How House Works

Why a mini-split stops heating for a few minutes in winter: defrost and hot start

How House Works EditorialChecked against 29 sources,
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HHW · HOW-IT-WORKS · MINI-SPLIT-DEFROST
Based on
Manufacturers' manuals and technical bulletins

If your mini-split stops blowing in heat mode for several minutes while the outdoor unit steams and drips, the manuals call that defrost: frost has formed on the outdoor unit, and the unit stops heating to melt it (Mitsubishi MSZ-GS18 operating instructions, p. En-13; Daikin OTERRA 115V operation manual, p. 23). Each brand prints its own time: Mitsubishi “max.10 minutes” for the MSZ-GL, MSZ-FS and MSZ-GS18 (Mitsubishi MSZ-GL operating instructions, p. En-11; Mitsubishi MSZ-FS operating instructions, p. En-13; Mitsubishi MSZ-GS18 operating instructions, p. En-13), Daikin “about 4 to 12 minutes” for the OTERRA 115V and FTXS30/36 (Daikin OTERRA 115V operation manual, p. 23; Daikin FTXS30/36 operation manual, p. 33), Fujitsu a “maximum of 15 minutes” for the KMAS and RLS3 and “between 7 and 15 minutes” for the RLB (Fujitsu KMAS operation manual, p. En-12; Fujitsu RLS3 operation manual, p. En-7; Fujitsu RLB operation manual, p. En-8), and MRCOOL “4 to 10 minutes” for the DIY 4th Gen multi-zone and “a maximum of 10 minutes” for the DIY E Star 3rd Gen (MRCOOL DIY 4th Gen multi-zone manual, p. 14; MRCOOL E Star 3rd Gen manual, p. 37), while MRCOOL’s 5th Gen owner’s manuals give no time. An error lamp, an error code or heat that stays weak after the defrost is a separate case, covered under “When it is not defrost” below.

How defrost works

What the owner’s manuals describe

Frost can form on the outdoor unit in heat mode “When outdoor temperature is very low with high humidity”, and “it could reduce the operating performance of the product” (Fujitsu KMAS operation manual, p. En-12). Mitsubishi names the same two conditions (Mitsubishi MSZ-GL operating instructions, p. En-11). None of the owner’s manuals gives the outdoor temperature at which a defrost begins.

During the defrost the warm air stops. Daikin: “During defrosting operation, hot air does not flow out of the indoor unit.” (Daikin OTERRA 115V operation manual, p. 25). Fujitsu: “The indoor unit stops for maximum of 15 minutes.” (Fujitsu KMAS operation manual, p. En-13). MRCOOL’s 4th Gen manual: “During defrosting, the indoor and outdoor fans will stop spinning.” (MRCOOL DIY 4th Gen multi-zone manual, p. 14).

Outside, the frost leaves as water and vapor. Mitsubishi says the defrost “makes ice forming on the outdoor unit melt and drip down”, and its vapor “looks like white smoke” (Mitsubishi MSZ-GS18 operating instructions, p. En-13). Daikin: “HEAT operation stops suddenly and a flowing sound is heard”, and “The frost on the outdoor unit melts into water or steam” (Daikin OTERRA 115V operation manual, p. 23). Fujitsu ties a “sizzling sound” to the automatic defrosting, along with steam and water from the outdoor unit (Fujitsu KMAS operation manual, p. En-13). MRCOOL adds that “When the unit restarts in heat mode after defrosting, white mist may be emitted” from both the indoor and the outdoor unit (MRCOOL DIY 5th Gen single-zone manual, p. 37).

None of the manuals lists this as a fault. Mitsubishi: “Outdoor unit is in defrost. Since this is completed in max.10 minutes, please wait.” (Mitsubishi MSZ-GL operating instructions, p. En-11). Daikin lists it under “Not a problem” (Daikin OTERRA 115V operation manual, pp. 23, 24). Fujitsu answers “Airflow is weak or stops” with “In this case, the Automatic defrosting operation is working.” (Fujitsu KMAS operation manual, p. En-13). MRCOOL’s mist row sits under “The following problems are not a malfunction” (MRCOOL DIY 5th Gen single-zone manual, p. 37).

Leave the defrost alone: “Do not use means to accelerate the defrosting process or to clean, other than those recommended by the manufacturer.” (Daikin OTERRA 115V operation manual, p. 3). Fujitsu and MRCOOL print the same rule (Fujitsu KMAS operation manual, p. En-2; MRCOOL DIY 5th Gen single-zone manual, p. 4); the Mitsubishi operating instructions used here do not.

