By Jake Lawson
When the temperatures start to drop and winter sets in, I get a staggering number of calls from homeowners who are absolutely convinced that something is wrong with their heat pump. They’ll tell me the outdoor unit is covered in frost, or they’ve seen what appears to be steam coming out of the top of the equipment or they’ve noticed the system suddenly stop blowing warm air for a few minutes. Some are even concerned that their brand-new heat pump already has a serious problem.
The interesting part is that, in many cases, the system is operating exactly as it was designed to.
A heat pump keeps working through the winter too, pulling heat from the outdoor air even when the temperatures feel uncomfortably cold, and that’s one of the biggest differences between a heat pump and a traditional air conditioner. While that’s a great piece of engineering, it also presents a unique challenge that homeowners don’t usually have to deal with when they use conventional cooling systems. Frost naturally forms on the outdoor coil under the right weather conditions . If you let that frost build up, it will eventually reduce airflow, it will reduce the efficiency of heating and it will be harder for the system to pick up heat from the outside air.
That's where the defrost cycle comes in.
When homeowners understand why defrost cycles happen, I find they are much less concerned the next time they see steam rising from their outdoor unit or hear the equipment temporarily change the way it operates. They don’t assume something has failed, but rather that the heat pump is in fact protecting itself and maintaining its performance.
Today’s most high-efficiency heat pumps, like the Goodman GLZS4BA3010 2.5-ton heat pump we featured in our pillar guide, have sophisticated controls that automatically monitor outdoor conditions and only initiate defrost cycles when necessary. The whole process is designed to take as little input from the homeowner as possible.
According to the U.S. Department of Energy, modern heat pumps are capable of providing efficient heating even in cold climates, and automatic defrost systems are an important part of maintaining reliable winter performance. U.S. Department of Energy – Heat Pump Systems
In this guide, I'd like to explain why frost develops in the first place, how modern heat pumps know when to begin a defrost cycle, what actually happens during the process, and why many of the things homeowners worry about are completely normal.
❄️ Why Does Frost Form on a Heat Pump?
One of the first things homeowners ask me is simple:
"If the system is heating my house, why is there ice on the outdoor unit?"
It's a great question because it seems backwards at first.
In the summer, your outside condenser pushes heat out into the outside air, so the outside coil gets warm. But in winter, a heat pump reverses the refrigeration cycle. It is not pushing heat out, it is trapping heat outside and bringing it in your house.
Which means the outdoor coil gets much colder than the air around it.
When saturated outdoor air comes in contact with a surface that is below freezing, the water vapour will naturally condense and freeze. This causes a light coating of frost to build up on portions of the outdoor coil.
In many winter conditions, especially when temperatures are around 30° to 40° F (-1° to 4° C) with relatively high humidity, frost can build up surprisingly quickly. Ironically, it is these moderate winter days that often produce more frost than the bitterly cold, dry ones, because there is more moisture available in the air.
I've had homeowners panic when they see a thin white coating covering the outdoor coil thinking the equipment has frozen solid. And often a light coating of frost is an indication that the heat pump is doing exactly what it is designed to do, which is extracting heat from the outside air.
The key difference is between normal frost and heavy ice build-up. Cold weather operation is expected to produce a thin frost. Thick ice covering the entire unit or blocking airflow can indicate the defrost system isn’t working correctly and will need to be inspected by a qualified HVAC technician.
🌨️ Why Can't the Heat Pump Just Keep Running?
Another question I hear regularly is:
"If frost is normal, why not simply let the system continue running?"
The answer comes down to efficiency.
A heat pump draws out the available heat from the outdoor air flowing over its coil. As frost builds up, it acts as insulation and reduces the amount of outdoor air that gets to the coil, making heat transfer more difficult.
Imagine trying to breathe through a scarf that gets thicker every few minutes. Airflow is not changed much initially. But the more layers pile up the more difficult it is to breathe. The same principle applies to the outdoor coil of a heat pump.
As the frost thickens, the system must work harder to extract the same amount of heat. If not addressed, heavy ice buildup can lead to decreased heating capacity, increased energy consumption, added strain on system components and ultimately disrupt normal operation.
That is why modern heat pumps do not just ignore frost.
Rather, they periodically remove it through an automatic defrost cycle before it becomes a major problem.
🧠 How Does the Heat Pump Know When to Defrost?
Many homeowners consider the heat pump to be on a timer, going into defrost every hour, regardless. That may have been so for some older equipment, but today's equipment is a lot smarter. Newer heat pumps have sensors throughout the system that continuously measure operating conditions. Depending on the manufacturer and equipment design, these controls may evaluate factors such as:
- 🌡️ Outdoor coil temperature
- 🌬️ Outdoor air temperature
- ⏱️ Compressor operating time
- ❄️ Temperature differences across the coil
- 📊 Overall system performance
Instead of initiating defrost on a fixed schedule, many systems determine whether frost is actually affecting performance. If little or no frost has developed, the heat pump simply continues heating your home without interruption.
