Heat Pump Defrost Cycle: How Long It Lasts and When It's a Problem

Quick answer

The defrost cycle is normal: in cold, damp weather, frost builds on the outdoor coil, and the heat pump briefly reverses itself to warm the coil and melt it. Steam over the outdoor unit, a whoosh, a stopped outdoor fan, and AUX on the thermostat (or a ductless indoor unit that stops blowing) are all expected, and manufacturers put a cycle at about 3 to 15 minutes, depending on the system. It signals a problem when ice is thicker than 1/4 inch or stays after defrost, or the unit defrosts constantly with cycles getting longer. Then leave the ice alone and call a licensed HVAC technician.

Why a heat pump has to defrost

In heating mode, the outdoor coil pulls heat out of the outdoor air. Trane says that coil usually runs 10 to 20°F colder than the air around it, so moisture freezes on it even in the low 40s, and defrost is more common in the 20s and 30s. Rheem’s use and care guide for its commercial packaged heat pumps puts the start of frost at about 45°F, with the heaviest frost on damp days around 35 to 40°F.

Frost acts like insulation. A frosted coil can’t absorb heat well, Lennox explains, so the system works harder for less heat.

What happens during a defrost cycle, step by step

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1. The control decides it’s time. In what Trane calls demand defrost, a microprocessor reads two sensors, one for outdoor air and one for the coil, and decides when to defrost. Other units defrost on a timer, at 30, 60, or 90 minute intervals (Lennox).
2. The reversing valve switches. It reverses the refrigerant flow to heat the outdoor coil, essentially running the heat pump in cooling mode for a few minutes (Trane). Carrier’s owner’s manual says you may hear a “whoosh” at the start and end.
3. The outdoor fan stops. Carrier, Trane, and Rheem all list this as normal. The compressor may also get louder or quieter (Trane).
4. Frost melts and steam rises. Trane describes the cloud as ice melting and evaporating. Melt water runs out of the outdoor unit (Mitsubishi Electric, Daikin).
5. Inside the house. In a ducted system with electric backup heat, the backup comes on and you may see AUX on the thermostat, so most owners notice nothing (Trane). Without backup heat, and in some dual-fuel systems, you may feel cool air for a few minutes, which Trane calls perfectly normal. Lennox says the indoor temperature may drop slightly.
6. Back to heating. The valve switches back and the outdoor fan restarts. Trane’s demand defrost ends as soon as the ice is gone; timed defrost on some other brands runs for a set time regardless, Trane says.

If the house stays cold after defrost ends, something else is wrong; see our guide to a heat pump not heating. For the backup heat that covers the gap, see aux heat vs. emergency heat.

Is steam from the outdoor unit normal?

Yes. Every manufacturer reviewed here describes it: “normal and harmless” in Rheem’s guide, vapor that “looks like white smoke” in Mitsubishi Electric’s manual, and not a fire or a malfunction, according to LG. Trane’s repair guide adds that there’s no need to call the fire department or your HVAC company over it, since heating resumes in a few minutes. Real smoke or a burning smell is another matter (see the safety box).

How long a defrost cycle lasts

Times vary by brand and by how much frost there is.

Manufacturer	System	Typical defrost length
Trane	Central heat pumps with Trane’s demand defrost	Usually 3 to 5 minutes
Carrier	Central heat pumps	About 5 to 10 minutes
Lennox	Central heat pumps	5 to 15 minutes, depending on weather and frost
Daikin	Atmosphera wall-mounted ductless (FTXM models)	About 4 to 12 minutes
Mitsubishi Electric	MSZ-GS wall-mounted ductless	About 10 minutes, at most
Fujitsu AIRSTAGE	Ductless mini-splits	7 to 15 minutes
LG	Ductless air conditioners and heat pumps	About 5 to 15 minutes; can take longer when it is far below 5°F (minus 15°C) at the outdoor unit

Your owner's manual gives the range for your model. Source: Trane, Carrier, Lennox, Daikin, Mitsubishi Electric, Fujitsu AIRSTAGE, and LG pages and manuals listed in the sources

One long cycle on an unusually icy day is no cause for panic, Lennox says, but repeated long cycles should be checked by an HVAC professional.

How often a heat pump defrosts

There’s no single right number. It depends on the weather and on how the unit decides when to defrost.

• Weather. Frost forms quickly on cold, damp days, so defrost runs more often; on dry or sunny days it may run rarely or not at all. Most systems check for frost every 30 to 90 minutes during heating, according to Lennox.
• Timed defrost. The older Rheem two-stage model offers 30, 60, and 90 minute settings that the installer picks for the climate: 90 minutes for dry southern Arizona, 30 for moist Seattle. The factory setting is 30 minutes.
• Demand defrost. Sensor-based defrost runs only when needed, so there are fewer cycles (Trane). Rheem’s commercial guide says electronic controls that sense coil and air temperatures may go as long as 6 hours between defrosts.

So a defrost every 30 to 90 minutes on a wet day near freezing can be normal. Defrosting constantly (Trane) or unusually often (Carrier) is not.

Mini-split defrost mode: what it looks like

On a ductless mini-split, defrost is easier to notice: the indoor unit stops heating and usually stops blowing. Here is how each brand shows it.

