An intermittent failure on an outdoor compressor installed on a roof
In Rivière-des-Prairies–Pointe-aux-Trembles, in east Montreal, AirGreen was called for a wall-mounted heat pump repair service because the operation had become irregular. The customer described a frustrating problem: the indoor unit responded to the remote control, the ventilation started, but the system did not always produce the expected cool or warm air. At times, the outdoor unit seemed to want to start, then quickly stopped. At other times, the heat pump worked but with significantly reduced performance.
The equipment involved was a mini-split wall-mounted heat pump, composed of a wall-mounted indoor unit and an outdoor compressor installed on a flat roof. The exact brand and model were not clearly visible in the service photo, but the configuration matched a common residential device, probably in the range of 12,000 to 18,000 BTU, used to cool and heat a main area of the home.
This type of rooftop installation is common in several areas of Montreal, Laval, Longueuil, on the North Shore and South Shore, especially when ground space is limited or when the facade does not allow for a traditional wall installation. However, a unit installed on a roof is more exposed to outdoor conditions: direct sun, rain, frost, snow, winds, vibrations, rapid temperature changes, and dirt accumulation. In this particular case, the visual clues pointed to an electrical and mechanical problem not to be underestimated.
Symptoms reported before our arrival
The customer was not facing a completely constant breakdown. This is precisely what made the service call more complex. An intermittent failure can come from a weakened electrical component, an oxidized connection, an unstable electronic board, a defective sensor, a communication problem between units, or a compressor protection that activates when certain conditions are no longer met.
The observed symptoms were as follows:
- the wall-mounted heat pump sometimes started normally;
- the indoor unit was ventilating, but not always producing sufficiently tempered air;
- The outdoor compressor seemed hesitant to start;
- some sequences ended with a premature shutdown;
- the customer had noticed less stable performance for several weeks;
- the outdoor noise seemed different, especially at startup;
- the machine seemed more sensitive on very hot days.
This kind of situation requires a step-by-step diagnostic method. Adding refrigerant randomly, replacing a board without testing, or concluding too quickly that the compressor is defective can be costly for the customer without solving the real cause. At AirGreen, we always start with the fundamentals: electrical power, connections, communication, visual condition, outdoor ventilation, temperatures, pressures, and system behavior under real demand.
An important first clue: the condition of the outdoor electrical box
During the inspection of the outdoor compressor, our technician noticed several concerning elements around the side panel and the electrical box. The screws showed signs of oxidation, the outdoor casing showed signs of aging, and the conduits connecting the unit appeared to have been exposed to the weather for a long time.
On a wall-mounted heat pump, the outdoor electrical box plays a crucial role. It protects the power connections, communication wires, control terminals, sometimes the capacitor, the contactor, or certain internal connections depending on the model. A slight water infiltration, a rusty screw, a loose connection, or a weakened wire can be enough to cause intermittent outages.
In this case, the appearance of the casing did not automatically mean it was the sole cause of the failure, but it was a logical starting point. An outdoor unit on a roof undergoes more vibrations and environmental stress than a unit installed on the ground on a stable base. Over time, this can affect connections, supports, electrical conduits, and the sealing of cable passages.
Checking the request between the indoor and outdoor units
We started by confirming that the indoor unit was indeed sending a request to the outdoor compressor. The remote control responded correctly, the modes were functional, and the wall unit initiated a normal sequence. The problem was therefore more likely in the continuity of the command to the outside or in the outdoor unit’s ability to complete its cycle.
Our technician then checked the communication connections between the two units. On several models of wall-mounted heat pumps, a communication error can be caused by:
- an oxidized terminal;
- a control wire that was loosely connected;
- incorrect polarity after a previous intervention;
- unstable voltage;
- an outdoor electronic board receiving irregular power;
- moisture infiltration into the box;
- a vibration that gradually loosened a connection.
In this case, tests confirmed that power was reaching the unit, but some control connections needed correction. The system did not show a clear and permanent failure; it behaved more like equipment whose external control became unstable depending on conditions.
Inspection of cables, ducts, and connections
The service photo shows ducts and an insulated line running along the roof. On an outdoor installation, these elements must remain well protected. The insulation of refrigerant lines must limit thermal losses, prevent excessive condensation, and protect the copper from direct exposure. Electrical conduits must maintain their integrity to prevent water from entering the connections.
We checked the condition of visible cables, fastening points, entries into the box, and areas where materials could be weakened by sun or frost. This step is especially important on flat roofs, where water, snow, and ice can remain longer than on the ground.
When insulation deteriorates or ducts become less sealed, several problems can arise: loss of performance, corrosion, electrical trips, unstable communication, partial short circuits, vibration noises, or premature compressor wear.
Cleaning and correction of electrical connections
After cutting the power and securing the unit, our technician proceeded to open the service panel. The connections were inspected, cleaned, tightened, and tested. Signs of oxidation around exposed points were taken seriously because corrosion doesn’t need to be severe to create enough electrical resistance and disrupt operation.
We corrected accessible weak points and secured the relevant connections. In this type of repair, precision is essential: a poorly fixed communication wire can give the impression of a defective board, while the problem simply comes from a bad contact. Conversely, a truly damaged board can sometimes be the result of infiltration or an uncorrected unstable power supply.
Our intervention aimed to restore a reliable connection, reduce the risk of outages, and confirm that the outdoor compressor could receive a stable command.
Complete diagnosis, restart, and HVAC maintenance recommendations
Inspection of the outdoor fan and coil
Once the connections were corrected, we continued with the inspection of the outdoor fan and coil. The wall-mounted heat pump must expel or capture heat through its outdoor exchanger. If the coil is dirty, if the fan runs poorly, or if the unit lacks clearance, performance drops quickly.