This intelligent approach helps minimize unnecessary defrost cycles while maintaining efficient winter operation. Organizations like the Air-Conditioning, Heating, and Refrigeration Institute emphasize that properly matched and certified heat pump systems are engineered to maintain reliable operation across a wide range of outdoor temperatures, including automatic defrost functions. AHRI Directory of Certified Product Performance
🔄 What Actually Happens During a Defrost Cycle?
This is the part that most homeowners are surprised by. To melt frost from the outdoor coil, the heat pump temporarily reverses its refrigeration cycle.For a few minutes the system will temporarily operate much as it does in cooling mode. Instead of pulling heat from outside, it circulates warm refrigerant through the outdoor coil, melting any accumulated frost and letting water drain away before it can freeze too much.
The sequence usually looks something like this:
- ❄️ Sensors detect excessive frost.
- 🔄 The reversing valve changes system operation.
- 🔥 Warm refrigerant flows through the outdoor coil.
- 💨 Frost quickly melts into water.
- 🌫️ Steam may rise as warm coils contact cold outdoor air.
- ✅ The system automatically returns to normal heating.
Homeowners typically spend just five to fifteen minutes on the entire process, depending on outdoor conditions and the amount of frost that has accumulated. One thing I always explain is that it’s not the system “switching to air conditioning” in the way many people imagine. It simply uses the refrigeration cycle temporarily to warm the outdoor coil enough to melt frost off and then returns to normal heating operation.
😲 Why Homeowners Sometimes Think Something Is Wrong
Because most people have never actually watched a heat pump go through a defrost cycle, the experience can be a little alarming the first time.
I've had service calls where homeowners reported:
- 🌫️ "Smoke" coming from the outdoor unit.
- 💧 Water dripping underneath the equipment.
- 🔊 Strange hissing or whooshing sounds.
- 🌬️ Cooler air briefly coming from indoor vents.
- ⏸️ The outdoor fan suddenly stopping.
Ironically, nearly every one of those observations can be perfectly normal during a defrost cycle.
The steam isn't smoke.
The water is melted frost.
The sounds are refrigerant changing direction.
The outdoor fan may temporarily stop while frost melts more efficiently.
Understanding these normal operating characteristics helps homeowners avoid unnecessary concern every time winter weather arrives.
🌫️ Steam Is Normal—And It's Usually a Good Sign
One of the most dramatic things that happen during a defrost cycle is when homeowners see what looks like smoke coming from the outdoor unit. I’ll admit, it can look alarming if you’ve never seen it before. I’ve taken calls where people thought their new heat pump was on fire because they walked outside and saw a cloud of white vapour coming out of the top of the equipment.
Fortunately, that's almost never what's happening. What you're actually seeing is steam.
The heat pump will run warm refrigerant through the outdoor coil briefly to melt the frost that has built up into water. When that warm coil meets the cold outdoor air, some of the moisture immediately becomes visible water vapour. It can be surprisingly dramatic on cold humid mornings, but it's all part of the normal defrost process.
I’ve had homeowners tell me that they almost went for a fire extinguisher before the steam cleared a minute or two later. When I arrived the heat pump was working beautifully.
You also may see water dripping from under the outdoor unit. In fact, it is a sign that the system is successfully removing frost rather than allowing ice to continue to build on the coil.
🔥 Why Auxiliary Heat Sometimes Turns On
Another question I hear regularly is: "Why does my heat pump seem to stop heating the house during defrost?"
The refrigeration system is temporarily reversed to heat the outdoor coil during the defrost cycle. This means that the heat pump cannot provide full heating indoors for a few minutes. Many systems also have auxiliary electric heat strips inside the air handler that come on automatically to prevent homeowners from feeling cool air during that short period. The auxiliary heaters continue to supply warm air while the outdoor unit defrosts itself. The process is completely automatic.
Most homeowners never touch a button or change a thermostat setting. The controls take care of all the background stuff and switch back to normal heat pump operation when the defrost cycle is done. If you’ve ever seen the “Aux Heat” or “Emergency Heat” light flash on your thermostat in the winter, it may be a defrost cycle. That doesn't necessarily mean a problem. It’s simply the HVAC system that maintains comfort inside and protects the equipment outside.
⏱️ How Long Should a Defrost Cycle Last?
One concern I often hear is whether a heat pump is spending too much time in defrost. Fortunately, modern systems are designed to keep these cycles relatively short.
