Quick answer
A heat pump defrost cycle is a short, automatic reversal of the system that sends warm refrigerant to the outdoor coil to melt frost that builds up while the heat pump is heating in cold, damp weather.
It usually lasts a few minutes. The outdoor fan stops, the unit may hiss and steam, and backup heat may come on indoors. All of that is normal.
Quick facts
The key facts about Heat Pump Defrost Cycle, with sources:
- Why it happens
- The outdoor coil runs below freezing and collects frost while heating
- How it works
- The reversing valve switches the system to cooling mode briefly 1
- One maker's start conditions
- Outdoor below 50°F and coil below about 32°F after a set run time 2
- One maker's timer options
- 30, 60, 90 or 120 minutes of run time between checks 2
- One maker's end point
- Coil reaches about 65°F 2
- DOE advice
- Choose a heat pump with demand-defrost control 1
- Normal signs
- Fan stops, hissing, steam, a whoosh when it switches back
Key takeaways
- Frost on the outdoor coil in winter is normal; the defrost cycle clears it.
- During defrost the system runs briefly in cooling mode using its reversing valve 1.
- Steam, a stopped fan and hissing during defrost are normal.
- DOE recommends demand-defrost controls to cut unneeded cycles 1.
- A coil that stays caked in ice is not normal; call a technician.
What the defrost cycle is
In heating mode, a heat pump pulls heat from outdoor air through its outdoor coil. To do that, the coil must be colder than the air. On cold, humid days the coil drops below freezing, and moisture in the air freezes onto the fins as frost. Frost blocks airflow and cuts heating output.
The fix is built in. DOE explains that a reversing valve changes the direction of refrigerant flow "for cooling and for the winter defrost cycle" 1. For a few minutes, hot refrigerant goes to the outdoor coil and melts the frost. The outdoor fan usually stops so the heat stays on the coil.
Indoors, the system is briefly moving heat the wrong way, so many systems turn on backup heat to avoid blowing cool air. When the coil is clear, the valve switches back and normal heating resumes.
Sources: [1]
One defrost cycle, step by step
- Heating mode → outdoor coil below freezing → frost builds on fins
- Control sees cold coil and enough run time → defrost starts
- Reversing valve shifts → hot gas to outdoor coil; outdoor fan off; backup heat may come on indoors
- Frost melts → water and steam → coil temperature rises to the end point → defrost ends
- Valve shifts back (often a loud whoosh) → fan restarts → heating resumes
How one manufacturer's control decides
Example of real settings, from the wiring notes for a Carrier 25HNB6 two-stage heat pump 2:
- Defrost is only performed at outdoor temperatures below 50°F.
- It starts when the selected run time has passed and the coil is below about 32°F (±2°F).
- The installer can choose 30, 60, 90 or 120 minutes between checks.
- Defrost ends when the coil reaches about 65°F (±5°F).
- After defrost, the outdoor fan waits 20 seconds before starting.
Other makers use different numbers, and many newer systems use demand defrost, which watches sensors and defrosts only when frost is actually building. DOE advises choosing a heat pump with demand-defrost control 1.
Types of defrost control
| Type | How it decides | Trade-off |
|---|---|---|
| Time-temperature | Checks the coil every set interval of run time 2 | Simple; may defrost when not needed |
| Demand defrost | Uses coil and outdoor temperature sensors to detect frost build-up | Fewer unneeded cycles; DOE recommends it 1 |
| Adaptive / variable-speed | Board learns timing; compressor speed changes | Common on inverter and cold-climate units |
| Forced (service) defrost | Technician triggers it to test the system 2 | Service use only |
Example: how often defrost can run on a cold, damp day
Example only, using the Carrier timer settings above 2. Assume 30°F outdoors with fog, and the heat pump running 75% of each hour. A time-temperature control defrosts after each set interval of run time if the coil is cold enough.
| Timer setting | Run time per day | Most defrosts per day |
|---|---|---|
| 30 minutes | 18 hours (75% of 24) | Up to 36 |
| 60 minutes | 18 hours | Up to 18 |
| 90 minutes | 18 hours | Up to 12 |
| 120 minutes | 18 hours | Up to 9 |
Each cycle costs energy, both the reversed heat and any backup heat. That is why DOE recommends demand defrost, which "will minimize defrost cycles, thereby reducing supplementary and heat pump energy use" 1. A technician picks the timer setting for your climate; it is not an owner adjustment.
