Fujitsu mini split not heating

Two more things look like a fault and are not. In the first 3 to 5 minutes of heating the indoor fan runs very slowly while the unit warms up inside. And a slowly blinking OPERATION lamp means automatic defrost, which can stop heat for up to 15 minutes.

Halcyon or AIRSTAGE: the lights mean the same thing

For years Fujitsu sold its home mini splits as Halcyon; the current range is AIRSTAGE. The operating manuals we read for both, the Halcyon RLS3/RLS3H and LZAS series and the current AIRSTAGE KP series, describe the same indicator logic:

Settings that quietly cap heat

• Min. Heat (the "10°C HEAT" button). It holds the room at 50°F and will not heat past that. On current models you cancel it by pressing START/STOP to turn the unit off, then start it again in HEAT.
• Economy. A few degrees below the setpoint, at about 70% of maximum output. It cannot run at the same time as Powerful.
• Energy Saving (the human sensor). On models that have it, such as the Halcyon RLS3 and LZAS series, it is switched on from the factory. If nobody is detected for about 20 minutes, the heating setpoint drops by up to about 8°F (4°C). A room that heats well and then cools while you sit still reading is often this.
• Quiet fan speed. Fujitsu says Quiet prioritizes noise reduction, and "performance may become lower."
• The setpoint itself. Fujitsu's manual is plain: in HEAT mode the setpoint has to be higher than the current room temperature, or heating may not start.

Powerful mode is the opposite of all of these. It runs at maximum and switches itself off when the room reaches the setpoint or after 20 minutes, whichever comes first.

Blowing cold air, or barely any air, in heat mode?

Work through it in this order:

1. Did it just start? The fan runs very slowly for 3 to 5 minutes to warm the unit up inside. After a sudden power cut the compressor also waits about 3 minutes before it runs. Give it five minutes before judging.
2. Is the room already at the setpoint? When it is, Fujitsu stops the outdoor unit and runs the indoor fan very slowly. That is normal.
3. Is the OPERATION lamp blinking? That is defrost: the indoor unit stops for up to 15 minutes, and the outdoor unit may sizzle, stop its fan, steam or drip water. Fujitsu lists all four as normal. For how the cycle works, see the defrost cycle explained.
4. Does it cool normally but never heat? That is not in the owner's manual at all. It points at the part that switches the system between heating and cooling, and it is a technician's diagnosis.

Know your Fujitsu's cold-weather floor

Fujitsu's manuals say heating performance falls as the outdoor temperature drops, and that outside the rated range "the protection circuits may be activated to stop the unit." They recommend another heat source if heating is not enough. How cold that range goes depends heavily on the model:

That is a 27-degree spread. Below its floor, a Fujitsu that slows or stops heating is protecting itself, not failing, and the answer is backup heat or a colder-rated system, not a repair. Look up your exact model in the product finder, or compare the lines that keep heating in our best cold-climate mini splits.

Safe checks, in order

1. Read the lamps against the table above.
2. Leave Min. Heat and Economy if the green lamp is on, and set the fan to anything but Quiet.
3. HEAT mode, setpoint above the room, then give it five minutes.
4. Clean the air filter and make sure nothing blocks the intake grille or the outlet. These are the first checks in Fujitsu's own list for poor heating.
5. Close windows and doors in the room being heated.
6. Clear snow from around the outdoor unit, with the power off. Sizzling, steam and dripping water during defrost are normal.
7. Check the outdoor temperature against your line's floor.

If only one room on a multi-zone system will not heat, see one room won't heat or cool. If the remote itself seems dead, see remote not working.

When to stop and call a technician

OPERATION and TIMER blinking while ECONOMY blinks fast is Fujitsu's error signal. The manual's instruction is to stop the unit immediately, turn off the breaker, and consult authorized service personnel. Do the same if the problem persists after the checks above.

On current AIRSTAGE KP-series models you can read the code first: press and hold TEST RUN on the remote for more than five seconds, and the unit shows a two-character code. Write it down for the technician. If the unit gives several beeps at "00", no error was detected. The owner's manual does not say what each code means, so do not try to act on the code yourself.

Call as well if:

• heat has faded gradually over weeks, which points to refrigerant charge, and refrigerant work is certification-gated;
• the outdoor coil stays iced over after a defrost has finished;
• it cools normally but never heats.

Fujitsu does not authorize online retail sales of its equipment, so repairs go through an authorized Fujitsu contractor. That is also who can tell you whether a failed part is still under warranty. Technicians reading the outdoor unit's own service codes will find all 36 of the ones we have catalogued in the Fujitsu error-code library. For what the lights mean on other brands, see blinking lights, and every brand's heating checks are in mini split not heating.