Heat pump defrost cycles
In cold, damp weather a heat pump periodically reverses for several minutes to melt frost off the outdoor coil, and steam, a whoosh and a stopped outdoor fan are normal parts of that.
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Contents
When an air-source heat pump heats in cold, humid weather, frost forms on the outdoor coil. To clear it, the heat pump runs a defrost cycle: it briefly switches to cooling mode so hot refrigerant flows through the outdoor coil and melts the frost.Source 1 This is normal.
Why frost forms
In heating mode the outdoor coil is colder than the outdoor air. NRCan explains that when the outdoor temperature is near or below freezing, "moisture in the air passing over the outside coil will condense and freeze on it," and "the amount of frost buildup depends on the outdoor temperature and the amount of moisture in the air."Source 1 Lennox says frost forms "when temperatures are below 45°F (7°C)."Source 2 Frost reduces the coil's ability to pick up heat, so it has to be removed.
What happens during defrost
NRCan describes the common approach: "the reversing valve switches the device to the cooling mode. This sends hot gas to the outdoor coil to melt the frost. At the same time the outdoor fan... is shut off."Source 1 Mitsubishi's service manual for its MSZ-FS wall units describes the sequence: "The compressor stops once, the indoor/outdoor fans stop, the 4-way valve reverses, and the compressor re-starts."Source 6
What you may notice:
| What you see or hear | Normal? | Source |
|---|---|---|
| Steam or fog rising from the outdoor unit | Yes | "you may notice steam rising from the outdoor unit. This is a normal occurrence."Source 2 |
| A "whoosh" at the start and end | Yes | "At the beginning and end of the defrost cycle, you may hear a 'whoosh' sound."Source 3 |
| Outdoor fan stops while the compressor runs | Yes | "the outdoor fan will stop, and the compressor will continue to run."Source 4 |
| Indoor ductless unit stops blowing or runs very low | Yes | "when defrosting is being done, the indoor fan will stop or rotate in Very Low speed."Source 6 |
| Water dripping or running from the outdoor unit | Yes | Melted frost has to drain; see What a good heat pump installation looks like |
| Backup heat comes on briefly (ducted) | Can be | "The thermostat may engage auxiliary heat during the defrost cycle."Source 2 |
Mitsubishi's operation manual for one wall unit puts it plainly: "The operation stops for about 10 minutes in the heating operation. Outdoor unit is in defrost. Since this is completed in max. 10 minutes, please wait."Source 5 It describes the vapor as looking "like white smoke."Source 5
How long and how often
Duration and frequency depend on the product, its control method and the weather.
- Duration. Lennox's guide says its control "activates a defrost cycle lasting 5 to 15 minutes at preset intervals."Source 2 Mitsubishi's MSZ-DM manual says a maximum of 10 minutes.Source 5
- Timed defrost. NRCan: "Time-temperature defrost is started and ended by a pre-set interval timer or a temperature sensor located on the outside coil. The cycle can be initiated every 30, 60 or 90 minutes, depending on the climate and the design of the system."Source 1 Rheem's manual lists the same 30, 60 and 90 minute options with a 30 minute factory setting, and suggests shorter intervals in moist climates and longer in dry ones.Source 4
- Demand defrost. "Demand-frost controls monitor airflow, refrigerant pressure, air or coil temperature and pressure differential across the outdoor coil to detect frost accumulation." NRCan says demand defrost "is generally more efficient since it starts the defrost cycle only when it is required."Source 1
How often defrost is needed depends on humidity as well as temperature, since frost buildup depends on both.Source 1
Why the house may feel cooler briefly
During defrost the heat pump is running in cooling mode. NRCan notes that "while this is happening, the heat pump is cooling the air in the ductwork," which the backup heat would normally offset.Source 1 NRCan also says you can have the installer set the indoor fan to turn off during defrost to reduce energy use, though "defrost may take a little longer with this set up."Source 1
When defrost is not normal
A light layer of frost that clears in each cycle is expected. Lennox's guidance: "A light layer of frost on the outdoor coils is completely normal... heavy ice buildup that covers the entire unit or blocks airflow signals a problem."Source 7 If ice is building up and not clearing, see Ice on the outdoor unit.
Sources
- 1.Heating and cooling with a heat pump. Natural Resources Canada. Accessed Oct 25, 2026
- 2.Heat Pump System Information, User Guide (507657-01). Lennox. Accessed Oct 25, 2026
- 3.25VNA Owner's Manual (OM25VNA-02). Carrier. Accessed Oct 25, 2026
- 4.Outdoor Heat Pump User's Information Manual (708163B). Rheem. Accessed Oct 25, 2026
- 5.MSZ-DM25VA, MSZ-DM35VA Operation Manual (JG79Y073H01). Mitsubishi Electric. Accessed Oct 25, 2026
- 6.Service Manual MSZ-FS06NA to MSZ-FS18NA (OBH872A). Mitsubishi Electric. Accessed Oct 25, 2026
- 7.What to Do If Your Heat Pump Is Frozen. Lennox. Accessed Oct 25, 2026
More in Living with a heat pump
- Heat pump backup heat and lockout temperaturesAux heat supplements the heat pump automatically, emergency heat replaces it by hand, and outdoor lockout settings decide at what temperatures each one is allowed to run.
- Heat pump filters and maintenanceCheck filters monthly and change or clean them at least every three months (ductless filters more often), keep the outdoor unit clear, and have a technician service the system once a year.
- Heat pump noiseOutdoor heat pump sound is rated in A-weighted decibels under AHRI Standard 270, variable-speed units are quieter at low speed and louder at full speed, and placement affects what you hear.
- Heat pump thermostat settings and setbacksWith a heat pump, keep setbacks small or use a thermostat with heat pump recovery logic, because a deep setback can make the system recover on backup heat.