steps ~60 minmoderate
Guide · Air Conditioner

PTAC error codes: how to read and clear them

Most PTAC brands store fault codes that can be shown with a button combination on the control (for example GE Zoneline uses a MODE + another key sequence on some models). Read the active code, fix the cause, then clear it.

4 steps ~20 mineasy

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Trane XR14 / XR16 and American Standard Silver 14 / 16 outdoor AC and heat pumps (4TTR4, 4TTR6, 4TWR4, 4TWR6): quick sheet

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Fujitsu Halcyon RLS3 / RLS3H wall-mount mini-splits (ASU9/12/15RLS3, AOU9/12/15RLS3): quick sheet · OnCall Tech