Inside the unit: what the service manuals print

Mitsubishi’s MSZ-GL indoor service manual, written for technicians, covers the cycle in three sentences: “Defrosting starts when the temperature of outdoor heat exchanger becomes too low. The compressor stops once, the indoor/outdoor fans stop, the 4-way valve reverses and the compressor re-starts. This mode continues until the temperature of outdoor heat exchanger rises or the fixed time passes.” (Mitsubishi MSZ-GL service manual, p. 17). On the MUZ-FS, a technician can change the defrost finish temperature by cutting or soldering a jumper wire on the outdoor inverter board (Mitsubishi MUZ-FS service manual, p. 24).

Daikin’s ATMOSPHERA, FTK-B/FTX-B and ENTRA service manuals all print the same line in their defrost sections: “Defrosting is carried out by the cooling cycle (reverse cycle).” (Daikin ATMOSPHERA service manual, p. 78; Daikin FTK-B/FTX-B service manual, p. 44; Daikin ENTRA service manual, p. 26). In the ATMOSPHERA manual a defrost starts after a minimum compressor run and an accumulated time since operation began or since the previous defrost, and “The defrosting time or outdoor heat exchanger temperature must be more than a certain value to finish defrosting.” (Daikin ATMOSPHERA service manual, p. 78). The ENTRA manual lists each part during defrost: compressor ON, 4-way valve OFF, EXV at a fixed opening, outdoor fan OFF, indoor fan OFF (Daikin ENTRA service manual, p. 26).

Fujitsu’s KMAS service manual starts a defrost when the outdoor heat exchanger sensor reads below set values; for the first defrost after start-up, the values depend on how long the compressor has run. It releases the defrost on a heat exchanger temperature or on “Compressor operation time 15 minutes”, whichever comes first; the KPAS manual has the same 15-minute release (Fujitsu KMAS service manual, p. 04-17; Fujitsu KPAS service manual, p. 04-34). That is the same figure as the “maximum of 15 minutes” in the owner’s manuals.

MRCOOL’s DIY 5th Gen service manual is drawn in the figure below. It lists eight entry conditions. The first combines the compressor’s running time, the T3 sensor, which the manual calls the condenser coil temperature sensor, and the T4 outdoor ambient sensor, with limits that are partly EEPROM settings whose values the manual does not print (MRCOOL DIY 5th Gen service manual, pp. 28, 31). In its wording, during defrost “the compressor continues to run, the indoor and outdoor motor will case operation”, with “case” as printed (MRCOOL DIY 5th Gen service manual, p. 31).

The compressor is where the documents differ: Mitsubishi’s stops once and restarts, Daikin’s ATMOSPHERA timing chart takes it to 0 Hz and back up and switches the four way valve (Daikin ATMOSPHERA service manual, p. 78), the ENTRA table lists it as ON, and MRCOOL’s keeps running. Both fans stop in the Mitsubishi, Daikin ENTRA and MRCOOL descriptions; the Fujitsu service pages on defrost, pp. 04-17 and 04-18, say nothing about the fans.

Fujitsu KMAS, ASUH09/12KMAS Operation manual, pp. En-5, En-12, En-13, En-14

  1. Heating startsIndoor fan at very low speed for 3-5 minutes, to warm up the internal parts

  2. Heating

  3. Frost forms on the outdoor unitVery low outdoor temperature with high humidity

  4. Automatic defrostingIndoor unit stops, maximum of 15 minutesOPERATION lamp blinksSizzling sound; steam and water from the outdoor unit

  5. Heating

The last box follows from “temporarily stop”, p. En-12, and “overrides the Heating operation”, p. En-5; the manual does not print “heating resumes”. The manual does not say whether the slow fan start repeats after a defrost. Lamp table, p. En-14: OPERATION and TIMER blinking while ECONOMY blinks fast is the error code display, not defrost.

MRCOOL DIY 5th Gen single-zone Service manual, pp. 30, 31

  1. Heating modeIndoor fan runs continuously; the anti-cold wind function has priority

  2. Any one of eight entry conditions is metCondition 1: compressor running time, coil sensor T3, outdoor sensor T4; some limits are EEPROM settings whose values the manual does not give

  3. Defrosting modeCompressor continues to runIndoor and outdoor motors stopDefrost light of the indoor unit on; a symbol on the display

  4. Defrost endsT3 rises above the exit value, or 15 minutes in defrosting mode; 10 minutes under one condition

  5. Normal heating mode

This section does not mention the 4-way valve.

Each strip follows one document; the strips are not combined. The Fujitsu strip is what the owner's manual tells you to expect; the MRCOOL strip is the service manual's account for technicians. Neither document gives a normal interval between defrosts, so no repeat loop is drawn.

Hot start: why the fan waits before blowing

At the start of heating the indoor fan stays off or runs at a low speed, so that the unit does not blow cold air into the room. Daikin’s service manual calls this the “Hot-Start Function”: “the temperature of the indoor heat exchanger is detected, and the airflow is either stopped or significantly weakened”, with “similar control when defrost control starts or when the thermostat is turned ON” (Daikin ATMOSPHERA service manual, p. 65).

