Repair of a Mitsubishi Electric wall-mounted heat pump in Boisbriand

A wall-mounted heat pump installed near the ceiling that no longer heated with the expected precision

In Boisbriand, our AirGreen team worked on a Mitsubishi Electric wall-mounted heat pump installed high up, very close to the ceiling, in a residential room with light-colored walls. The device looked clean from a distance, with a modern and discreet front, but the customer contacted us because its operation had become irregular: the room took longer to reach the requested temperature, the blown air varied in intensity for no apparent reason, and the unit sometimes produced a slight fan noise accompanied by a damp smell at startup.

This type of situation is common with wall systems that have been operating for several years without thorough internal cleaning. A wall-mounted heat pump can still respond to the remote control, open its flap, start its fan, and give the impression of working normally, while having lost a significant part of its actual efficiency. The device does not always break down suddenly. Often, performance gradually decreases until the customer notices that comfort is no longer stable.

In this specific case, the Mitsubishi Electric unit was installed in an upper corner of the wall, with limited clearance above the device. This setup requires special attention because the air drawn in by the unit circulates near the ceiling. If dust accumulates on the filters, coil, or fan, the machine has to work longer to transfer heat or produce cooling. In Boisbriand, on the North Shore, where temperature variations are marked between winter and summer, a wall-mounted heat pump must be able to react quickly and maintain constant ventilation.

Our goal was clear: to perform a complete diagnosis, identify the real cause of the performance drop, fix internal problems, and restore a reliable, quiet, and more efficient device to the customer.

A Mitsubishi Electric wall unit of about 12,000 BTU

The observed device was a residential Mitsubishi Electric wall-mounted heat pump of the mini-split type, with an estimated capacity of around 12,000 BTU, a common power level for a master bedroom, an office, a closed living room, or a medium-sized living area. This type of unit is known for its precision and quiet operation, but like all HVAC equipment, it heavily depends on internal cleanliness and drainage quality.

The installation had several important particularities:

• The unit was positioned very close to the ceiling.
• Access above the device was limited.
• The air intake could be influenced by dust accumulation near the ceiling.
• The lower flap seemed functional, but the air was distributed unevenly.
• The device was installed in an indoor space where the ventilation noise was easily noticeable.
• The client used the system for both heating and cooling.

Positioning high up is not necessarily a problem in itself. However, when a wall unit is very close to the ceiling, even the slightest airflow restriction can have a more noticeable effect. Accumulation on the filters or fan can reduce the unit’s ability to draw in enough air, which directly harms performance.

Symptoms reported by the client

The client explained that the Mitsubishi Electric wall-mounted heat pump no longer maintained the temperature as well as before. In heating mode, the blown air sometimes seemed hot at startup, then became less consistent. In cooling mode, the room took longer to cool, and the device seemed to run longer for a less convincing result.

A slight ventilation noise was also noticed, especially when the unit switched from one speed to another. The client did not describe a loud squeak or snap, but rather an intermittent vibration, as if the fan wheel was no longer perfectly balanced. A damp smell at startup completed the picture.

These symptoms pointed us toward several possible causes:

• Clogged filters.
• Indoor fan loaded with dust.
• Partially clogged indoor coil.
• Slowed condensate drain.
• Indoor sensor influenced by poor airflow.
• Slightly misaligned motorized flap.
• Communication or control problem, less likely but to be checked.
• Refrigerant charge to be validated only after basic checks.

At AirGreen, we avoid quick conclusions. A serious HVAC repair always starts with symptom analysis and concrete tests.

AirGreen Diagnosis: confirm the airflow before suspecting the refrigerant

The first step was to operate the heat pump in different modes. We tested the response to the remote control, the opening of the flap, the variation of fan speeds, the temperature of the intake air, and the temperature of the blown air. The device responded correctly to commands, indicating that the control board was not completely broken.

The main problem was actually on the airflow side. The flow seemed weaker than expected, and the ventilation was not perfectly even along the entire air outlet. This observation was essential because a drop in heating or cooling does not automatically mean a refrigerant leak. If air no longer passes properly through the indoor coil, the unit loses efficiency even when the refrigeration circuit is still working.

After carefully opening the front panel, we inspected the filters. They were loaded with fine dust. The indoor coil also showed buildup in some sections, especially near areas where air flows the most. The cylindrical turbine, harder to see without partial disassembly, was unevenly dirty. This buildup explained the vibration noise reported by the customer.

Technical cleaning of the turbine and coil

We performed a thorough technical cleaning of the accessible areas of the indoor unit. On a Mitsubishi Electric wall-mounted heat pump, the turbine plays a crucial role. When the blades of the fan wheel accumulate dust, they move less air and can lose their balance. The motor keeps running, but the volume of air blown decreases and the noise increases.

The turbine cleaning was done gradually to remove deposits between the blades. This step requires precision because incomplete cleaning can leave the wheel unbalanced. We also protected the wall and surfaces around the unit to prevent any splashing during the work.

The indoor coil was then cleaned using a method suited to the condition of the unit. The fins must remain straight to allow good airflow. Cleaning that is too aggressive can damage them; cleaning that is too light does not fix the problem. Our intervention aimed to restore effective heat exchange without weakening the unit.

After this step, the airflow was already more stable. The unit was breathing better, and the fan noise had decreased.

Fixing a slow drain and cleaning the condensate pan

The musty smell reported by the customer led us to check the condensate pan and the drainage line. In cooling mode, the indoor unit removes moisture from the air. This water must drain properly outside or to a designated drainage point. When a drain becomes partially blocked, water can stagnate in the pan, create odors, and eventually cause a leak.

