Heat Pump Defrost Mode: Why It Steams, Stops, and Blows Cooler Air
Heat Pump Defrost Mode: Why It Steams, Stops, and Blows Cooler Air
Ben Amina
Aire One East Heating & Cooling, 1910 Dundas St E #117, Whitby
Heat pump defrost mode is a normal, automatic cycle that melts the frost that builds up on your outdoor unit when it heats your home in cold, damp weather. For a few minutes the heat pump reverses itself, the outdoor fan stops, the unit may hiss or whoosh, and you may see a cloud of steam rise from it. Inside, the air from your vents can feel cooler until the cycle ends. All of that is expected in a Durham winter. What is not normal is ice that never clears, a unit encased in ice, or defrost cycles that run constantly. This guide explains how defrost works, how often it should happen, what you should and should not see, and when to call a technician. If your heat pump has stopped heating altogether, see our heat pump not working guide.
- Defrost is normal when the outdoor temperature is near or below freezing and the heat pump is heating.
- During defrost the heat pump briefly switches to cooling mode and the outdoor fan stops, so steam and a short pause are expected.
- Many systems defrost on a timer every 30, 60, or 90 minutes, while demand-defrost systems run only when frost is detected.
- Light frost that clears after each cycle is fine; thick ice that stays, or a unit covered in ice, is not.
- Keep snow and ice clear around the outdoor unit and never chip ice off the coil with a sharp tool.
Short answer: heat pump defrost mode is a short, automatic cycle that melts frost off the outdoor coil in cold weather. Steam, a brief stop of the outdoor fan, a whooshing sound, and slightly cooler air indoors for a few minutes are all normal.
Call a technician if the unit stays covered in ice, the coil never clears, the defrost runs nonstop, or your home is not staying warm. Aire One East services heat pumps across Durham Region. Call 905-576-7600.
- What is heat pump defrost mode?
- How heat pump defrost mode works
- How often does heat pump defrost mode run?
- Normal vs not normal at a glance
- When ice means heat pump defrost mode has a problem
- What you can do safely
- How to help your heat pump through winter
- Key terms in this article
- Frequently asked questions
What is heat pump defrost mode?
An air source heat pump heats your home by pulling heat out of the outdoor air through the coil in the outdoor unit. That coil gets very cold, colder than the air around it. When the outdoor temperature is near or below freezing and the air is damp, moisture freezes on the coil as frost. Frost acts like insulation, so the heat pump has to work harder and heats less efficiently. Heat pump defrost mode is the built-in fix: every so often the system stops heating for a few minutes, melts the frost away, and then goes back to heating. It is a sign the heat pump is protecting its efficiency, not a sign that something is broken.
How heat pump defrost mode works
When the control decides it is time to defrost, a reversing valve switches the heat pump into cooling mode for a short time. That sends hot refrigerant to the outdoor coil to melt the frost. At the same time, the outdoor fan turns off so the heat stays on the coil instead of being blown away. As the frost melts, water drips from the unit and some of it turns to steam in the cold air, which is why you may see a cloud rising from the outdoor unit. Because the system is briefly moving heat out of your home instead of into it, the air from your vents can feel cooler. Most systems turn on supplementary heat during heat pump defrost mode to keep that air comfortable, such as electric backup heat or, in a dual fuel system, your gas furnace. When the coil is clear, the valve switches back, the fan restarts, and normal heating resumes.
How often does heat pump defrost mode run?
It depends on the weather and the type of control. Older and simpler systems use time-temperature defrost, which can start a cycle every 30, 60, or 90 minutes when the coil is cold enough, whether or not much frost has formed. Most modern heat pumps use demand defrost, which senses frost and runs only when it is actually needed, so it is more efficient. A typical heat pump defrost mode cycle lasts a few minutes. You will usually see heat pump defrost mode more often on damp days around the freezing mark, when there is lots of moisture in the air, and less often on very cold, dry days. Cycles that run back to back, or that last much longer than usual, are worth having checked.