Brand and series Wait printed for the owner Name of the control
Daikin OTERRA 115V, ATMOSPHERA, FTXS30/36 (Daikin OTERRA 115V operation manual, p. 25; Daikin ATMOSPHERA operation manual, p. 47; Daikin FTXS30/36 operation manual, p. 33) “1 to 4 minutes” Hot-Start Function in the ATMOSPHERA service manual; the owner’s manuals say “warming up” or “To prevent the release of cold air”
Fujitsu KMAS, KPAS, RLB (Fujitsu KMAS operation manual, p. En-5; Fujitsu KPAS operation manual, p. En-5; Fujitsu RLB operation manual, p. En-3; Fujitsu KMAS service manual, p. 04-10) Indoor fan at very low speed for “3-5 minutes” Cool air prevention control (heating mode), in the KMAS service manual
Mitsubishi MSZ-GL (Mitsubishi MSZ-GL operating instructions, p. En-11; Mitsubishi MSZ-GL service manual, p. 17) No time: “Please wait as the unit is preparing to blow out warm air.” Cold air prevention control, in the service manual
MRCOOL DIY 4th Gen multi-zone, E Star 3rd Gen, DIY 5th Gen (MRCOOL DIY 4th Gen multi-zone manual, p. 14; MRCOOL E Star 3rd Gen manual, p. 6; MRCOOL DIY 5th Gen service manual, p. 30) No time Anti-Cold Air Function (4th Gen); Anti-Cold Air feature, shown as cF (3rd Gen); anti-cold wind (5th Gen service manual)

Daikin prints two longer waits for the OTERRA 115V, under operation right after the circuit breaker is turned on: “Wait for about 3 to 10 minutes”, and, when the outdoor temperature is below 40°F (5°C) and heating is started, “Wait for about 10 to 25 minutes.” (Daikin OTERRA 115V operation manual, p. 25). The page does not say whether these apply when the breaker has been on all along.

Fujitsu gives the reason for its slow fan: it turns at very low speed “temporarily to warm up the internal parts of the unit” (Fujitsu KMAS operation manual, p. En-13).

Mitsubishi’s service manual stops the indoor fan or runs it “in Very Low speed” when the compressor is off or starting with a cold indoor coil or room, “or when defrosting is being done” (Mitsubishi MSZ-GL service manual, p. 17). The words “hot start” do not appear in the Mitsubishi manuals used here. Two other Mitsubishi waits can look like one. On a multi system, if operation starts while the outdoor unit is defrosting, “it takes a few minutes (max. 10 minutes) to blow out warm air”; and “The unit cannot be operated for about 3 minutes when restarted” (Mitsubishi MSZ-GL operating instructions, p. En-11).

MRCOOL’s 4th Gen manual says the unit “is designed to not blow cold air while in HEAT mode” when it “has just begun heating”, “While the unit is defrosting” and in “Low temperature heating” (MRCOOL DIY 4th Gen multi-zone manual, p. 14).

Preheat is a different control

In the Mitsubishi and Fujitsu service manuals, preheat means warming the compressor. Mitsubishi’s “pre-heat control” energizes the compressor because moisture in the refrigerant cycle “may interfere with the start-up of the compressor at low outside temperature”; on the MUZ-FS it is off as shipped and is switched on with a jumper wire (Mitsubishi MUZ-FS service manual, p. 24). Fujitsu’s KMAS service manual: “turn on the power for the outdoor unit at least 12 hours before the operation. Especially in cold climate regions, the compressor may fail if the outdoor unit is on for less than 12 hours.” (Fujitsu KMAS service manual, p. 04-21).

What the indoor unit shows during defrost

On some units the lamp that marks a defrost also marks a fault, in another pattern or color. Read the row for your series.

