A heat pump stuck in defrost mode is one that stays in its defrost cycle far longer than a normal one, or keeps starting new cycles back to back. Trane says a normal cycle usually lasts 3 to 5 minutes (Trane, February 2026), and the Consortium for Energy Efficiency (CEE) says 5 to 15 minutes (CEE, January 2024). A cycle that runs past 15 minutes is outside every range we found.
This guide answers the question most owners search first: how long does a heat pump defrost last, and how do you tell a normal cycle from a heat pump stuck in defrost mode? It shows what a normal cycle looks and sounds like, how to time your own unit, the usual causes and when to call a technician. For how defrost works and why units freeze, read our heat pump frozen guide. More fixes are in our heat pump troubleshooting section.
A heat pump in defrost mode normally stops its outdoor fan, steams for a few minutes, then restarts. Trane says that usually takes 3 to 5 minutes, and the CEE owner guide says 5 to 15 minutes. A heat pump stuck in defrost mode stays there longer than 15 minutes, never seems to finish, or still has thick ice afterward. Call a licensed HVAC technician if yours does. Time it with a phone stopwatch first.
Typical defrost, Trane
3–5 min
Trane’s usual cycle length; the unit then returns to heating in a few minutes (Trane, February 2026)
Typical defrost, CEE
5–15 min
Range from the owner guide; stuck in defrost means call a professional (CEE, January 2024)
Time between cycles
30–90 min
How often Lennox says a heat pump should defrost itself (Lennox, undated page, checked October 2026)
In this guide
- What a heat pump in defrost mode looks and sounds like
- How long does a heat pump defrost last?
- How often does a heat pump go into defrost mode?
- Is your heat pump stuck in defrost mode? Time it
- Signs of a heat pump stuck in defrost mode
- What is usually behind a heat pump stuck in defrost mode
- What to do while it is in defrost mode
- When to call a technician
- What a service call costs
- Frequently asked questions
What a heat pump in defrost mode looks and sounds like
In winter the outdoor coil gets colder than the air, so frost builds on it. To clear it, the heat pump reverses the flow of refrigerant for a few minutes and warms the outdoor coil. That is defrost mode, and it is normal. Trane lists what you may notice: a cloud of steam from the outdoor unit, a swooshing sound as the refrigerant reverses, the outdoor fan stopping, and the compressor noise rising or falling (Trane, February 2026).
Inside the house you may notice little or nothing. Trane says that the thermostat can show AUX, which means the electric heat strips are running, and that homes without backup heat may feel cool air from the vents for a few minutes. The CEE owner guide adds that on a ductless system the indoor unit may put off some cooling while the outdoor coil is warmed (CEE, January 2024).
If what you see matches that picture, your heat pump in defrost mode is doing its job. A heat pump stuck in defrost mode looks different: the fan stays off, the steam does not stop or the ice keeps growing. Our guide to heat pump steam in winter explains how to tell harmless steam from smoke.
How long does a heat pump defrost last?
Most published answers fall between 2 and 15 minutes. Trane says its cycles are usually 3 to 5 minutes (Trane, February 2026). The CEE owner guide says defrost typically takes 5 to 15 minutes (CEE, January 2024). PickHVAC, a contractor resource, gives 2 to 10 minutes and says some models allow up to 15 in heavy frost (PickHVAC, August 2025).
The answers differ because heat pumps differ. HVAC School, a technician training site, explains that defrost ends when a coil sensor reaches a set temperature, and that many boards also stop defrost after a time limit, such as about 10 minutes on the Carrier board it uses as an example. It adds that the limit can be adjusted on universal boards but is built into many original-equipment boards (HVAC School, November 2024). Your model’s manual is the final word on your own unit.
So how long does a heat pump defrost last in practice? Plan on a few minutes, most often under 10, with up to 15 as the top of the published ranges. We found no source that gives a single figure that fits every brand, so a heat pump stuck in defrost mode is best judged against your own unit’s normal.
How often does a heat pump go into defrost mode?
Lennox says a heat pump should automatically defrost itself every 30 to 90 minutes (Lennox, undated page). That fits the way timed controls work. HVAC School says that on time-and-temperature defrost the unit must run for a set interval first, usually 30, 60 or 90 minutes. On demand defrost, the unit starts a cycle only when the temperature difference between two sensors passes a set point, and the longest wait between cycles is commonly six hours (HVAC School, November 2024).