In this case, the drain was not completely clogged, but the flow was too slow. We cleaned the accessible section of the pan, cleared accumulated deposits, and confirmed that water drained faster after intervention. This correction was important, especially in a wall installation located high up. A leak on a unit placed near the ceiling can damage the wall, paint, furniture, or floors before the client even understands the source of the problem.

We also recommended the client monitor the following signs after the repair:

• Return of a damp smell at startup.
• Water drops under the unit.
• Bubbling or flowing noise in the drain line.
• Sudden drop in airflow.
• Flap that no longer opens fully.

Replacement of an unstable indoor thermistor

During the tests, we also noticed that the temperature reading varied less stably than expected. On a wall-mounted heat pump, indoor thermistors transmit essential information to the control board. They allow the device to understand the return air temperature and the state of the indoor coil.

A thermistor that reads poorly can cause irregular cycles. The device may reduce its power too early, unnecessarily prolong some cycles, or seem unresponsive to the setpoint. After verification, we replaced an unstable indoor thermistor, then restarted the operation tests.

The result was more consistent: the wall-mounted heat pump responded better to setpoint variations, the ventilation adjusted more naturally, and the overall behavior was more predictable. This replacement completed the repair, as cleaning alone would have improved airflow but would not have corrected the unstable temperature reading.

Final check in heating and cooling

Once cleaning was completed, the drain corrected, and the thermistor replaced, we tested the device in heating and cooling modes. The ventilation was more uniform, the noise had decreased, the flap worked properly, and the blown temperature was more stable.

We also observed the unit's behavior during a full cycle. This step is essential because a device may seem to work during the first few minutes, then reveal a problem when pressure, temperature, or ventilation stabilize. In this case, the operation remained steady.

No signs justified an immediate addition of refrigerant. The main problem came from the indoor unit: restricted airflow, dirty fan, slow drain, and unstable temperature reading. By addressing these elements in the right order, we avoided unnecessary intervention on the refrigeration circuit.

An HVAC repair that demonstrates the importance of diagnosis on wall systems

Why a wall-mounted heat pump can lose performance without showing an error code

Many customers expect a device to display a code as soon as a problem appears. However, a wall-mounted heat pump can lose a lot of efficiency without generating a code. A dirty fan, a clogged coil, or a slow drain do not always trigger an alarm. The device keeps running but heats less effectively, cools more slowly, and becomes noisier.

In this intervention in Boisbriand, the Mitsubishi Electric unit was not completely broken. It was weakened by a combination of internal problems. This is precisely the kind of situation that requires a full diagnosis rather than a quick glance.

Mistakes to avoid when the heat pump blows less strongly

Cleaning only the filters

Filters are accessible and should be cleaned regularly, but they represent only part of the system. The fan and coil can be dirty even when the filters look acceptable.

Assuming an immediate refrigerant shortage

A drop in efficiency can be caused by poor airflow. Adding refrigerant without a full diagnosis can create additional problems.

Ignoring humidity odors

A smell at startup can indicate stagnation in the condensate tray or a slow drain line. This sign deserves a check.

Waiting for the device to stop completely

A heat pump that has been malfunctioning for several weeks can put extra strain on certain components. Early intervention often helps avoid a more costly breakdown.

Forcing the settings

Increasing the setpoint significantly does not fix an air restriction or an unstable sensor. It can simply prolong cycles and increase consumption.

The importance of maintenance in homes in Boisbriand and the North Shore

In Boisbriand and on the North Shore, wall-mounted heat pumps are often used almost year-round: heating in spring and fall, air conditioning in summer, and supplemental heating in winter depending on the system's capacity. This intensive use requires more thorough maintenance than just occasional filter cleaning.

Internal cleaning every 12 to 18 months can make a big difference, especially when the unit serves a heavily used room. Dust, humidity, pet hair, fine particles, and cooking residues can accumulate inside the device. Over time, these deposits reduce performance and alter the system's behavior.

We see the same type of situation in Montréal, Laval, Longueuil, on the South Shore, and in many areas of the North Shore. A well-installed wall unit can lose efficiency if it is not deeply maintained.

What the customer gained after our intervention

At the end of the service call, the customer had a quieter, more stable, and more efficient wall-mounted heat pump. The airflow was significantly better. The damp smell was reduced thanks to cleaning the tray and fixing the drain. The temperature reading was more reliable after replacing the thermistor. The unit responded better to controls and maintained the room more consistently.

The customer also received clear explanations about the system's condition. The unit did not need to be replaced immediately. It required reconditioning, technical cleaning, and targeted correction. This distinction is important: a good HVAC repair must protect the customer's budget while restoring real comfort.

Our AirGreen approach to wall-mounted heat pump repairs

At AirGreen, we work on wall-mounted heat pumps, wall air conditioners, multi-zone systems, central heat pumps, and other residential and commercial HVAC equipment. Every service call is handled with a comprehensive approach: symptoms, diagnosis, checks, repair, testing, and recommendations.

In this repair of a Mitsubishi Electric wall-mounted heat pump in Boisbriand, the solution was not found in a single part. The problem was a combination of reduced airflow, internal clogging, slow drainage, and unstable temperature readings. By correcting these elements, we restored much more reliable operation.

For customers in Montréal, Laval, Longueuil, the North Shore and the South Shore, this type of intervention shows the importance of not waiting for a complete breakdown. A heat pump that heats less, blows less strongly, smells damp, or becomes noisier is already sending signals. A quick diagnosis often helps extend the system's lifespan and avoid higher costs.