Brand and series During defrost, as printed Printed as a fault
Mitsubishi MSZ-GL (Mitsubishi MSZ-GL operating instructions, pp. En-5, En-11) No lamp state for defrost. The lamp table covers only running toward the set temperature, approaching it, and standby on a multi system Upper operation indicator lamp blinking: “stop using the air conditioner and consult your dealer”
Daikin OTERRA 115V, ATMOSPHERA (Daikin ATMOSPHERA operation manual, p. 5; Daikin OTERRA 115V operation manual, pp. 23, 27) No lamp state for defrost; HEAT stops “and a flowing sound is heard” OPERATION lamp blinking: “Check the error code”. For codes U0 and F3, a blink interval of about 0.5 seconds “is a notification of an abnormality”
Daikin ENTRA FTK(X)B/N (Daikin ENTRA service manual, pp. 31, 32) LED red, blinking (red and steady in heating) LED blue, blinking; the code is read from the remote controller
Fujitsu KMAS, KPAS (Fujitsu KMAS operation manual, p. En-14; Fujitsu KPAS operation manual, p. En-13) OPERATION lamp blinks; on the KMAS, TIMER and ECONOMY in any state. “Wait until finishing the defrosting (maximum of 15 minutes).” OPERATION and TIMER blink while ECONOMY blinks fast: error code display, “Consult authorized service personnel.”
Fujitsu RLS3, RLB (Fujitsu RLS3 operation manual, p. En-2; Fujitsu RLB operation manual, p. En-8) OPERATION indicator (green) “blinks slowly in Automatic defrosting operation” (RLS3); “the OPERATION Indicator Lamp will flash” (RLB) Not covered in the pages used here
MRCOOL DIY 5th Gen single-zone, Advantage 5th Gen (MRCOOL DIY 5th Gen single-zone manual, pp. 10, 39; MRCOOL DIY 5th Gen service manual, pp. 31, 36; MRCOOL Advantage manual, p. 46) dF on the display, “When defrosting.”; the service manual adds that “the defrost light of the indoor unit will turn on” The codes in the “Error Display” tables, which also carry dF; see the note below
MRCOOL DIY 5th Gen multi-zone (MRCOOL DIY 5th Gen multi-zone manual, p. 46) Operation light and PRE-DEF indicator lit: “the unit’s anti-cold wind is activated in order to defrost the unit” Not covered in the pages used here
MRCOOL E Star 3rd Gen (MRCOOL E Star 3rd Gen manual, pp. 6, 37) dF on the display; the troubleshooting table says the “defrost-light blinks”. cF is the Anti-Cold Air feature, not defrost Not covered in the pages used here

On a Mitsubishi, do not read a blinking lamp as defrost. The operating instructions print no lamp state for defrost and put a blinking upper lamp on the list of cases to stop using the unit (Mitsubishi MSZ-GL operating instructions, p. En-11). In the service manual a blinking OPERATION INDICATOR lamp means the unit “detected an abnormality”, and the blink pattern identifies it (Mitsubishi MSZ-GL service manual, p. 27). The blink codes are on the Mitsubishi codes page.

A Fujitsu KMAS can also defrost after you switch heating off. The service manual gives the lamp for that case: “if the indoor unit operation lamp flashes slowly (6 sec on/2 sec off), the outdoor unit allow the heat exchanger to defrost, and then stop.” (Fujitsu KMAS service manual, p. 04-18).

MRCOOL’s 5th Gen documents disagree about dF. The DIY 5th Gen owner’s manual shows dF “When defrosting.” on its display list, yet puts DF, “Defrost”, in its error table with no note (MRCOOL DIY 5th Gen single-zone manual, pp. 10, 39). The service manual for the same series and the Advantage manual mark the row “Normal Display, not error code” (MRCOOL DIY 5th Gen service manual, p. 36; MRCOOL Advantage manual, p. 46). The DF page has the wording from other MRCOOL manuals.

Cold weather: the lowest outdoor temperature for heating

The bottom of the heating range depends on the series, and for some on the outdoor unit or an added heater.

Brand and series Lowest outdoor temperature for heating, as printed Below it, the manual says
Mitsubishi MSZ-GL (Mitsubishi MSZ-GL operating instructions, p. En-12) -4°F (-20°C) dry bulb “the outdoor unit may stop operation until the outdoor temperature exceeds the lower limit”
Mitsubishi MSZ-FS (Mitsubishi MSZ-FS operating instructions, p. En-14) -13°F (-25°C) dry bulb Same note as the MSZ-GL
Mitsubishi MSZ-GS18 with outdoor unit MUZ-GS18NA(H) or MUY-GS18NA (Mitsubishi MSZ-GS18 operating instructions, p. En-15) -4°F (-20°C) dry bulb Same note as the MSZ-GL
Mitsubishi MSZ-GS18 with outdoor unit MUZ-GS18NAHZ (Mitsubishi MSZ-GS18 operating instructions, p. En-15) -13°F (-25°C) dry bulb Same note as the MSZ-GL
Daikin OTERRA 115V (Daikin OTERRA 115V operation manual, p. 26) 5°F (-15°C) “A safety device may activate to stop the operation.”
Daikin ATMOSPHERA (Daikin ATMOSPHERA operation manual, p. 48) 5°F (-15°C); -13°F (-25°C) “if a drain pan heater (sold separately) is installed” For the heater: “Please consult your dealer.”
Daikin 19 series (Daikin 19 series operation manual, p. 28) 5°F (-15°C) with an RX or RXN outdoor unit; -13°F (-25°C) with an RXL Not covered here
Fujitsu KMAS (Fujitsu KMAS operation manual, p. En-12) 5°F (-15°C), as the “Suction temperature of the outdoor unit” “the air conditioner operation may stop because of the automatic protection circuit working”
MRCOOL DIY 5th Gen single-zone (MRCOOL DIY 5th Gen single-zone manual, p. 13) -13°F (-25°C) “certain safety protection features may activate and cause the unit to disable”

None of the manuals puts a number on the heat lost in the cold. Mitsubishi: “the heating effect may not be sufficient” (Mitsubishi MSZ-GL operating instructions, p. En-11). Daikin: “the heating capacity becomes smaller in lower outdoor temperatures” (Daikin FTXS30/36 operation manual, p. 11). Fujitsu: “the operating performance is reduced as outdoor air temperature drops” (Fujitsu KMAS operation manual, p. En-12). Daikin and Fujitsu suggest another heating appliance alongside the unit, and MRCOOL’s answer to poor heating when “The outdoor temperature is extremely low” is “Use auxiliary heating device.” (MRCOOL DIY 5th Gen single-zone manual, p. 38).

