An outdoor rooftop unit losing efficiency just when comfort became essential
In the Southwest sector of Montreal, our AirGreen team was called for a HVAC service call regarding a wall-mounted heat pump whose performance had noticeably dropped. The client reported a problem easy to describe but often complex to diagnose: the indoor unit was still blowing air, the control responded, but the house no longer cooled as before. On sunny days, the device ran for a long time without reaching the requested temperature.
The outdoor unit was installed on a flat roof, a common setup in residential and semi-commercial buildings in the Southwest, especially in multiplexes, condominiums, and renovated buildings. In the photo, you can clearly see a compact condenser of a wall-mounted heat pump, mounted on wooden blocks, with its outdoor coil exposed, insulated refrigerant lines, and communication wiring. The machine was accessible, but the rooftop installation added several constraints: direct sun exposure, stronger winds, temperature variations, possible dust accumulation, roof membrane protection, and more delicate tool handling.
From our visual inspection, several elements caught our attention. The outdoor unit appeared generally stable, but the insulation around the refrigerant lines showed signs of advanced wear. Some sections were hardened, cracked, or partially degraded by the sun. This situation is very common on rooftop installations: UV radiation and freeze-thaw cycles eventually damage the insulation, which can harm system performance and promote condensation on the lines.
The outdoor coil, visible behind the grille, also showed a buildup of fine dust. It was not completely blocked, but sufficiently loaded to reduce heat exchange. On a wall-mounted heat pump, especially when it operates in cooling mode during hot periods, the condenser must efficiently release the heat extracted from inside. If air does not pass well through the coil, the compressor works harder, pressure increases, and the system can lose efficiency.
Symptoms reported by the client
The client described a gradual failure rather than a sudden stop. The device initially seemed less powerful, then the cycles became longer and longer. The main room remained lukewarm despite a low setpoint. At times, the outdoor unit started normally, but the cold produced inside remained insufficient.
The symptoms noted during the service call included:
- low cooling despite active demand;
- a functional outdoor unit, but noisier than usual;
- prolonged cycles;
- an insufficient temperature difference between return air and supply air;
- deteriorated refrigerant line insulation;
- a partially dirty outdoor coil;
- a risk of efficiency loss due to direct exposure of the piping to the sun.
This type of situation requires a comprehensive diagnostic method. A wall-mounted heat pump may seem to be working but struggle due to a combination of small issues: reduced heat transfer, poor airflow, inaccurate sensor, refrigerant charge to be checked, weakened electrical connection, or less stable outdoor fan motor.
Inspection of the outdoor unit on the roof
Before any technical intervention, we secured access to the roof and protected the membrane around the work area. On a flat roof, the priority is twofold: properly diagnose the HVAC equipment and avoid damaging the roof. The support blocks under the unit were functional, but their aging deserved special attention. An outdoor unit must remain level to limit vibrations, ensure proper drainage, and reduce stress on the pipes.
We then inspected:
The outdoor coil
The coil had a layer of fine dust and accumulated particles. On rooftops, condensers often receive pollen, urban dust, membrane debris, leaves, and wind-blown particles. Even if the grille looks clean from a distance, the coil fins can be sufficiently clogged to restrict airflow.
The refrigerant lines
The black insulation around the lines showed visible deterioration. This wear may seem minor, but it directly affects performance. Poorly insulated lines can absorb heat even before the refrigerant returns to the compressor, reducing the machine's overall efficiency. In air conditioning, proper insulation also helps limit condensation and protects the pipes from extreme temperature variations.
Electrical and communication connections
The wiring was exposed to roof conditions, which always requires careful inspection. We checked accessible connections, the condition of the electrical box, cable passages, and signs of overheating or oxidation. No major damage was apparent, but some ties and protections needed to be restored to prevent the cable from moving in the wind.
Refrigeration and thermal diagnosis
After the visual inspection, we started the unit in cooling mode to take measurements. The outdoor fan was running, but heat rejection seemed less effective than expected. Temperature readings confirmed a performance below what is expected from a wall-mounted heat pump in good condition.
We compared the return air temperature and the supply air temperature inside. The difference was present but too small to provide adequate comfort. This observation could indicate a lack of heat exchange, a refrigerant charge to monitor, or a restriction caused by coil fouling.
Before concluding there was a leak or a need for a recharge, we cleaned the outdoor coil and verified air circulation. This step is essential. Too often, a performance drop is too quickly attributed to the refrigerant, while the system is simply unable to dissipate heat due to a dirty or poorly ventilated condenser.
