When an outdoor heat pump unit is installed on a flat roof in Montreal, it's subjected to the harshest aspects of a Quebec winter: wind, frost, frequent defrost cycles, blown snow, accumulated ice, and rapid temperature fluctuations. In Ville-Marie , with its many renovated plexes, condos, and accessible rooftops, this type of installation is common… and the need for preventative maintenance is even greater.
At AirGreen , we regularly service Carrier wall-mounted heat pumps in dense urban environments. The photo clearly shows a Carrier outdoor unit installed on a roof, with frost/ice on the cabinet and icicles near the ducts/refrigerants. This type of situation is not uncommon: the unit is functioning, but certain conditions (drainage, defrosting, air circulation, unit level, insulation condition, etc.) can cause excessive icing or intermittent symptoms (reduced efficiency, excessively long cycles, noise, safety shutdown).
In this article, we show you how we approach a service call on a Carrier wall-mounted heat pump (mini-split type), explaining what we check, what we correct, and most importantly, how to prevent the problem from recurring.
What the photo tells us: typical service context in Montreal
Several elements are useful for understanding the type of intervention:
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Carrier outdoor unit installed on a roof (bituminous surface), on wooden boards .
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Presence of frost and ice on the bodywork and around the exit of the lines/connections.
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Ice buildup at the bottom of the unit and on nearby components often indicates: imperfect drainage, intensive defrost cycles, or stagnant moisture.
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Context exposed to wind , very common on the roofs of Ville-Marie.
These signs don't automatically indicate a major breakdown. A heat pump in heating mode can frost up: that's normal. What's not normal is when the ice builds up continuously , grows larger, or eventually damages the fan, the heat exchanger, or the components.
System type: Carrier wall-mounted heat pump (mini-split)
This is a wall-mounted heat pump : an indoor unit (not visible in the photo) and a compact outdoor unit. This format is very popular in Ville-Marie because it offers:
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Efficient heating and air conditioning
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Flexible installation (walls, roofs, balconies)
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Comfort by zone (main room, bedroom, office)
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Good energy efficiency in mid-season
In return, an outdoor rooftop unit must be maintained with special care: access, safety, exposure to the elements, and especially water management (melting/defrosting).
Model: Carrier 38MARB (ductless series, consistent estimate)
The exact model isn't clearly visible in the photo, but the size and configuration correspond to an outdoor unit from the Carrier ductless range, type 38MARB (or equivalent from the same family). At AirGreen, when a specific model isn't perfectly identifiable in an image, we stick to a reference consistent with the brand and dimensions, without inventing fanciful specifications.
What matters for the service: the operating logic, the control points, and how the unit reacts to cold and humidity.
Why does an outdoor unit frost up so much in winter?
In heating mode, the heat pump extracts heat from the outside air. The outdoor unit becomes cold, and the humidity in the air can freeze on it. The unit then initiates a defrost cycle to melt this ice.
The problem arises when:
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Defrosting is less efficient (probes, circuit board, parameters, intermittent faults)
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The defrost water is not draining properly and is refreezing at the bottom
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Air circulation is poor around the unit (blown snow, insufficient space, obstacles)
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The unit is too low or improperly raised: the ice rises and eventually reaches sensitive areas.
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There is a potential issue with refrigerant charge or pressure (less frequent, but possible).
Our AirGreen approach: a complete, step-by-step service
When we intervene in Ville-Marie on a Carrier wall-mounted heat pump in the middle of winter (or during a period of freezing), we follow a clear methodology.
1) Functional diagnosis under real-world conditions
Before "touching everything", we observe:
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Indoor air supply temperature (if accessible)
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Heating stability (short cycles, stops, restarts)
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Presence of error codes (remote control, indoor unit, outdoor board)
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Frequency and duration of defrosting
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Abnormal noises (fan, vibration, contact with ice)
The goal: to understand if the icing is normal , excessive , or a symptom of another problem .