Trane’s systems use demand defrost, which Trane says means fewer defrost cycles (Trane, February 2026). PickHVAC says frequent cycles in mild, dry weather are a reason to call for service (PickHVAC, August 2025). In other words, a heat pump in defrost mode often is not automatically a fault, and frequent cycles alone do not make a heat pump stuck in defrost mode. Frequent cycles in mild, dry weather deserve a second look. If you are also seeing the unit run nonstop, read why a heat pump runs constantly in cold weather.
Is your heat pump stuck in defrost mode? Time it
The simplest test for a heat pump stuck in defrost mode is a stopwatch. No source gives an owner test, so the method below is our own. It only uses the sources’ cycle lengths as the yardstick.
- Pick a cold, damp day and a spot where you can see the outdoor unit safely, such as a window. Do not open any panel or reach near the fan guard.
- Start the stopwatch when the outdoor fan stops. Steam usually rises at about the same time.
- Stop it when the fan restarts and the steam fades. That is one cycle.
- Write down three cycles and the time between them, then compare them with the chart below.
- Look at the ice about two hours later. This is our rule of thumb, not a published figure: if the ice is the same or bigger, the defrost is not keeping up.
Here is where the bands come from. About 10 minutes is the top of PickHVAC’s usual range and the time limit on some control boards (HVAC School). Fifteen minutes is the top of every range we found, from CEE and from PickHVAC in heavy frost. One 12-minute cycle in a snowstorm is not a fault. A 40-minute cycle, or one that never ends, is outside all of the published ranges.
Signs of a heat pump stuck in defrost mode
A heat pump stuck in defrost mode rarely announces itself. The sources describe these signs of a heat pump in defrost mode that is not finishing:
- Defrost keeps starting. Trane lists constantly switching into defrost mode and cycles that last longer and longer as warning signs (Trane, February 2026).
- Ice stays or builds. Trane also lists frost and ice starting to cover the unit. PickHVAC says to get service if the unit never clears ice, or if cycles run back to back with little melting (PickHVAC, August 2025).
- The cycle never ends. Lennox lists a defrost cycle that never kicks in after several hours of running as a reason to call, and the CEE owner guide says to consult a professional if the unit is stuck in defrost or covered in ice (CEE, January 2024).
- Steam does not stop. PickHVAC says continuous steaming, or thick ice after defrost, warrants troubleshooting.
- Cool air indoors that does not go away. A few minutes of cool air is normal during defrost (Trane). If the house stays cold, see heat pump not heating.
Short version
Under 10 minutes and the fan restarts: normal. 10 to 15 minutes: note it and watch. Over 15 minutes, back-to-back cycles or ice that stays: call a technician.
What is usually behind a heat pump stuck in defrost mode
We found no manufacturer page that ranks the causes of a unit staying in defrost mode. Most pages describe the related failure where defrost does not work and ice builds up. The table lists the parts and conditions the sources name. Our heat pump frozen guide covers the full list of causes and repair costs.
| Possible cause | What the sources say | Who handles it |
|---|---|---|
| Snow, ice or debris around the unit | Lennox says to clear at least two feet on all sides, and that heavy ice blocking airflow signals a problem. CEE says to clear obstructions through the year. | You, with a soft broom |
| Coil temperature sensor or its wiring | HVAC School (November 2024) says sensor problems usually involve damaged conductors, shorts, loose wiring or poor contact on the coil. | Licensed HVAC technician |
| Defrost board, timer or control | Lennox says failed defrost controls, timers or sensors can let ice build up. Some boards end defrost on a time limit (HVAC School). | Licensed HVAC technician |
| Reversing valve | Defrost works by reversing the refrigerant, which is the swoosh Trane describes. A valve that does not reverse is one more part a technician checks (our reading; no source ranks it). | Licensed HVAC technician |
| Low refrigerant or a leak | Our frozen heat pump guide explains how a leak can make the coil frost faster than defrost can clear it. Refrigerant work is not an owner job. | EPA Section 608 certified technician |
| Outdoor fan not spinning | Lennox says to call a technician if the fan is not spinning even after the ice melts. | Licensed HVAC technician; licensed electrician for wiring |
Only the first row is an owner job. Everything else involves controls, wiring or refrigerant, and needs a professional.
What to do while it is in defrost mode
If the cycle is a normal length, do nothing. Let it finish. While a heat pump stuck in defrost mode waits for a technician, keep to the safe basics:
- Clear at least two feet around the unit with a soft broom and check the indoor air filter. Both are on Lennox’s list of immediate steps.
- Do not chip the ice or pour water on it. Lennox says hot water can crack components and tools can damage the coils, fins and fan blades.
- Keep the meltwater draining, so it does not pool and refreeze on a walkway (CEE, January 2024).