• LG: a Defrost or Preheating message, or a water drop light. The indoor fan stops, louvers on wall-mounted models close, and the outdoor fan stops while the compressor keeps running. LG asks owners not to turn off the power and to wait for warm air to return.
• Mitsubishi Electric (MSZ-GS): operation stops for up to about 10 minutes, and the horizontal vane moves to a level position. The manual also lists an occasional hissing sound when the refrigerant flow switches.
• Fujitsu AIRSTAGE: the OPERATION lamp flashes, the outdoor fan may stop, steam may rise outside, and you may hear an occasional sizzling sound.
• Daikin (Atmosphera): heat stops suddenly with a flowing sound. There may also be a blowing sound as the refrigerant changes direction at the start, and a whistling sound while defrost runs.

Defrost or preheat? If the indoor unit doesn’t blow right after you switch on the heat, that is usually a warm-up delay, not defrost. LG’s fan waits until the indoor piping warms up, and Daikin says air may not come out for 1 to 4 minutes after heating starts, to avoid blowing cold air.

Normal or a problem? Quick check

What you notice	Normal	Get it checked
Steam or a white cloud	Rises for a few minutes during defrost (Trane, LG)	Smoke or a burning smell (Lennox)
Frost on the outdoor coil	Thin frost under 1/4 inch that clears during defrost (Carrier). Rheem says the coil can be almost completely frosted at times.	Ice thicker than 1/4 inch, a solid coat, or ice that stays after defrost (Carrier). A thickening layer with patches of hard, clear ice (Rheem).
Length of each cycle	Within the range in your manual (see the table above)	Stays in defrost much longer, or repeated long cycles (Lennox). Cycles that last longer and longer while ice spreads (Trane).
How often	More often on damp days near freezing	Seems to defrost constantly (Trane) or unusually often (Carrier)
Sounds	A whoosh at the start and end (Carrier); hissing, sizzling, or whistling on mini-splits (Mitsubishi Electric, Fujitsu, Daikin)	Banging, screeching, rattling, or grinding (Trane). Excessively loud or constant noise from the reversing valve (Carrier).
Outdoor fan	Stops during defrost, then restarts (Carrier, Trane, LG)	Stays stopped while the unit runs and ice builds (Carrier lists a defective fan motor as a cause)
Inside the house	AUX on the thermostat, brief cool air, or a mini-split that stops blowing for a few minutes	Cold air that continues after defrost ends
Water	Melt water dripping from the unit (Mitsubishi Electric, Daikin)	Water or ice collecting under the unit (Carrier says to have it drained away)

For ducted heat pumps and ductless mini-splits. Your owner's manual may give different limits for your model. Source: Carrier, Trane, Lennox, Rheem, Mitsubishi Electric, Daikin, Fujitsu AIRSTAGE, and LG pages and manuals listed in the sources

Why defrost runs too often, too long, or not enough

These are explanations, not repairs. Apart from clearing debris and fixing a dripping gutter, each is a job for a licensed technician.

• Blocked airflow. Dirty or blocked coils make frost form faster (Lennox), and so do leaves and snow (Carrier).
• Water dripping onto the unit. Water from a leaking gutter can drip onto the coil and freeze, and defrost may not clear a severe freeze-over (Carrier). Fixing the leak removes the cause.
• Sensors or controls. Lennox mentions faulty sensors or controls that misread conditions. When ice won’t go away, Trane points to a faulty defrost control or sensor, and Carrier to a malfunctioning defrost timer or controls.
• Low refrigerant (Lennox, Carrier, Trane). Testing and charging refrigerant is technician work.
• Outdoor fan motor. A defective motor means little or no airflow, which can freeze the coil (Carrier).
• Reversing valve. A gentle whoosh is normal, but loud or constant noise is a reason to call (Carrier). A stuck or failing valve can leave the system blowing cold air in heat mode (Lennox).

For scale, Trane’s homeowner repair guide gives ballpark replacement costs of $200 to $650 for a defrost control board and $1,000 to $2,300 for a reversing valve. Trane stresses that these are estimates, labor varies, and most companies quote only after a technician visits.

If thick ice stays on the outdoor unit after defrost, see heat pump freezing up in winter.

What you can safely do

• Let it finish. Give a cycle the time listed in your manual. LG specifically says not to turn the power off during defrost.
• Keep the unit clear. With the system off at the thermostat, clear heavy snow from the top and sides, and keep plants and debris at least two feet away (Trane’s repair guide, Lennox). Don’t cover the unit or set things on or next to it (Carrier).
• Give melt water somewhere to go. Rheem’s commercial guide says defrost water must drain away and snow drifts must be kept clear, or the water can build up as ice in the coil. Carrier says the unit should sit level so it drains; if the pad has settled, ask your dealer to re-level it.
• Stop drips from above. Fix any gutter or roof edge dripping onto the unit (Carrier).
• In an ice storm, heavy snow and ice on the top and the fan blade can cause vibration and damage. Carrier’s Infinity owner’s manual says it may be appropriate to turn the heat pump off and run on auxiliary heat until they clear.
• Wait a few hours before calling. If you suspect the unit is iced up, Trane suggests waiting a few hours, without doing anything, to see if it defrosts itself.
• Ask for a defrost check at the fall tune-up. Trane’s professional maintenance includes a fall check of the defrost control and an inspection of the reversing valve. Lennox says technicians clean coils and check sensors and the defrost system.