2) Detailed visual inspection of the outdoor unit (as shown in the photo)
Specifically, we check:
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Condition of the heat exchanger fins (deformed, clogged, obstructed)
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Localized ice pack (bottom of the toilet, around the drain, on the lines)
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Insulation of refrigeration pipes (cracked, detached, damp)
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Passage of conduits/cables (waterproofing, water infiltration)
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Fixings and supports (level, stability, corrosion)
On a roof, wind can push snow and create invisible "plugs" that disrupt ventilation. A simple environmental adjustment can make a huge difference.
3) Technical cleaning (adapted to the season)
In service, cleaning is not limited to simply "watering." It is adapted according to the context:
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Dusting and cleaning of accessible surfaces
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Careful cleaning of the fins if necessary
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Checking the air intake/exhaust
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Safe removal of problematic ice (without damaging the heat exchanger)
The idea: to restore the heat pump's heat exchange capacity, which is essential to reduce repeated defrosting.
4) Defrost control: probes, logic, behavior
A unit that defrosts poorly can accumulate ice even if everything else is working correctly. We are testing:
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Outdoor temperature sensor
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Coil sensor
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Consistency of measured values
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Control board response (defrost timing, triggering)
When a probe “reads” incorrectly, the machine may defrost too late, too early, or not for long enough.
5) Electrical verification and safety
On the roofs, humidity and frost put the connections to a severe test:
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Tighten the connections (without overtightening)
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Cable condition
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Circuit breaker/disconnector check
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Checking the protections
An electrical anomaly can manifest itself through random stops, particularly during defrosting (when the machine changes mode and puts different stress on its components).
6) Pressure/performance check (if required)
If the symptoms warrant it (low efficiency, abnormal frost despite defrosting, insufficient heating), we will go further:
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Operating measurements (temperatures, superheating/subcooling depending on configuration)
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Indirect assessment of a possible leak or inadequate charge
We always favour a logical diagnosis: we don't "add gas" without reason. We identify the cause.
Frequent adjustments that we recommend for roofs in Ville-Marie
On an exposed installation like the one in the photo, here are some very common service improvements:
Properly elevate and stabilize the unit
Visible boards can be a temporary solution, but Montreal winters often demand:
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Sufficient elevation to prevent the defrost water from refreezing in one block beneath the unit
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Suitable supports (stability, leveling, vibration reduction)
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Optimal airflow around the fan
Optimize the drainage of defrost water
Defrosting produces a lot of water… which freezes quickly. We check that nothing is creating a “pocket” of water:
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Unit position
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Ice accumulation at the bottom
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Areas where water can collect and form stalactites
Check and redo the insulation of the lines
Damaged insulation on refrigeration pipes can lead to:
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Internal condensation/infiltration
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Localized freeze
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Loss of yield
We provide clean, durable, and well-sealed insulation.
Why perform this service before it "breaks down"?
A heat pump that fights ice:
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consumes more electricity,
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It doesn't heat up as well.
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defrosts more often (therefore less stable comfort).
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wears out faster.
A well-done preventative service (even in cold season, depending on conditions) can avoid a breakdown at the worst time — when everyone is calling at the same time.
AirGreen in Ville-Marie: Rigorous and human-centered residential HVAC service
Our difference lies in our approach: we don't do a "quick lap". We aim for a measurable result: performance, stability, reduced icing, and real comfort inside.
Whether your Carrier wall-mounted heat pump is on a roof, balcony, backyard or alley, we adapt our interventions to the Montreal context: weather, access, safety, building configuration, and urban constraints.
Good practices to remember (especially for winter in Montreal)
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Keep a clear area around the outdoor unit (snow, objects, benches, bins, etc.)
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Never hit the interchange to remove the ice
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Watch for signs: unusual noise, weakening heating, excessively frequent cycling.
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Regular maintenance is often cheaper than emergency repairs.