Where you will see the term
- On the thermostat: "defrost" or "aux heat" shown during a cycle.
- On the outdoor unit's control board: defrost timer pins and fault lights 2.
- In heat pump specs: "demand defrost."
- On service invoices: "defrost board," "defrost sensor" or "reversing valve."
Why defrost is needed, and its cost
What defrost does for you
- Restores airflow and heating output.
- Protects the outdoor coil from heavy ice.
- Runs automatically; no owner action needed.
What it costs
- Heating pauses for a few minutes.
- Backup heat may run, raising energy use.
- Steam and noise can alarm people who do not expect it.
- Too-frequent cycles waste energy 1.
What defrost cannot handle
Defrost clears frost from the coil; it cannot clear snow piled around the unit, ice dripping onto it from a roof, or water that refreezes in a pan below it. DOE notes outdoor units may need to be elevated due to snow and protected from high winds, which "can cause defrosting problems" 1.
It also cannot fix a fault. A bad sensor, a failed defrost board, a stuck reversing valve or low refrigerant can leave the coil iced. Then the heat pump loses output and may lean on backup heat all day.
Cost drivers
HyreHVAC lists no repair prices without enough dated quotes. Defrost repairs range from a sensor or board to a reversing valve, which involves refrigerant work. Running cost depends on how often defrost runs and how much backup heat comes on.
Compare heat pump running costs with the heat pump savings calculator, and see heat pump repair for what a defrost repair involves.
How a technician checks defrost
- Look at frost patterns on the coil; uneven frost can point to low charge or airflow.
- Check outdoor and coil sensors against a thermometer.
- Trigger a forced defrost to test the board and valve, as the maker describes 2.
- Confirm the reversing valve shifts fully and the fan stops and restarts.
- Check refrigerant charge if frost returns quickly. Refrigerant work requires EPA 608 certification 5.
- Set the defrost timer to suit the climate, and check controls and the start-up cycle, which ENERGY STAR lists in every check-up 4.
What owners can do
- Keep snow cleared around and on top of the outdoor unit, with power off for anything near the fan.
- Keep gutters and roof runoff from dripping onto the unit.
- Make sure the unit sits on a raised stand where snow is common 1.
- Change the indoor filter; low indoor airflow affects the whole cycle.
- Never chip ice off the coil or pour hot water on it.
Normal vs not normal
- Normal: steam rising from the unit, fan stopped, hissing, a whoosh at the end, a few minutes of cool air indoors on systems without backup heat.
- Not normal: coil fully encased in ice for hours.
- Not normal: defrost every few minutes all day.
- Not normal: backup heat on constantly in mild weather.
- Not normal: loud grinding or banging, not just a whoosh.
Ratings and rules
No federal rule governs defrost controls on their own. Heat pump efficiency is regulated through DOE's ratings, which moved to SEER2 and HSPF2 in January 2023 to better reflect real duct systems 1. Defrost energy is one reason real-world winter performance varies.
ENERGY STAR's cold-climate heat pump criteria require third-party verified performance testing down to 5°F 3, which is where defrost behavior matters most. See cold-climate heat pump performance.
Defrost vs related terms
| Term | How it relates |
|---|---|
| Defrost cycle | Clears frost from the outdoor coil in heating |
| Frozen evaporator coil | Ice on the indoor coil in cooling; never normal |
| Auxiliary heat | Backup heat that may run during defrost |
| Cold-climate heat pump | Built to heat at low temperatures; still defrosts |
| HSPF2 | Heating efficiency rating used since January 2023 1 |
Common misconceptions
- Myth Steam from the unit means it is on fire or broken.
