Repair of a wall-mounted heat pump in Rosemont–La Petite-Patrie

An outdoor unit on a flat roof losing its efficiency due to a set of neglected technical details

In Rosemont–La Petite-Patrie, our AirGreen team was called for a wall-mounted heat pump repair installed on a residential flat roof. The outdoor unit, visible on a wooden base with anti-vibration pads, was connected inside by a set of insulated refrigerant lines running over the roof before entering the building. The customer reported a progressive loss of performance: the indoor wall head was working, the system responded to the remote control, but the air conditioning became less consistent and the heating seemed less effective on cold days.

The installation had several important particularities. Unlike an outdoor unit installed on the ground or on a wall bracket, a wall-mounted heat pump placed on a flat roof undergoes more demanding conditions: stronger winds, direct sun exposure, snow accumulation, freeze-thaw cycles, vibrations transmitted by the structure, more complex access for maintenance, and accelerated aging of the refrigerant line insulation. In this specific case, the unit appeared well cleared, but several signs showed that performance had been affected by the roof environment.

The exact model was not clearly readable from the outdoor unit, but the device corresponded to a residential Inverter wall-mounted heat pump of compact to medium capacity, typically in a range of 12,000 to 18,000 BTU depending on the indoor configuration. This category is very common in multiplexes, condos, and houses in Montreal, especially in areas like Rosemont, Villeray, the Plateau, Hochelaga, Laval, Longueuil, the North Shore, and the South Shore, where flat roofs often offer the most logical space to install the outdoor condenser.

Symptoms described by the customer

The customer explained that the problem had not appeared suddenly. For several weeks, the device had operated irregularly. Some days, the system cooled properly. Other days, especially when demand was higher, the indoor temperature dropped slowly and the outdoor unit seemed to work harder than usual.

The symptoms observed before our intervention were as follows:

• less cold air at the indoor unit outlet in cooling mode;
• less consistent heating at lower temperatures;
• longer cycles to reach the setpoint;
• more noticeable outdoor noise on the roof;
• slight vibration transmitted to the structure;
• intermittent shutdown after a long operating period;
• presence of moisture around the refrigeration line insulation;
• customer concern about a possible refrigerant leak.

In an HVAC service call, these signs can point to several causes. A drop in performance can indeed come from a lack of refrigerant, a leak, an electronic board, a sensor, a worn compressor, a weakened outdoor fan, or a dirty condenser. However, on a rooftop installation, you also need to check often underestimated elements: pipe insulation, line fastening, condition of exposed wiring, support stability, and actual clearance of the unit.

Inspection of the outdoor unit on the roof

Our technician started by inspecting the outdoor unit and its immediate surroundings. The device was installed on two beams, themselves placed on a roof protection layer. This method is common to avoid direct contact with the membrane, but it requires regular checks. Over time, the wood can slightly deform, absorb moisture, create uneven support, or transmit vibrations.

We found that the unit was stable, but some anti-vibration mounts were unevenly compressed. This situation can create a more noticeable noise when the Inverter compressor modulates its speed. The customer had indeed noticed a more pronounced humming at certain times. It was not a sign of a defective compressor but rather a vibration amplified by the outdoor mount.

We also checked the condenser coil. Even on an open roof, dust, pollen, plant fibers, and windborne particles can accumulate in the fins. The outdoor unit looked clean from a distance, but a closer inspection showed a partial blockage of the coil. This level of dirt did not stop the system but reduced heat exchange, especially when the machine had to run longer.

Refrigerant lines: a detail that greatly affects efficiency

One of the most important points of this intervention concerned the insulation of the refrigerant lines. In the photo, you can clearly see the pipes running across the roof inside insulated ducting. On the roof, this insulation is exposed to sun, wind, rain, frost, and temperature changes. When it cracks, compresses, or lets moisture in, system performance can decrease.

We inspected the duct and accessible sections of the insulation. Some parts were weakened, with areas where the insulation no longer fully protected the line. This problem can cause several effects:

• loss of energy efficiency;
• excessive condensation in cooling mode;
• moisture buildup around the pipes;
• risk of accelerated degradation of the insulation;
• temperature variation of the refrigerant before it reaches the indoor unit;
• false impression of a gas shortage.

In this case, temperature readings and the system’s overall behavior did not initially indicate a major leak. The loss of efficiency was more related to a combination of reduced heat exchange, worn external insulation, and vibrations. We corrected these issues before recommending a more extensive intervention.

Electrical and communication check

Modern wall-mounted heat pumps use electronic communication between the indoor and outdoor units. On a flat roof, external connections can be exposed to moisture, cable movement, vibrations, and temperature changes. We opened the service compartment to check the connections, communication terminals, and power supply.

One communication connection was not tightened enough. It wasn’t a clear failure, but it could explain the intermittent shutdowns observed during long operating periods. A slightly unstable connection can cause a temporary code, a protective shutdown, or a restart without the customer always being able to identify the cause.

We retightened connections, cleaned accessible points, checked continuity, and secured the wiring to limit movement caused by the wind. This step stabilized communication between the two units and reduced the risk of intermittent interruptions.