Technical cleaning of the condenser
We performed cleaning suitable for an outdoor wall-mounted heat pump unit installed on a roof. The pressure used must be controlled: a jet that is too strong can bend the coil fins and further reduce air exchange. We therefore removed deposits gradually, respecting the coil direction and the device’s limits.
After cleaning, the airflow through the unit improved. The fan expelled heat more evenly, and the compressor seemed to work with less strain. This improvement allowed for more stable readings during the second test cycle.
Repair of line insulation
An important part of our intervention involved repairing the insulation of the refrigeration lines. The damaged sections were replaced or reinforced with appropriate insulation, then protected to better withstand outdoor exposure. On a roof, this step is especially important because the lines are exposed to direct sunlight for long hours.
Clean insulation reduces losses, protects piping, limits condensation, and improves operational stability. In a wall system, especially when the distance between the indoor and outdoor units is significant, the quality of the refrigerant lines has a real impact on performance.
Outdoor fan check
We also checked the operation of the outdoor fan motor. The noise reported by the client mainly came from a vibration amplified by the block installation and the general condition of the unit. After cleaning, repositioning some protections, and checking fastenings, the noise diminished.
On roof installations, vibrations can transmit differently than on the ground. A small vibration can seem more significant when the unit rests on aging supports or a rigid surface. We therefore checked the unit’s stability during operation and recommended medium-term monitoring of the supports.
Result after intervention
After cleaning the condenser, restoring the insulation on the lines, and electrical checks, the wall-mounted heat pump behaved much more stably. The air blown inside was cooler, cycles were shorter, and the outdoor unit rejected heat better.
The client noticed a tangible difference: the main room reached the set temperature faster, and the unit no longer seemed to run without results. We also explained the limits of the intervention. A refurbishment improves performance when major components are still functional, but an older machine or one heavily exposed to roof conditions must be regularly monitored.
Why wall-mounted heat pumps on roofs require special attention
Flat roof installations are very common in several areas of Montreal, including Le Sud-Ouest, but they do not age the same way as a unit installed on the ground or on a protected wall mount. Exposure is more intense, access is sometimes more difficult, and outdoor components are more affected by sun, wind, snow, and rapid temperature changes.
Common mistakes to avoid
Neglecting the insulation of refrigerant lines
Cracked or missing insulation is not just an aesthetic detail. It can reduce system efficiency, cause condensation, and accelerate wear on certain pipe sections.
Letting the outdoor coil get dirty
A dirty coil forces the compressor to work longer. This increases electricity consumption, reduces comfort, and can lead to safety shutdowns.
Using excessive pressure to clean the unit
Poor cleaning can bend the coil fins. Once the fins are crushed, air flows less efficiently and performance decreases.
Ignoring vibrations
A slight vibration can indicate a problem with the support, fan, fastening, or contact with a pipe. On a roof, intervention is needed before the vibration damages the lines or fittings.
Practical tips to extend the system’s lifespan
For owners of wall-mounted heat pumps installed on roofs, we recommend regular checks, especially before periods of intense heat. Maintenance should include inspecting the outdoor coil, checking the supports, the condition of the insulation, electrical connections, indoor unit drainage, and performance readings.
Preventive maintenance often helps avoid emergency calls. It also allows for the quick detection of problems that worsen over time: insulation damaged by UV rays, clogged coil, noisy fan, oxidized connection, loss of efficiency, or beginning of a leak.
AirGreen expertise tailored to Greater Montreal
At AirGreen, we regularly work on systems installed in complex urban settings: flat roofs, narrow alleys, condominiums, multiplexes, multiple wall units, MultiZone installations, and older equipment. This type of intervention requires more than just a simple part replacement. It is necessary to understand the environment, protect the building, make an accurate diagnosis, and propose a durable solution.
Our team serves Montreal, Laval, Longueuil, the North Shore, and the South Shore for repairs of wall-mounted heat pumps, wall air conditioners, MultiZone systems, central heat pumps, and other residential HVAC equipment. Every service call is handled with the same rigor: symptoms, diagnosis, solution, performance tests, and clear recommendations.
In this case in the Southwest, the intervention restored comfort without a complete replacement of the unit. Cleaning the condenser, correcting the insulation of the lines, and performing operational checks gave the machine better efficiency. For the client, the result was immediate: a cooler home, a more stable system, and a better understanding of the actual condition of their installation.