Normal vs not normal at a glance
| What you notice | Normal? | What to do |
|---|---|---|
| Steam rising from the outdoor unit | Yes, melting frost | Nothing |
| Outdoor fan stops for a few minutes | Yes, part of defrost | Nothing |
| Whooshing or clicking sound | Yes, reversing valve switching | Nothing |
| Cooler air from vents for a few minutes | Usually, during defrost | Nothing if it passes quickly |
| Light frost that clears after a cycle | Yes | Nothing |
| Thick ice that never clears | No | Turn heat down, call a technician |
| Unit or fan encased in ice | No | Switch to backup heat, call |
| Defrost running back to back | No | Call for a sensor or control check |
| Home not staying warm | No | Call a technician |
What to expect from heat pump defrost mode in a Durham Region winter, verified November 2026.
When ice means heat pump defrost mode has a problem
A thin layer of frost that melts away during each heat pump defrost mode cycle is normal. A heat pump that stays covered in ice, or where ice keeps growing on the top, sides, or fan, is not. Common causes include a failed defrost sensor or control board, a stuck reversing valve, low refrigerant, a weak outdoor fan motor, water dripping onto the unit from a roof or gutter above, and a unit that sits too low, so melted water refreezes around the base. Restricted airflow indoors, such as a clogged filter, can also make the outdoor coil run colder and frost faster.
What you can do safely
- Watch one heat pump defrost mode cycle. If steam rises, the fan restarts, and the coil clears within several minutes, the system is working.
- Clear snow from around and on top of the outdoor unit with a soft brush, and keep the area around it open.
- Check that water from the roof or gutters is not dripping onto the unit.
- Replace the indoor air filter if it is dirty.
- If the unit is encased in ice, set the thermostat to emergency or auxiliary heat, or let your furnace take over, and call a technician.
Never chip or pry ice off the coil with a shovel, ice pick, or screwdriver, and never pour boiling water on the unit. The thin aluminum fins and refrigerant tubes are easy to damage, and a puncture means an expensive repair.
How to help your heat pump through winter
A few simple steps keep heat pump defrost mode working the way it should. Make sure the outdoor unit sits on a stand or wall bracket high enough to stay above typical snow levels, with clear space around it so melted water can drain away. Keep gutters above the unit clear and repaired. Book a heat pump service before winter so a technician can test the sensors and controls that run heat pump defrost mode, check the refrigerant charge, and clean the coil. Our heat pump maintenance guide lists what a visit covers, and our maintenance and protection plans include it every year. If you are choosing a new system, cold climate heat pumps are designed to handle Durham winters better, and our heat pump vs furnace guide compares the options. We install and service heat pumps in Whitby, Oshawa, Pickering, and across Durham Region.
Key terms in this article
- Defrost cycle: a short, automatic cycle that melts frost off the outdoor coil.
- Reversing valve: the valve that switches a heat pump between heating and cooling, and runs the defrost.
- Demand defrost: a control that senses frost and defrosts only when needed.
- Time-temperature defrost: a control that defrosts on a set schedule when the coil is cold enough.
- Supplementary or auxiliary heat: backup heat, electric or a furnace, that keeps indoor air warm during defrost or extreme cold.
Frequently asked questions
| Your heat pump questions, answered |
|---|
| Is it normal for my heat pump to steam in winter? Yes. Steam is a normal part of heat pump defrost mode. The unit briefly switches to cooling mode to melt frost off the outdoor coil, and the warm meltwater turns to steam in the cold air. It should stop within a few minutes when the cycle ends. |
| How often should a heat pump go into defrost mode? It depends on the weather and the control. Time-temperature systems can defrost every 30, 60, or 90 minutes when the coil is cold enough, while demand-defrost systems run only when frost is detected. Defrost is most frequent on damp days around the freezing mark. |
| Why does my heat pump blow cold air in winter? During defrost, the heat pump briefly moves heat out of the house to melt frost outdoors, so the air from the vents can feel cooler for a few minutes. Most systems add backup heat during defrost. If the air stays cold for long, call a technician. |
| What should I do if my heat pump is covered in ice? Do not chip the ice off. Set the thermostat to emergency or auxiliary heat, or let your furnace take over, clear snow around the unit with a soft brush, and call a technician. A unit that stays iced may have a defrost control, sensor, refrigerant, or fan problem. |
| How long does a heat pump defrost cycle last? A typical heat pump defrost cycle lasts a few minutes, until the outdoor coil is clear. Cycles that run back to back or last much longer than usual can point to a sensor or control problem and should be checked. |
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References and further reading
