Quick answer
A heat pump’s defrost cycle is normal, and it’s how the system keeps working in winter. Every 30 to 90 minutes in cold, damp weather, the heat pump briefly reverses to melt frost off its outdoor coil. A normal defrost lasts about 5–15 minutes and ends on its own. It’s a problem only if it happens far more often, never seems to happen while ice builds up, runs much longer than 15 minutes, or leaves the outdoor unit encased in ice.
What a normal defrost cycle looks and sounds like
You may notice some or all of these, all normal:
- A loud whoosh or hiss when the reversing valve switches, at the start and end of the cycle
- The outdoor fan stops while the compressor keeps running
- Steam or “smoke” rising from the outdoor unit. It’s water vapor from melting frost, not smoke
- Water dripping and draining from under the unit
- Cooler air from your vents for a few minutes, unless backup heat kicks in
- “Aux heat” or “Defrost” showing on your thermostat
- A clunk and then normal operation when it switches back to heating
If it stops after 5–15 minutes and heating resumes, everything is working as designed.
How a heat pump defrosts
In heating mode, the outdoor coil gets colder than the outdoor air so it can absorb heat. When the coil drops below freezing, moisture in the air freezes on its fins, the same way frost forms on a cold drink can. A frosted coil can’t absorb heat well, so the heat pump periodically clears it:
- The control decides it’s time. Sensors and a timer (or smart logic) detect that frost has built up.
- The reversing valve switches. The system briefly runs in cooling mode, sending hot refrigerant to the outdoor coil. That’s the whoosh you hear.
- The outdoor fan stops, so the heat stays on the coil instead of blowing away.
- Backup heat may turn on indoors (electric strips or a furnace in a dual-fuel system) to offset the cool air the indoor coil would otherwise deliver.
- The frost melts and drains off; steam rises in cold air.
- The cycle ends when the coil reaches a set temperature or after a maximum time (often around 10–15 minutes), and the system switches back to heating.
Two kinds of defrost control
| Type | How it decides | Pros and cons |
|---|---|---|
| Time-temperature | When the coil is below freezing, defrosts on a fixed interval (often settable to 30, 60 or 90 minutes) | Simple and reliable, but may defrost when there’s little frost, wasting some energy |
| Demand (adaptive) | Uses coil and outdoor temperature sensors (sometimes pressure) to defrost only when frost has actually built up | More efficient; standard on most modern and variable-speed heat pumps |
Many inverter heat pumps and mini splits use adaptive logic that changes timing based on conditions, so the interval you see will vary from hour to hour.
How often should a heat pump defrost?
Frequency depends mostly on temperature and humidity. Surprisingly, the most frequent defrosting happens near freezing in damp weather, not on the coldest days.
| Outdoor conditions | Typical defrost frequency | Why |
|---|---|---|
| Above ~40°F | Rarely or never | Coil usually stays above freezing |
| ~25–40°F and humid, foggy, drizzly or snowing | Most often: every 30–60 minutes | Lots of moisture freezing onto the coil |
| ~25–40°F and dry | Every 60–90 minutes or less | Less moisture to freeze |
| Below ~15°F | Often less frequent | Very cold air holds little moisture |
| Freezing rain or wet snow | Frequent; may struggle to keep up | Ice forms from precipitation, not just humidity |
How long: a normal cycle runs about 5–15 minutes, usually closer to 5–10. Most controls cap it at a maximum time even if the coil hasn’t fully warmed.
A simple check: note the time when you hear the whoosh and see steam, and again at the next one. Cycles every 30–90 minutes in cold, damp weather, each lasting under 15 minutes, are normal.
