The first real mild spell of January arrives, you look out the window, and the outdoor unit is covered in white. A cloud of vapor escapes from it. Your first instinct is to turn off the unit and call someone. In most cases, that would be a mistake. If your heat pump ices up in winter, it’s doing exactly what it was designed to do.
The government portal on heat pump maintenance reminds us that the unit’s durability primarily depends on proper maintenance, preferably performed in the fall. In other words, what you observe in January is determined in October. The challenge remains to distinguish between normal frost and a real symptom, because from the living room window, they look very similar.
Why a Heat Pump Ices Up, Mechanically Speaking
In heating mode, the outdoor coil is colder than the ambient air. This is the very condition of its operation: the unit captures heat from the outside air by circulating a fluid that is even colder than the air itself.
As a direct consequence, the moisture in the air condenses on this coil. And when the surface is below freezing, this condensation freezes. Nothing abnormal so far.
Frost accumulates mainly within a specific temperature range, around the freezing point, between approximately minus 5 and plus 5 degrees Celsius, when the air is laden with humidity. It’s the gray, wet weather of November and March that produces the most frost, not the dry cold of February. Many homeowners draw the opposite conclusion and worry at the wrong time.
To remove this ice, the unit triggers a defrost cycle. It briefly reverses its operation and sends hot fluid into the outdoor coil. The ice melts, water drains away, and the unit resumes heating. During this short interval, the outdoor fan stops, white vapor rises from the unit, and the air blown indoors becomes lukewarm. Three symptoms that cause concern, three perfectly normal symptoms.
What a Normally Icing Heat Pump Looks Like
A thin layer of white frost on the coil fins, which appears and disappears throughout the day. Vapor escaping from the unit for a few minutes, several times a day. A little ice on the ground under the unit, formed by defrost water. A clicking or whooshing sound that changes when the cycle reverses.
This is what a healthy unit looks like in March.
What is not normal, however, is recognized by a simple criterion: persistence. Normal frost melts; problematic frost accumulates.
| Observation | Interpretation | What to Do |
|---|---|---|
| Thin layer of frost that disappears in a few hours | Normal operation | Nothing |
| White vapor for a few minutes, several times a day | Defrost cycle in progress | Nothing |
| Opaque block of ice enveloping the unit for several days | Defrost failure or insufficient charge | Service call |
| Ice on top of the casing and on the fan | Often water infiltration from above, not frost | Check and correct |
| Unit base encased in a block of ice | Blocked drainage, defrost water no longer flows | Clear, then correct the base |
| Unit blowing cold air continuously | Supplemental heating engaged, defrost blocked | Service call |
| Abundant frost in very cold and dry weather | Abnormal, dry air produces little frost | Service call |
| Snow accumulated halfway up the unit | Airflow obstruction | Immediate clearing |
Dry Cold Is Not the Enemy You Imagine
We largely overestimate the severity of our winters when it comes to judging a heat pump. Hydro-Québec provides a figure that puts things into perspective: in southern and central Quebec, there are, on average, fewer than fifteen days per winter below minus 20 degrees Celsius. The rest of the season, the unit operates within its comfort range.
However, it is precisely on mild and humid days that frost accumulates the most. A dry February at minus 18 degrees Celsius often produces less ice than a November week at minus 2 degrees Celsius with freezing rain. If your unit becomes covered in ice during an intense, dry cold spell, the diagnosis points to a real problem rather than a seasonal phenomenon.
Another useful benchmark for winter 2026 and beyond: units sold today have little in common with those from the 2000s in this regard. Cold climate models retain a significant portion of their capacity down to minus 25 degrees Celsius, or even minus 30 degrees Celsius for some, and their defrost sequences are controlled by electronic boards that adapt the frequency of cycles to actual conditions rather than following a fixed timer. A fifteen-year-old unit that defrosts every thirty minutes regardless loses a lot of energy in the process.
This doesn’t change the basic rule: supplemental heating is required during cold peaks. No air-source heat pump completely eliminates the need for a complementary source at our latitudes, and any contractor who tells you otherwise is selling you an imaginary electricity bill.
What Turns Normal Frost into a Real Problem
Four causes consistently recur, and three out of four relate to installation or environment, not the unit itself.
The unit installed too low. A heat pump installed a few centimeters from the ground will be buried by the first heavy snowfall, and defrost water that cannot drain will refreeze, forming a base that blocks the fan. Supports must keep the unit above the usual snow level for the area, and the surface under the unit must allow for drainage. We have seen otherwise correct installations ruined by this single detail.
Lack of clearance. A unit stuck between a wall and a hedge, or installed under a gutter that pours water onto it, accumulates ice where it shouldn’t. Ice on top of the casing is almost never operational frost: it’s water that came from elsewhere.
Insufficient refrigerant charge. A leak causes pressure to drop, the coil cools too much, and frost accumulates faster than defrosting can remove it. Adding refrigerant without looking for the leak is like filling a leaky bucket, and unfortunately, it’s a common practice.
Faulty defrost sensor or board. The unit no longer detects when to switch cycles, or remains stuck in a cycle. In this case, it’s a repair, and it’s not dramatic if caught early.
A detail that almost everyone overlooks: the location chosen at the time of installation determines a good part of the unit’s winter behavior. A unit placed on the north side, in the permanent shade of a wall, dries poorly between cycles. The same unit moved three meters, exposed to the late morning sun, gets rid of some of its ice without even needing defrosting. This kind of trade-off is made in ten minutes during the evaluation, and it’s not easily corrected once the refrigerant line has been run through the wall.
The same logic applies to the support height. In areas where snow accumulates easily, fifteen centimeters of clearance is not enough. Often double that is needed, sometimes more on windy terrain where blowing snow forms drifts against the house.
What You Can Do Yourself, and Where the Limit Is
Clear snow around the unit after each heavy snowfall, without ever hitting the fins or the casing. Remove accumulated leaves, branches, and debris. Check that nothing is pouring water onto the unit. Replace or clean the indoor filters, because a clogged filter reduces airflow and disrupts the entire system.
What you should not do: pour hot water on the unit, scrape ice with a tool, or cover the unit with a tarp while it’s running. Hot water refreezes and worsens the situation, scraping bends the fins irreversibly, and a tarp suffocates the airflow.
Contrary to a persistent belief, you should also not turn off the heat pump in winter to protect it. It is made for this season, and it is precisely in winter that it saves you money. Turning it off amounts to paying for an expensive unit to heat with baseboard heaters.
If frost persists despite proper clearing, the technician’s check involves: pressures, charge, defrost cycle, temperature difference between return air and supply air. This is exactly the content of an annual maintenance visit for an existing heat pump, which costs a fraction of the price of a compressor replaced in an emergency in February.
The Schedule That Prevents All This
The real question, in reality, is not what to do when the unit ices up, but how to organize the fall so that the question arises as infrequently as possible.
October: complete maintenance, coil cleaning, charge verification, defrost cycle check before it’s needed. November: clear the perimeter, trim vegetation, check that gutters don’t drain onto the unit. December to March: a visual inspection after each significant snowfall; thirty seconds from the window is usually enough.
A unit checked in the fall frosts normally, defrosts normally, and gets through winter without you having to think about it. A neglected unit reminds you of its existence exactly on the day when service is most difficult to obtain.