The Mitsubishi documents here do not use the names Hyper-Heat or H2i. Of the three Mitsubishi manuals above, only the GS18 one separates its outdoor units, by model suffix.

The outdoor unit in winter

The Mitsubishi and Fujitsu operating instructions used here do not mention snow. The Daikin OTERRA 115V manual says “Take snow protection measures. Contact your dealer for the details of snow protection measures, such as the use of a snow protection hood.” and “Do not place objects in direct proximity of the outdoor unit.” (Daikin OTERRA 115V operation manual, p. 4); the ATMOSPHERA and 19 series manuals print the first sentence too, the FTXS30/36 manual does not. Fujitsu asks you to “Keep the surroundings of the outdoor unit clean and tidy, and do not place items around it” (Fujitsu KMAS operation manual, p. En-2). Mitsubishi warns against appliances or furniture under the unit because “Water may drip down from the unit” (Mitsubishi MSZ-GL operating instructions, p. En-2).

None of these owner’s manuals tells you to clear snow or ice off the outdoor unit, and none mentions covering it for winter. Two safety lines forbid blocking air openings without naming a unit: Fujitsu’s “Do not block or cover the intake grille and the outlet port” (Fujitsu KMAS operation manual, p. En-2) and Daikin’s “Do not block air inlets nor outlets” (Daikin FTXS30/36 operation manual, p. 2). Two troubleshooting tables ask you to remove an obstruction from the air inlet or outlet of the outdoor unit (Daikin OTERRA 115V operation manual, p. 24; MRCOOL DIY 5th Gen single-zone manual, p. 38). The snow advice is in the installation instructions, written for whoever places the unit:

  • Mitsubishi GS: “Please install it in an area not affected by snowfall or blowing snow. In areas with heavy snow, please install a canopy, a pedestal and/or baffle boards.” (Mitsubishi GS installation manual, PDF p. 1).
  • Fujitsu KMAS: “Install the outdoor unit on a raised platform that is higher than drifting snow.” (Fujitsu KMAS installation manual, p. En-1). The KNAS manual asks for “a raised stand or rack, at or above the anticipated snow depth for the region”, recommends “snow hoods and drift prevention fencing” where snow blows and drifts, and warns that in an area with heavy snowfall, if snow blocks the intake and outlet of the outdoor unit, “it might become difficult to get warm and it is likely to cause of the breakdown” (Fujitsu KNAS installation manual, p. En-14).
  • MRCOOL DIY 5th Gen: “Where snowfall is anticipated, take appropriate measures to prevent ice buildup & coil damage.” For heavy rain or snow: “Build a shelter above the unit to protect it from the rain or snow. Be careful not to obstruct air flow around the unit.” (MRCOOL DIY 5th Gen single-zone manual, p. 20).
Mini-split outdoor unit on a raised stand above the snow, with a canopy above itFront view, not to scale. A canopy sits above the outdoor unit, with open space between them. The unit stands on the two posts of a raised stand, above a dashed line for the anticipated snow depth; hatched snow lies below the line. Snow also lies on the canopy. Water drips from the drain port under the unit and falls between the posts, so no part of the stand frame is under the drain port. A dashed outline under the port marks the open space the water falls through. Numbers 1 to 6 match the key. 1 2 3 4 5 6
  1. Canopy, shelter or snow hood Mitsubishi GS installation manual, PDF page 1, in areas with heavy snow: “a canopy”. MRCOOL DIY 5th Gen single-zone manual, page 20: “Build a shelter above the unit to protect it from the rain or snow.” Fujitsu KNAS installation manual, page En-14, where snow blows and drifts: “snow hoods”. The sentence does not say where a hood goes.
  2. Air flow around the unit MRCOOL DIY 5th Gen single-zone manual, page 20: “Be careful not to obstruct air flow around the unit.” Fujitsu KNAS installation manual, page En-14: in an area with heavy snowfall, if snow blocks the intake and outlet of the outdoor unit, “it might become difficult to get warm and it is likely to cause of the breakdown”.
  3. Raised stand Fujitsu KNAS installation manual, page En-14: “a raised stand or rack, at or above the anticipated snow depth for the region”. Mitsubishi GS installation manual, PDF page 1, in areas with heavy snow: “a pedestal”.
  4. Anticipated snow depth Fujitsu KNAS installation manual, page En-14, in the same sentence as the stand: “the anticipated snow depth for the region”. The dashed line stands for that depth and has no height on it. Hatching on the drawing is snow.
  5. Drain water Fujitsu KNAS installation manual, page En-14: “During heating operation, drain water flows from the outdoor unit.” At 32°F (0°C) or less the manual says not to use the accessory drain pipe, because the water in it “may freeze in extremely cold weather”, so none is drawn.
  6. No stand frame under the drain port Fujitsu KNAS installation manual, page En-13: do not place the stand’s frame under the drain port, where dripping water “repeats freezing and accumulating, and may block the drain port”. The dashed outline marks the open space under the port; it has no width.
A generalized drawing from the words of three installation manuals: Mitsubishi GS, Fujitsu KNAS and MRCOOL DIY 5th Gen. It is not a drawing of any one model and has no scale: it shows no heights or distances, and not how the canopy is held up. The baffle boards and drift prevention fencing named in the Mitsubishi and Fujitsu sentences are not drawn. The Mitsubishi, Daikin and Fujitsu owner’s manuals above do not tell you to clear snow or ice off the outdoor unit, and none mentions covering it for winter.