On this unit, the visible coil showed some dirt buildup, normal for a machine exposed on a rooftop. Even when the unit looks clean from afar, the fins can trap dust, pollen, fine debris, and urban residues. In Montreal, this buildup is common, especially in areas more exposed to wind or traffic.
So we checked:
- the fan rotation;
- the presence of abnormal vibrations;
- the condition of the grille;
- airflow through the coil;
- signs of overheating;
- the noises at startup;
- the general condition of the fins.
The fan was running, but the unit needed preventive cleaning to reduce pressure on the system. A partially clogged coil can raise the compressor temperature, increase electrical consumption, and cause protection shutdowns.
Startup test after correction
After restoring the connections and checking the housing, we restarted the system under real demand. The goal was to observe the complete sequence: indoor unit request, outdoor unit response, fan startup, compressor startup, stabilization, and cycle maintenance.
This time, the wall-mounted heat pump completed its sequence more correctly. The outdoor compressor responded without immediate premature shutdown, and the indoor air production became more stable. We let the unit run long enough to confirm that the intervention was not just fixing a temporary symptom.
A good service call does not end with the first successful startup. The system must be monitored for several minutes, variations checked, noises listened to, thermal response measured, and confirmed that the equipment does not revert to the same behavior after a few cycles.
Performance check in air conditioning and heating
Even though the call mainly concerned a loss of performance and intermittent shutdowns, we validated the two main functions of a wall-mounted heat pump: air conditioning and heating. A heat pump may seem fine in air conditioning but show weakness in heating, or vice versa. The valves, sensors, probes, and control boards do not always respond the same way depending on the selected mode.
We observed the discharge temperatures and the behavior of the outdoor unit. The system regained a more consistent response after the electrical correction. The client immediately noticed that the unit seemed less hesitant and that the air blown better matched the demand.
Why a small electrical failure can create major symptoms
In the HVAC field, some visible failures seem more serious than they really are. A wall-mounted heat pump that stops producing cold or heat can suggest a finished compressor, a refrigerant leak, or a major electronic board. Yet, a simple oxidized connection can cause similar symptoms.
A bad contact can cause:
- irregular communication between units;
- a compressor shutdown;
- an erroneous reading by the electronic board;
- an incomplete start;
- an intermittent failure difficult to reproduce;
- accelerated wear of components;
- a perception of refrigerant shortage.
That is why our technical approach avoids quick conclusions. Before talking about costly replacement, what can be measured, observed, and corrected must be checked.
Recommendations given to the client to prevent recurrence
After the repair, we gave the client simple but important recommendations. A wall-mounted heat pump installation on a roof must be monitored more closely than a unit easily accessible on the ground.
We recommended:
- to have the outdoor unit inspected at least once a year;
- to visually check the condition of ducts and insulation;
- to keep the roof clear around the compressor;
- to report any intermittent shutdown quickly;
- not to let a communication error repeat for weeks;
- to have the outdoor coil cleaned when performance drops;
- to document noises or shutdowns with a video;
- to have connections checked after severe weather or after winter.
These tips are especially relevant in Montreal, Laval, Longueuil, on the North Shore and South Shore, where outdoor equipment undergoes demanding seasonal cycles.
Common mistakes to avoid
The first mistake would be to continue using the heat pump despite repeated shutdowns. An intermittent failure is not a minor detail. It often indicates that a component is deteriorating or that an operating condition is no longer stable.
The second mistake would be to add refrigerant without a complete diagnosis. In this specific case, the main problem was not initially a matter of charge, but of electrical reliability and external control. An unnecessary recharge would not have fixed the shutdowns.
The third mistake would be to neglect the outdoor electrical enclosure. Even if it seems secondary, this enclosure protects essential connections. A rusty screw, a weakened cable entry, or poor sealing can cause a major breakdown.
Finally, it is important not to believe that a rooftop unit is “protected” just because it is off the ground. In reality, it is often more exposed to sun, wind, rain, and temperature fluctuations.
Result of the intervention
By the end of the service call, the wall-mounted heat pump was operating more stably. Correcting the connections, checking the outdoor enclosure, controlling the startup, and inspecting the overall condition restored a reliable operating sequence.
The customer received a clear answer: the machine did not necessarily need to be replaced immediately, but it needed maintenance and monitoring, especially due to its rooftop exposure and visible signs of oxidation. This type of analysis allows for a rational decision: repair what can be fixed now, prevent recurrence, and plan replacement only if major components begin to show end-of-life signs.
AirGreen’s expertise for wall-mounted heat pumps in East Montreal
AirGreen regularly services wall-mounted heat pumps, wall air conditioners, MultiZone systems, central heat pumps, and other HVAC equipment in Rivière-des-Prairies–Pointe-aux-Trembles, Montreal, Laval, Longueuil, the North Shore, and the South Shore. Each service call is different because every installation has its own environment, history, and constraints.
In this case, the most important element was not just the machine itself, but its context: an outdoor rooftop unit, an exposed enclosure, aging connections, intermittent symptoms, and unstable performance. By combining visual inspection, electrical diagnosis, mechanical verification, and operational testing, we were able to restore the system and provide the customer with a concrete solution.
A successful wall-mounted heat pump repair is not just about getting the device running again. It must explain why the breakdown occurred, reduce the risk of the problem recurring, and help the customer extend the life of their equipment. This is exactly what our team accomplished during this service call in Rivière-des-Prairies–Pointe-aux-Trembles.