- Switch off the outdoor disconnect before going near the fan guard. This is our precaution. If you cannot find the disconnect, leave the unit alone.
If ice has built up over hours rather than minutes, switch to our step-by-step guide on how to defrost a frozen heat pump. If your thermostat shows AUX or you are deciding about Emergency heat, our emergency heat vs auxiliary heat guide explains the difference.
When to call a technician
Call a licensed HVAC technician for a heat pump stuck in defrost mode, meaning any of these (Lennox, undated page; Trane, February 2026):
- A defrost cycle runs longer than 15 minutes, or never seems to end (our band; CEE says to consult a professional if the unit is stuck in defrost).
- Cycles are getting longer, or the unit is constantly switching into defrost (Trane).
- Ice is still there 6 to 8 hours later, or it refreezes quickly after thawing (Lennox).
- The outdoor fan does not spin, even after the ice melts (Lennox).
- You hear grinding, squealing or banging (Lennox).
Anything involving refrigerant needs a technician with EPA Section 608 certification (U.S. EPA), and wiring problems need a licensed electrician. If you smell burning, switch the system off at the thermostat and the outdoor disconnect. PickHVAC says to call a professional right away for a burning smell. If you see smoke or flames, call 911.
What a service call costs
A visit for a heat pump stuck in defrost mode usually starts with a flat call-out fee. Fixr puts it at $75 to $250, which often covers diagnosis, with time past the first hour at about $75 to $150 an hour. Fixr’s average heat pump repair is about $400 (Fixr, January 2026). The final price depends on the part. See the repair cost table in our heat pump frozen guide.
Frequently asked questions
How long should a heat pump be in defrost mode?
A heat pump in defrost mode usually finishes in a few minutes. Trane says its defrost cycles usually last 3 to 5 minutes, and the CEE owner guide says 5 to 15 minutes. PickHVAC gives 2 to 10 minutes, up to 15 in heavy frost. Longer than 15 minutes is outside every range we found, so call a technician.
How often should a heat pump go into defrost mode?
Lennox says a heat pump should defrost itself every 30 to 90 minutes. HVAC School says timed controls use intervals of 30, 60 or 90 minutes, while demand controls start a cycle only when sensors see frost, with six hours between cycles common as a maximum.
Why is my heat pump blowing cold air in defrost mode?
During defrost the unit reverses the refrigerant to warm the outdoor coil, so homes without backup heat can feel cool air from the vents for a few minutes (Trane, February 2026). If the cool air lasts beyond the cycle, read our heat pump not heating guide.
Is steam coming off my heat pump normal?
Yes, during defrost. Trane says a cloud of steam from the outdoor unit is normal as the ice melts and evaporates. Steam that never stops, or steam with a burning smell, is not normal: PickHVAC says to shut the system off and call a professional right away for a burning smell.
Can I turn off a heat pump that is stuck in defrost mode?
If the cycle has only run a few minutes, let it finish. If the unit is covered in ice, Lennox’s first step is to turn the heat pump off at the thermostat. Then follow our guide on how to defrost a frozen heat pump and call a technician if the ice stays 6 to 8 hours.
What does a heat pump stuck in defrost mode look like?
The outdoor fan stays off and steam keeps rising well past the usual few minutes, or the unit keeps starting new cycles while ice builds up. Trane lists constant defrost cycles, cycles that grow longer and ice covering the unit as warning signs (Trane, February 2026).
Why does the outdoor fan stop during defrost?
Trane says the outdoor fan may stop in defrost mode. Our reading is that the unit is warming the coil rather than pulling in cold air. It should restart when the cycle ends. A fan that does not spin even after the ice melts needs a technician, according to Lennox.
Sources
- Trane: What is a heat pump defrost cycle? (February 2026)
- Consortium for Energy Efficiency: You Installed an Air Source Heat Pump. Now What? (January 2024)
- Lennox: Heat pump frozen (undated, checked October 2026)
- HVAC School: Heat Pump Defrost Troubleshooting Tips (November 2024)
- PickHVAC: Heat Pump Defrost Mode: How It Works, What’s Normal, and Fixes (August 2025)
- Fixr: Heat Pump Repair Cost (January 2026)
- U.S. EPA: Section 608 Technician Certification
About this guide
Published by HomeHeatAdvisor. This guide is based on manufacturer, efficiency-program and industry sources, checked in October 2026; each figure names its source and date, and our own readings are labeled as ours. Use a licensed HVAC technician for anything beyond the owner checks above. Our editorial policy explains how we research, fact-check and update our guides. Spotted an error? Let us know. More troubleshooting guides.