An outdoor unit on a flat roof that no longer delivered the expected performance
In Rosemère, on the North Shore, we responded to a service call regarding a wall-mounted heat pump whose outdoor unit was installed on a flat roof. The client noticed a drop in cooling performance, longer cycles than usual, and an indoor temperature that became difficult to stabilize despite a constant thermostat demand. The device was still working, but comfort was no longer guaranteed.
This type of intervention requires a different approach than ground-level service. An outdoor unit installed on a roof is more exposed to sun, large temperature swings, wind, snow, ice, and UV rays. The refrigerant lines, insulation, communication wiring, and supports must be carefully inspected, as external deterioration can directly impact the performance of the heat pump.
On site, the unit was stable, placed on supports suitable for the flat roof, with proper clearance around the condenser. However, the condition of the refrigerant lines immediately caught attention: the black insulation surrounding the pipes showed signs of advanced aging, cracking, and prolonged exposure to the elements. This detail may seem minor, but in an HVAC repair, it can explain a significant part of the efficiency losses.
Symptoms reported by the client
The client described a system that started normally but seemed to take too long to cool the rooms. The wall-mounted indoor unit was blowing air, but the air was not cold enough to quickly reach the setpoint. Some days, the heat pump ran almost continuously, especially in the afternoon when the roof accumulated a lot of heat.
The main symptoms were as follows:
- cooling slower than before;
- prolonged operation of the indoor unit;
- perceived loss of power;
- slight performance variation depending on sunlight;
- Normal outdoor noise, but insufficient performance;
- Customer concern about a possible refrigerant leak.
These signs can be related to several causes: dirty outdoor coil, poor heat exchange, insufficient refrigerant, faulty sensor, electronic board problem, weakened capacitor, less effective outdoor fan, deteriorated refrigerant line insulation, or a combination of several factors.
A single-zone wall-mounted heat pump exposed to the challenges of a flat roof
The device in question was a wall-mounted heat pump of the mini-split type, connected to an indoor unit installed in the residence. This type of system is very popular in Rosemère, Montreal, Laval, Longueuil, on the North Shore and the South Shore, as it offers an effective solution to heat and cool a specific area without ductwork.
In this case, the outdoor unit was installed on a flat roof, a common setup when ground space is limited or when preserving the facade’s aesthetics is necessary. This solution can be very effective but requires special attention:
Sun exposure
A flat roof can become very hot in summer. The outdoor unit must then reject heat into an already overheated environment. If the coil is dirty or the lines are poorly insulated, performance can quickly decrease.
Insulation wear
UV rays, frost, snow, and rain gradually degrade the insulation of refrigerant lines. When the insulation cracks or disintegrates, thermal losses increase, especially on the cold line in cooling mode.
Service access
A rooftop unit requires safe and structured intervention. The membrane must be protected, unnecessary equipment movement avoided, and it must be ensured that the supports do not create damaging pressure points.
Drainage and water accumulation
Even if the outdoor unit does not produce the same drainage as an indoor unit, water, snow, and ice around the device can accelerate corrosion or affect the supports.
Initial diagnosis: observe before concluding
Our first step was to check the general condition of the installation. We inspected the outdoor condenser, accessible refrigerant connections, line passages, insulation condition, visible connections, supports, and clearance around the unit.
The outdoor unit showed no major damage to its casing. The protective grille was in place and the coil seemed relatively clean at first glance, but a closer inspection revealed a fine accumulation of dust and particles on certain areas of the coil. On a flat roof, even without leaves or vegetation, the wind often carries dust, membrane granules, pollen, and small particles that eventually reduce the efficiency of heat exchange.
We then paid particular attention to the refrigerant lines. The insulation was visibly worn, and some sections no longer provided optimal thermal protection. This can cause a loss of efficiency, but also unwanted condensation or temperature variation on the line, depending on the mode of operation.
Verification of operation in cooling demand
We started the system in cooling mode to observe a complete cycle. The outdoor fan started, the compressor responded to the demand, and the indoor unit produced a constant airflow. However, the temperature difference between the return air and the supply air was not as stable as expected at the beginning of the cycle.
This observation led us to check several points:
- supply air temperature at the indoor unit;
- return temperature;
- compressor behavior;
- condition of the outdoor coil;
- speed of the outdoor fan;
- condition of refrigerant lines;
- signs of frost or abnormal condensation;
- electrical stability at startup;
- device response after cleaning and correction of visible elements.
A wall-mounted heat pump repair should never rely on a single measure. It is necessary to understand the complete behavior of the system, especially when it is still functional but less efficient.
Technical cleaning of the outdoor condenser
We performed a controlled cleaning of the outdoor condenser. On a wall-mounted heat pump unit, the coil must remain clean to allow the system to efficiently reject heat in summer and capture energy from the outside air in winter.