Water leaving the outdoor unit has to drain away, and it can freeze. Fujitsu’s KNAS manual: “During heating operation, drain water flows from the outdoor unit.” At 32°F (0°C) or less it says not to use the accessory drain pipe, because the water in it “may freeze in extremely cold weather” (Fujitsu KNAS installation manual, p. En-14), and not to place the stand’s frame under the drain port, where dripping water “repeats freezing and accumulating, and may block the drain port” (Fujitsu KNAS installation manual, p. En-13). For the MRCOOL Advantage in colder climates, the drain hose should run “as vertical as possible”, since slow-draining water “can freeze inside the hose and flood the unit” (MRCOOL Advantage manual, p. 30).

Some outdoor units have a heater in the base, tied to particular models:

  • Mitsubishi lists a “defrost heater (base pan heater)” for the MUZ-GL NAH models (Mitsubishi MUZ-GL service manual, p. 59). The MUZ-FS service manual marks its defrost heater “MUZ-FS•NAH only” (Mitsubishi MUZ-FS service manual, pp. 23, 42), and in the GS series it is for NAH and NAHZ only (Mitsubishi MUZ-GS service manual, p. 57).
  • Daikin’s drain pan heater for the ATMOSPHERA is sold separately (Daikin ATMOSPHERA operation manual, p. 48). The service manual recommends it “In high humidity areas or heavy snow areas” to “prevent ice build-up from the bottom frame”, and sets it up on the circuit board (Daikin ATMOSPHERA service manual, p. 163).
  • MRCOOL’s DIY 5th Gen outdoor board has outputs for a “compressor heater” and a “chassis heater” (MRCOOL DIY 5th Gen service manual, p. 22). The owner’s manual: “For Outdoor Units with Auxiliary Electric Heater: When the outside temperature is below 32°F (0°C), we strongly recommend keeping the unit plugged in at all times” (MRCOOL DIY 5th Gen single-zone manual, p. 13). It does not say which models have one.
  • The Fujitsu K series manuals describe no heater in the base of the outdoor unit.

When it is not defrost

None of these documents says what to do if a defrost runs past the printed time or comes back often, and none gives a normal number of defrosts per hour. For weak heating, the Mitsubishi and Fujitsu troubleshooting tables also ask whether the filters are clean, whether anything blocks an air inlet or outlet (of the indoor or outdoor unit on the Mitsubishi, of the indoor unit on the Fujitsu), and whether a window or door is open (Mitsubishi MSZ-GL operating instructions, p. En-11; Fujitsu KMAS operation manual, p. En-13). Past those checks, the steps they print are for error lamps, error codes and heat that stays weak:

  • Mitsubishi MSZ-GL: its troubleshooting table opens with “Even if these items are checked, when the unit does not recover from the trouble, stop using the air conditioner and consult your dealer.” A separate list, “In the following cases, stop using the air conditioner and consult your dealer”, names among others water leaking or dripping from the indoor unit, the upper operation indicator lamp blinking and the breaker tripping frequently (Mitsubishi MSZ-GL operating instructions, p. En-11).
  • Daikin OTERRA 115V, with the OPERATION lamp blinking: “Turn off the power with the circuit breaker and restart operation with the remote controller. If the OPERATION lamp is still blinking, check the error code and consult your dealer.” (Daikin OTERRA 115V operation manual, p. 24).
  • Fujitsu KMAS: “Immediately stop operation and turn off the electrical breaker” and consult authorized service personnel when “The problem persists even if you perform these checks or diagnostics” or the error lamp pattern shows (Fujitsu KMAS operation manual, p. En-14).
  • MRCOOL DIY 5th Gen, if the indicator lamps keep flashing or error codes appear: “wait for about 10 minutes”, then disconnect the power, connect it again and turn the unit on; “If the problem persists, disconnect the power and contact MRCOOL®.” For poor heating, the same table also lists “Cold air is entering through doors and windows” and “Low refrigerant due to leak or long-term use. Call technician to check for leaks, re-seal if necessary, and top off refrigerant.” (MRCOOL DIY 5th Gen single-zone manual, p. 38).