- Reality It is melting frost; it is normal during defrost.
- Myth Any frost on the coil is a problem.
- Reality Light frost between cycles is normal; heavy ice that will not clear is not.
- Myth Heat pumps do not work in the cold because of defrost.
- Reality Modern units, including ENERGY STAR cold-climate models tested at 5°F, heat well with defrost 3.
- Myth Covering the unit stops frost.
- Reality It blocks airflow and stops it heating.
Climate and defrost
Defrost runs most in cold, damp climates, around freezing with high humidity, such as the Pacific Northwest and the Ohio Valley. Very cold, dry air frosts coils less. Snowy areas need raised stands and clear space around the unit 1.
When it matters to you
It matters if you have or are buying a heat pump in a cold or damp climate. When buying, ask whether it has demand defrost, as DOE advises 1. If yours stays iced, call a technician.
Next step: compare cold-weather options in best cold-climate heat pumps, and check costs with the heat pump vs furnace tool.
Questions about Heat Pump Defrost Cycle
Why is my heat pump steaming?
It is most likely in defrost. The system sends warm refrigerant to the outdoor coil to melt frost, and the melting frost gives off steam. The outdoor fan usually stops during this time. It is normal and lasts a few minutes.
How long does a heat pump defrost cycle last?
Usually a few minutes. It ends when the coil warms to the control's end point; on one Carrier model that is about 65°F. If the unit stays in defrost or stays iced for a long time, call a technician.
How often should a heat pump go into defrost?
It depends on weather and the control. Time-temperature controls check after a set run time, such as 30, 60, 90 or 120 minutes on one Carrier model. Demand-defrost controls run only when frost is building, often less often. Cold, damp weather triggers the most cycles.
Is it normal for my heat pump to blow cold air in winter?
Briefly, during defrost, yes, on systems that do not turn on backup heat. If cool air lasts long, or happens without the outdoor unit defrosting, have a technician check the system.
Why is my heat pump covered in ice?
Light frost is normal and should clear in defrost. Heavy ice that does not clear can come from a failed defrost sensor or board, a stuck reversing valve, low refrigerant, blocked airflow, or water dripping onto the unit. Call a technician.
What is demand defrost?
It is a control that uses sensors to start defrost only when frost is actually building, rather than on a fixed timer. DOE recommends choosing a heat pump with demand-defrost control to minimize defrost cycles and energy use.
Should I pour hot water on my frozen heat pump?
No. It can refreeze, damage fins and send water into electrical parts. Turn the system to emergency heat if needed and call a technician. Keep snow and roof runoff away from the unit.
Why does my heat pump make a loud whoosh?
That sound is usually the reversing valve shifting at the start or end of defrost as refrigerant pressures equalize. It is normal. Grinding, banging or screeching is not.
Does defrost use a lot of energy?
Each cycle uses some, both for the reversed heat and any backup heat. Too-frequent cycles waste energy, which is why demand defrost and correct timer settings matter. Over a winter, heat pumps still usually deliver more heat than they use.
Sources
- U.S. DOE Energy Saver, Air-Source Heat Pumps (archived copy, 2025), retrieved .
- Carrier, 25HNB6 Infinity 16 Two-Stage Heat Pump wiring diagram and defrost notes, retrieved .
- ENERGY STAR, Air Source Heat Pumps, retrieved .
- ENERGY STAR, Maintenance Checklist (heating and cooling), retrieved .
- U.S. EPA, Stationary Refrigeration and Air Conditioning (Section 608), retrieved .
Expert review
Written by the HyreHVAC Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Defrost start, timer and end settings are copied from one Carrier model's wiring notes and labelled as such; demand-defrost and siting advice is DOE's, from an archived Energy Saver page. The frequency example multiplies the published timer options by an illustrative run time.
Suggest a correction. We fix errors and say what changed.