Cleaning, insulation repair, and full recommissioning

After diagnosis, the intervention focused on three objectives: restoring airflow through the condenser, properly protecting the refrigeration lines, and stabilizing the outdoor unit to reduce vibrations and protective shutdowns.

Technical cleaning of the outdoor condenser

We cleaned the outdoor coil using a method suited to the condenser fins. On a wall-mounted heat pump, the outdoor coil must allow a large volume of air to pass through. When it is even partially blocked, the compressor has to work harder to produce the same result. In cooling mode, the unit rejects heat less effectively. In heating mode, it captures the available energy in the outside air less efficiently.

Cleaning removed a layer of fine dust and light debris accumulated on the fins. We avoided excessive pressure to prevent bending the coil. Once the condenser was cleared, the outdoor fan could move air more freely, and the machine regained better heat exchange capacity.

Restoration of refrigeration line insulation

We replaced and reinforced the weakened insulation sections on the accessible lines. This part of the repair was essential because a poorly insulated line on a roof can reduce efficiency even if the compressor and electronic board are in good condition. We used insulation suitable for outdoor use, well fitted around the pipes, then protected the joints to reduce water infiltration.

This correction also helped reduce condensation around the pipes in cooling mode. For a client, a line that condenses or a damp duct can raise concerns about a refrigerant leak. In reality, it is often an insulation weakness, but it must be fixed quickly to prevent further damage.

Unit stabilization and vibration reduction

We then rebalanced the supports of the outdoor unit and adjusted the contact points with the anti-vibration pads. The goal was to prevent the compressor from transmitting its vibrations to the beams, and then to the roof structure. On an Inverter unit, the compressor speed constantly varies. A slightly unstable support can therefore be quiet at one speed and noisy at another.

After adjustment, the outside noise was more consistent, less resonant, and better absorbed by the supports. This type of correction is especially important in urban buildings in Montreal, where a rooftop unit can be close to a bedroom, a neighboring apartment, or an inner courtyard.

Performance tests after repair

Once the corrections were completed, we restarted the wall-mounted heat pump in cooling mode, then checked its behavior over an extended cycle. The indoor unit blew more stable air, the outdoor unit modulated normally, and no communication interruptions were observed during the tests.

We validated several points:

• start-up of the outdoor unit;
• response of the indoor unit;
• communication stability;
• temperature of the blown air;
• speed of the outdoor fan;
• no excessive vibration;
• condition of the refrigerant lines;
• normal condensate drainage on the indoor side;
• no immediate sign of refrigerant leak.

The client noticed that the device operated more consistently and that the outside noise was less noticeable. The system did not need a complete replacement or an automatic refrigerant recharge. The solution was rather in a precise restoration of the outdoor installation.

Why you should not add refrigerant without a diagnosis

This intervention in Rosemont–La Petite-Patrie illustrates a common mistake: believing that a heat pump that cools less effectively necessarily lacks refrigerant. In some cases, this is true. But adding refrigerant without confirming a leak or checking the heat exchange condition can worsen the problem. Overcharging refrigerant can harm compressor operation, increase pressures, and reduce equipment lifespan.

At AirGreen, we first check mechanical, electrical, and environmental causes. On this installation, the dirty condenser, weakened insulation, and unstable communication connection explained the observed symptoms much better. A recharge would not have fixed these problems.

Mistakes to avoid with a wall-mounted heat pump installed on a flat roof

Roof installations are effective when well designed and well maintained, but they require special attention. The most common mistakes we observe are:

• leaving refrigerant lines exposed with damaged insulation;
• neglecting to clean the condenser because it looks clean from a distance;
• placing the unit on an unstable or aging support;
• ignoring vibrations that appear over time;
• letting cables move freely in the wind;
• blocking service access with roofing elements;
• waiting for a complete breakdown before requesting an inspection;
• having refrigerant added without a complete diagnosis.

An HVAC installation on a flat roof should be inspected regularly, especially after winter and before the heat of summer. The outdoor conditions in Montreal, Laval, Longueuil, the North Shore, and the South Shore can accelerate the wear of exposed materials.

Advice given to the client after the intervention

We recommended that the client have the outdoor unit checked at least once a year, especially the condition of the insulation, the cleanliness of the condenser, and the stability of the support. We also advised monitoring for new noises, intermittent shutdowns, and any signs of moisture around the lines.

For a unit on a flat roof, it is also important to maintain safe access for technicians. HVAC repair becomes faster, more accurate, and less costly when the unit is accessible, well cleared, and installed on a stable base.

Final result: a more stable system and restored comfort

At the end of the service call, the wall-mounted heat pump was operating with improved efficiency, stabilized communication, and less external vibration. Cleaning the condenser restored airflow, repairing the insulation reduced heat loss, and tightening connections eliminated the risk of intermittent shutdowns.

This repair shows that a loss of performance is not always caused by a major breakdown. In many cases, comfort is restored by addressing details that directly affect system operation: air, insulation, electricity, support, and maintenance.

AirGreen provides wall-mounted heat pump repair, performance diagnostics, wall air conditioner maintenance, multi-zone systems, and central equipment services throughout the greater Montreal area, including Laval, Longueuil, the North Shore, and the South Shore. For rooftop outdoor units, a professional inspection helps avoid incorrect conclusions, extends the equipment's lifespan, and restores reliable comfort in the home.