Most defrost cycles typically last:
| Outdoor Conditions | Typical Defrost Time |
|---|---|
| Light Frost | 3–5 minutes |
| Moderate Frost | 5–10 minutes |
| Heavy Frost | 10–15 minutes |
The exact duration depends on several factors, including:
- 🌡️ Outdoor temperature
- 💧 Humidity levels
- ❄️ Amount of frost accumulation
- 🌬️ Wind conditions
- 🏡 Heat pump design
Homeowners shouldn't expect their system to enter defrost on a predictable schedule every day. Some cold, dry winter days may require very few defrost cycles. Meanwhile, damp mornings with temperatures hovering near freezing may trigger them more frequently because frost develops much faster under those conditions.
📊 Normal Defrost vs. Signs That Need Attention
Understanding what's normal can help homeowners know when it's time to call a professional.
| Observation | Usually Normal? |
| Light frost on outdoor coil | ✅ Yes |
| Steam during defrost | ✅ Yes |
| Water draining beneath unit | ✅ Yes |
| Outdoor fan temporarily stops | ✅ Yes |
| Short auxiliary heat operation | ✅ Yes |
| Thick ice remaining for hours | ❌ No |
| Outdoor coil completely encased in ice | ❌ No |
| Defrost lasting unusually long every cycle | ❌ No |
| No heating after defrost completes | ❌ No |
One thing I always remind homeowners is that temporary frost is normal—permanent ice is not.
If the outdoor unit becomes completely covered in thick ice that never seems to melt, or if heating performance noticeably declines, it's worth scheduling an inspection.
Possible causes could include:
- Low refrigerant charge
- Faulty defrost control board
- Defective temperature sensor
- Reversing valve problems
- Restricted airflow
- Drainage issues allowing water to refreeze
Fortunately, these situations are relatively uncommon in properly installed and maintained systems.
🧰 Simple Winter Maintenance Tips
Although the defrost process is automatic, homeowners can still help their heat pump perform efficiently throughout winter.
Here are a few habits I recommend:
- ✔️ Keep leaves and debris away from the outdoor unit.
- ✔️ Maintain at least two feet of clearance around the equipment.
- ✔️ Replace indoor air filters regularly.
- ✔️ Avoid stacking snow against the outdoor coil.
- ✔️ Never chip or scrape ice from the coil with sharp tools.
- ✔️ Schedule annual professional maintenance.
A mistake I see sometimes is when homeowners dump hot water over the outdoor coil to “help” the heat pump defrost. It's an understandable sentiment, but usually not necessary, and can sometimes add to the ice if temperatures remain below freezing. If possible, allow the equipment to undergo its own defrost cycle.
According to the ENERGY STAR, routine maintenance and proper installation play an important role in helping heat pumps maintain efficient year-round performance. ENERGY STAR Heating & Cooling Guide
📋 Homeowner Winter Heat Pump Checklist
Before assuming your heat pump has a problem during cold weather, ask yourself:
- ✔️ Is there only a light layer of frost on the outdoor coil?
- ✔️ Did the steam disappear after a few minutes?
- ✔️ Is water draining beneath the outdoor unit?
- ✔️ Did the system return to normal heating afterward?
- ✔️ Is the outdoor unit free from leaves, snow, and debris?
- ✔️ Has the air filter been replaced recently?
- ✔️ Has the system received annual maintenance?
- ✔️ Is my thermostat operating normally?
The more confidently you answer "yes," the more likely your heat pump is simply doing exactly what it was designed to do.
👨🔧 Jake's Final Thoughts
One of the biggest lessons I've learned over the years is that most homeowners tend to worry most about the things that are actually signs of a healthy HVAC system. A little frost on the outdoor coil, a puff of steam on a chilly morning, or a short burst of auxiliary heat can definitely grab your attention—but in many cases, that’s exactly what modern heat pumps are built to do.
Today's heat pumps don't have to worry about that, with features like automatic defrost cycles working to keep them operating efficiently even in harsh winter weather. Instead of letting frost slowly degrade performance, the system periodically clears the coil and returns to normal operation with very little homeowner involvement.
Obviously there is a difference between normal frost and excess ice accretion. That’s why regular maintenance is so important. A qualified HVAC technician can ensure that the sensors, controls, refrigerant levels, and airflow all work together as the manufacturer designed it.
If I could leave one message with every homeowner, it’s this: watching your heat pump go through a defrost cycle usually means it’s protecting itself, not breaking down. Knowing that simple fact will save you unnecessary worry and help you appreciate just how intelligently modern heat pump systems operate throughout the winter months.
Jake Lawson
HVAC Insights Specialist
The Furnace Outlet