Refrigerant work and anything on the circuit boards belong to a technician: the defrost finish temperature, the compressor pre-heat and the drain pan heater setup are jumper or switch settings in the service manuals (Mitsubishi MUZ-FS service manual, p. 24; Daikin ATMOSPHERA service manual, p. 163), and Mitsubishi checks its defrost heater with a troubleshooting chart in the outdoor service manual (Mitsubishi MUZ-GL service manual, p. 59). Have a licensed HVAC technician do this work.

Mini-split codes on this site

What these documents do not cover

  • Manuals for the Mitsubishi MSZ-FX, GX, HX and JX were not part of this set.
  • The Daikin OTERRA 115V service manual was not read, so the defrost conditions for that series are not covered. Daikin’s defrost LED is described only for the ENTRA.
  • Fujitsu gives the fan limits of its cool air prevention control and the conditions for second and later defrosts as diagrams and tables that are not transcribed here.
  • MRCOOL’s 5th Gen troubleshooting table has the row “The unit changes from COOL/ HEAT mode to FAN mode”, explained as “The unit may change its setting to prevent frost from forming on the unit” (MRCOOL DIY 5th Gen single-zone manual, p. 37). The row does not say whether it means heating or cooling, so this page does not treat it as defrost.
  • The owner’s manuals do not say when a base pan, drain pan or chassis heater switches on. The only conditions are in the Mitsubishi service chart for its defrost heater (Mitsubishi MUZ-GL service manual, p. 59), a technician’s test that runs the unit in HEAT mode with both thermistors cold. The Daikin, Fujitsu and MRCOOL manuals used here give none.
  • Each quote belongs to the series and document named with it. MRCOOL replaces files without changing the address, and both the Advantage manual and the Fujitsu KNAS installation manual were updated in October 2026; the pages quoted here did not change. Check the manual for your own model.