Signs of a defrost problem
| What you notice | What it may mean |
|---|---|
| Thick ice builds up and the unit never seems to defrost (no whoosh, no steam, fan never pauses) | Defrost isn’t starting: failed defrost control board, faulty sensor, or a stuck reversing valve |
| Defrost every 10–20 minutes, even in dry weather | Faulty sensor or control, low refrigerant, dirty coil or restricted airflow |
| A cycle lasts well over 15 minutes, or the unit seems stuck in defrost | Sensor or control fault, or a reversing valve that won’t switch back |
| Ice remains on part of the coil after each defrost, building up over days | Bad sensor placement or failure, low refrigerant, or poor drainage refreezing at the base |
| Ice on the copper refrigerant lines | Often low refrigerant — call a technician |
| Outdoor fan keeps running during defrost | Fan relay or control problem; defrost won’t be effective |
| House gets noticeably cold during every defrost | Backup heat isn’t engaging during defrost (heat strip, breaker or thermostat setting) |
Likely causes and typical repair costs
| Cause | DIY or pro | Typical cost |
|---|---|---|
| Dirty or blocked outdoor coil | DIY clean (power off) or tune-up | $0–$200 |
| Poor drainage, meltwater refreezing under the unit | DIY: clear the base; raise the unit | $0–$400 for a riser |
| Water dripping from gutters onto the unit | DIY: fix gutters | Varies |
| Faulty defrost sensor or thermistor | Pro | $150–$400 |
| Failed defrost control board | Pro | $250–$700 |
| Stuck reversing valve or failed solenoid | Pro | $150–$450 (solenoid) to $1,000–$2,500 (valve) |
| Low refrigerant (leak) | Pro | $400–$1,500+ |
| Outdoor fan relay or motor | Pro | $150–$800 |
| Backup heat not running during defrost | Check breaker; pro for heat strips or settings | $0–$700 |
If the unit is already encased in ice, follow our step-by-step guide to safely defrosting a frozen heat pump, then watch whether normal defrost cycles resume.
How to make defrost less noticeable
- Make sure backup heat runs during defrost. Most ducted systems turn on electric strips (or the furnace, in dual-fuel) during defrost so you don’t feel a cool draft. If you do, ask a technician to check the thermostat and control settings.
- Use a heat-pump-aware thermostat. It should recognize defrost and manage auxiliary heat correctly, without leaving strips on longer than needed.
- Expect quieter defrost from inverter models. Many variable-speed heat pumps and mini splits ramp the compressor down before switching, reducing the whoosh.
- Mini splits pause the indoor fan. Ductless heads often stop blowing during defrost to avoid cold drafts. A pause of a few minutes with a blinking light is often normal; check your manual.
- Locate the outdoor unit thoughtfully at replacement time, away from bedroom windows, if the defrost noise bothers you.
How to prevent defrost problems
- Keep at least 2 feet of clearance around the outdoor unit, and clear snow after storms
- Raise the unit on a stand or bracket above typical snow depth
- Keep the base and drain openings clear so meltwater can escape
- Fix gutters or roof edges that drip onto the unit
- Rinse leaves and dirt off the coil each fall (power off)
- Replace the indoor filter every 1–3 months in winter
- Book a fall tune-up that tests the defrost board, sensors, reversing valve and refrigerant charge
Frequently asked questions
How long should a heat pump defrost cycle last?
Usually 5–15 minutes, often 5–10. Most controls end it once the coil warms up or after a maximum time.
How often should a heat pump go into defrost?
Typically every 30–90 minutes when it’s cold and damp, and less often in dry or very cold weather. Defrosting more often than every 20–30 minutes in dry conditions is worth having checked.
Why is steam coming out of my heat pump?
That’s water vapor from frost melting off the warm coil during defrost. It’s normal and not smoke.
Is it normal for my heat pump to blow cold air during defrost?
Briefly, yes, if your system doesn’t run backup heat during defrost. Many systems do turn on backup heat to prevent it; if you feel cold air every cycle, have the settings checked.
Can I force my heat pump to defrost?
Some thermostats and outdoor control boards have a test or forced-defrost function, usually meant for technicians. Homeowners shouldn’t open the outdoor unit’s control panel. If ice is building up, turn the system off and follow safe de-icing steps instead.
What happens if a heat pump doesn’t defrost?
Ice builds up, airflow through the coil drops, heating output falls, and the system works harder and leans on backup heat. Left long enough, ice can damage the fan and strain the compressor.
Related guides
- Heat Pump Frozen Outside: How to Defrost It Safely
- Heat Pump Making Loud Noises: Banging, Buzzing and Whooshing Explained
- Heat Pump Blowing Cold Air in Heat Mode: 9 Causes and Fixes
- Heat Pump Running Constantly in Winter: Normal or a Problem?
- Emergency Heat on a Heat Pump: When to Use It and What It Costs
- Do Heat Pumps Work Below 0°F? Data From Cold-Climate Models
This guide is general information. Turn off power before inspecting the outdoor unit, and leave electrical, refrigerant and control repairs to a licensed HVAC technician. Repair prices are approximate 2026 national ranges.