The cleaning was done carefully, without excessive pressure, to avoid damaging the fins. We removed accumulated particles and checked that air flowed better through the coil after the intervention. This step improved the unit’s breathing and reduced the effort required from the compressor.
Correction of refrigerant line insulation
The most important point of this intervention was restoring the insulation of the lines. The deteriorated insulation was removed from the problematic sections and replaced with insulation suitable for outdoor exposure. We also secured the lines to avoid direct rubbing against the roof surface and excessive movement caused by the wind.
This correction is essential for several reasons:
Reducing thermal losses
A poorly insulated line can lose part of the energy transported between the indoor and outdoor units. This reduces the overall system efficiency.
Protecting pipes against outdoor conditions
Insulation helps limit the impact of sun, frost, and humidity on refrigerant lines.
Preventing condensation
Poor insulation can promote condensation on certain sections, especially on humid days.
Improving installation durability
Well-protected lines age better and reduce the risk of premature deterioration.
Inspection of fittings and search for leak clues
Since the client feared a refrigerant leak, we inspected the visible fittings and areas around the service valves. A refrigerant leak often leaves oily traces around a fitting because oil circulates with the refrigerant in the system.
In this case, no obvious signs of a major leak were observed during the intervention. We still explained to the client that some micro leaks can be slow and require a more thorough test if performance drops again. Adding refrigerant without clear evidence is not good practice: it can increase costs, skew the diagnosis, and not fix the real cause of the problem.
Electrical check and compressor behavior
We also checked the accessible electrical components. The compressor start-up was stable, the outdoor fan operated normally, and no obvious anomaly pointed to a faulty electronic board. The system showed no active error code at the time of our visit.
This step ruled out several possible causes, including intermittent electrical failure or difficult startup related to a weakened capacitor. The main problem was rather on the side of heat exchange and protection of the refrigerant lines.
A targeted intervention that restored system performance
After cleaning the condenser, correcting the line insulation, and performing operational checks, we restarted the heat pump through a full cycle. The blowing temperature became more consistent, the system responded more efficiently to demand, and the outdoor unit operated without abnormal noise.
The client noticed a concrete improvement in comfort. The air produced by the indoor unit was more stable, and the machine no longer needed to run as long to reach the desired temperature.
What this repair demonstrates
This intervention in Rosemère shows that a drop in performance does not always mean a major part is broken. A wall-mounted heat pump can lose efficiency due to details that accumulate over time: damaged insulation, slightly clogged coil, exposed lines, installation on a roof exposed to UV and temperature fluctuations.
A complete diagnosis helps avoid three costly mistakes:
- Replacing a unit that is still functional;
- Adding refrigerant without evidence of a leak;
- Ignoring external deterioration that reduces efficiency.
In this case, the solution was not a complete replacement. The installation needed to be restored to good working order, the lines protected, and proper heat exchange reestablished.
Practical tips for wall-mounted heat pumps installed on roofs
Units installed on flat roofs need to be monitored more closely than those installed on the ground or on easily accessible wall mounts.
Check the insulation every year
The insulation on the refrigerant lines must remain continuous, well sealed, and UV resistant. If it cracks, deteriorates, or detaches, it should be replaced quickly.
Keep the unit clear
Even on a roof, debris can accumulate. Pollen, dust, granules, wind-blown leaves, and small debris can damage the coil.
Avoid unnecessary unit movements
An outdoor unit should not be pushed or moved without reason. Refrigerant lines can be weakened by repeated movements.
Protect the roof membrane
Supports must distribute weight properly and avoid pressure points. Poor installation can damage the roof.
Have it inspected before the heatwave
Pre-summer maintenance helps detect problems before the system is heavily stressed.
Why AirGreen intervenes differently
At AirGreen, we regularly see wall systems installed in very varied conditions: alleys in Montreal, backyards in Laval, triplex facades in Longueuil, flat roofs on the North Shore, residential installations on the South Shore. Each location affects how a heat pump ages and performs.
In this intervention in Rosemère, the difficulty was not due to a dramatic breakdown. It was due to a gradual loss of efficiency, harder to diagnose because the machine was still running. This is precisely the type of situation where field experience makes the difference.
We combined observation, measurements, cleaning, line corrections, and a full cycle check. The result was a tangible improvement without unnecessary replacement.
Result: improved comfort and better-protected installation
At the end of the service call, the wall-mounted heat pump was operating more efficiently. The outdoor unit was breathing better, the refrigerant lines were better protected, and the customer knew exactly what to monitor in the coming months.
This repair extended the system's lifespan, improved indoor comfort, and reduced the risk of a more serious breakdown during a heatwave.
For AirGreen, a good HVAC repair is not just about restarting a machine. It involves understanding why it lost performance, fixing the real causes, and providing the customer with a clear, lasting, and tailored solution for their installation.