See also

Error codes: Daikin, Fujitsu, Mitsubishi Electric, MrCool

Sources

29 documents
  1. Mitsubishi Electric. Operating Instructions, MSZ-GL06/09/12/15NA, MSY-GL09/12/15NA. Printed code DG79A01LH01; no publication date printed (layout stamp 2021/10/01); pp. En-2, En-5, En-11, En-12. Accessed Oct 12, 2026.
  2. Mitsubishi Electric. Operating Instructions, MSZ-FS06/09/12/15/18NA. Printed code JG79Y766H01; no publication date printed (layout stamp 2020/04/10); pp. En-13, En-14. Accessed Oct 12, 2026.
  3. Mitsubishi Electric. Operating Instructions, MSZ-GS18NA, MSY-GS18NA. Printed code DG79A05BH01; no publication date printed (layout stamp 2022/02/17); outdoor units MUZ-GS18NA(H), MUY-GS18NA, MUZ-GS18NAHZ; pp. En-13, En-15. Accessed Oct 12, 2026.
  4. Mitsubishi Electric. Installation Manual, MSZ(Y)-GS. Printed code JG79N458H03; installer document with no printed page numbers; PDF p. 1. Accessed Oct 12, 2026.
  5. Mitsubishi Electric. Indoor Unit Service Manual, MSZ-GL.NA / MSY-GL.NA Series (OBH732, Revised Edition-D). MSZ-GL06-24NA; issued Feb. 2022; technician manual; pp. 17, 27. Accessed Oct 12, 2026.
  6. Mitsubishi Electric. Outdoor Unit Service Manual, MUZ-FS.NA / MUZ-FS.NAH Series (OBH873, Revised Edition-B). MUZ-FS06-18NA and NAH; issued Aug. 2021; technician manual; pp. 23, 24, 42. Accessed Oct 12, 2026.
  7. Mitsubishi Electric. Outdoor Unit Service Manual, MUZ-GL.NA / MUY-GL.NA Series (OBH733, Revised Edition-J). Distributor copy of the Mitsubishi document, hosted by Baker Distributing / Gemaire (resource.bakerdist.com); MUZ-GL09-24NA and NAH; issued Jun. 2021; technician manual; p. 59. Accessed Oct 12, 2026.
  8. Mitsubishi Electric. Outdoor Unit Service Manual, MUZ-GS.NA/NAH/NAHZ, MUY-GS.NA Series (OBH875, Revised Edition-B). Distributor copy of the Mitsubishi document, hosted by Meier Supply (meiersupply.com); MUZ-GS09-36NA, NAH and NAHZ; issued Oct. 2022; technician manual; p. 57. Accessed Oct 12, 2026.
  9. Daikin. Operation Manual, FTXQ09/12ASBU9 (OTERRA 115V). 3P741457-13J; print stamp 2024/11; pp. 3, 4, 23, 24, 25, 26, 27. Accessed Oct 13, 2026.
  10. Daikin. Operation Manual, FTXM09/12/18/24WVJU9 (ATMOSPHERA). 3P686871-1A; print stamp 2022/05; pp. 4, 5, 47, 48. Accessed Oct 13, 2026.
  11. Daikin. Operation Manual, FTX/FTXN-NMVJU (19 series). 3P379751-4C; print stamp 2015; outdoor units RX, RXN, RXL; pp. 4, 28. Accessed Oct 13, 2026.
  12. Daikin. Operation Manual, FTXS30/36HVJU, FTXS30/36LVJU. 3P228444-4K; print stamp 2013; pp. 2, 11, 33. Accessed Oct 13, 2026.
  13. Daikin. Service Manual, Inverter Pair Heat Pump, Wall Mounted Type FTXM-W/V Series (ATMOSPHERA). SiUS042113EA; specification update 01/2023; technician manual; pp. 65, 78, 163. Accessed Oct 13, 2026.
  14. Daikin. Service Manual, Inverter Wall Mounted Single Split FTK(X)B/N-AXVJU (ENTRA). SM-5WM-Y-NA-B1; no date printed; technician manual; pp. 26, 31, 32. Accessed Oct 13, 2026.
  15. Daikin. Service Manual, Wall Mounted FTK-B & FTX-B Series. DAMA-SM-23-006; first edition 04/2023; technician manual; p. 44. Accessed Oct 13, 2026.
  16. Fujitsu General. Operation Manual, ASUH09/12KMAS. PART No. 9387602524-01; pp. En-2, En-5, En-12, En-13, En-14. Accessed Oct 12, 2026.
  17. Fujitsu General. Operation Manual, ASUH07/09/12/15KPAS. PART No. 9387602548-01; pp. En-5, En-13. Accessed Oct 12, 2026.
  18. Fujitsu General. Operation Manual, ASU18/24RLB. PART No. 9315345615; R-410A; pp. En-3, En-8. Accessed Oct 12, 2026.
  19. Fujitsu General. Operation Manual, ASU9/12/15RLS3. PART No. 9320455026; R-410A; pp. En-2, En-7. Accessed Oct 12, 2026.
  20. Fujitsu General. Service Manual & Parts List, ASUH09/12KMAS, AOUH09/12KMAS1. SR_AS134ES_03; printed stamp 2024.11.11; technician manual; pp. 04-10, 04-17, 04-18, 04-21. Accessed Oct 12, 2026.
  21. Fujitsu General. Service Manual & Parts List, ASUH09-36KPAS/KPBS, AOUH09-36KPAS1. SR_AS142ES_09, revision 03; printed stamp 2026.06.18; technician manual; p. 04-34. Accessed Oct 12, 2026.
  22. Fujitsu General. Installation Manual, ASUH09/12KMAS (KM series). PART No. 9387603385-01; installer document; p. En-1. Accessed Oct 12, 2026.
  23. Fujitsu General. Installation Manual, ASUH09/12/18/24KNAS (KN series). PART No. 9387603460-01; installer document; pp. En-13, En-14; the file on the site was updated in October 2026 and the quoted pages did not change. Accessed Oct 12, 2026.
  24. MRCOOL. DIY Series Single-Zone Mini-Split System, Installation & Owner's Manual (5th generation). Version date August 5, 2026; DIY-*-HP-WMAH-115D25-O and -230D25-O; pp. 4, 10, 13, 20, 37, 38, 39. Accessed Oct 13, 2026.
  25. MRCOOL. DIY Series Single-Zone Mini-Split System, Service Manual (5th generation). Version date April 20, 2026; technician manual; pp. 22, 28, 30, 31, 36. Accessed Oct 13, 2026.
  26. MRCOOL. MRCOOL Advantage Series, Installation & Owner's Manual (5th generation). Version date September 19, 2025 in the copy used; the file on the site is dated October 29, 2026 and the quoted pages did not change; pp. 30, 46. Accessed Oct 13, 2026.
  27. MRCOOL. DIY Series Multi-Zone Mini-Split System, Installation & Owner's Manual (5th generation). Version date January 8, 2026; p. 46. Accessed Oct 13, 2026.
  28. MRCOOL. Installation & Owner's Manual, 4th Generation DIY Multi-Zone. Version date 06-06-2023; p. 14. Accessed Oct 13, 2026.
  29. MRCOOL. Installation & Owner's Manual, DIY E Star Series (3rd generation). Version date 03-21-23; DIY-24-HP-C-230B, DIY-36-HP-C-230B; pp. 6, 37. Accessed Oct 13, 2026.