A Mitsubishi wall unit installed in a narrow space that had lost its efficiency
At the Old Port of Montreal, our AirGreen team was called for a service call on a Mitsubishi Mr. Slim wall-mounted heat pump installed in a particularly demanding indoor area: a narrow, high space between two close walls, with the unit positioned very close to the ceiling. This type of location requires a more careful analysis than a standard wall installation in a living room or bedroom, as air circulation, maintenance access, and condensate drainage can all become critical factors.
The client contacted us because the device was still working but no longer delivering the expected performance. The room did not cool as quickly as before, the airflow seemed weak, and a musty smell appeared at startup. The indoor unit, visibly older, showed typical signs of a wall system that had served for several years: yellowed casing, dust-loaded grilles, limited access above the device, and restricted space around the front.
In the Old Port buildings, HVAC installations often face particular constraints. Thick walls, high ceilings, narrow stairs, successive renovations, and atypical spaces sometimes complicate equipment maintenance. Here, the wall-mounted heat pump was installed in a vertical recess above a passageway, which limited direct access to the front and above the unit. Our mandate was therefore clear: diagnose the loss of performance, check if a repair was possible, and restore reliable operation without recommending an unnecessary replacement.
Symptoms reported by the client
Before our arrival, the client had noticed several changes in the device's behavior:
- The airflow was much weaker than before.
- The blown air sometimes seemed lukewarm or irregular.
- A musty smell appeared at startup.
- The system ran for a long time without quickly reaching the setpoint.
- A slight vibration noise was heard inside the casing.
- The air outlet flap no longer distributed air as well in the space.
- The device seemed noisier when switching to a higher ventilation speed.
These symptoms are common on an older Mitsubishi Mr. Slim wall heat pump when internal maintenance has not been done thoroughly for some time. Filters can be cleaned regularly by the client, but the turbine, coil, condensate tray, and drain require a much more comprehensive technical intervention.
Visual inspection of the indoor unit
Upon arrival, we began by observing the installation in its real context. The device was placed very high, almost at the level of the ceiling moldings. This proximity reduces the available space for air intake. A wall unit must draw in ambient air, pass it through its internal coil, then redistribute it through the lower flap. If air has difficulty entering the device, efficiency drops even if the compressor and outdoor unit are still working.
The tight space around the unit also posed a challenge for cleaning. In a standard room, the technician can stand directly in front of the device with enough clearance. Here, the intervention had to be done precisely, protecting the walls, moldings, decorative elements, and the area under the unit.
An old casing, but a machine still recoverable
The aging appearance of the device did not automatically mean it needed to be replaced. At AirGreen, we always assess the actual condition of the machine before drawing conclusions. In this case, the front panel was worn, but the unit still responded to commands. The ventilation motor started, the flap reacted, and the system still produced a temperature difference. The problem therefore seemed more related to internal blockage, slow drainage, or lack of technical maintenance than a major compressor failure.
HVAC diagnosis: understanding the loss of efficiency
Our diagnosis was carried out in stages to avoid confusing a performance drop caused by dirt with a refrigeration or electronic failure.
Filter check
The filters were loaded with dust. They were not completely blocked, but they already limited airflow. On a wall-mounted heat pump, the filters are the system's first line of defense. When too dirty, dust more easily reaches the coil and turbine, gradually reducing efficiency.
Inspection of the indoor coil
The evaporator coil showed a buildup of fine dust between the fins. This deposit prevents air from properly passing through the exchanger. The result is often misleading: the device seems to produce cold or heat, but the volume of air distributed is insufficient. The customer therefore experiences reduced comfort, even if the machine is not completely broken.
Inspection of the blower turbine
The turbine was one of the most affected components. Its blades were clogged with a mixture of dust and moisture. On a wall-mounted unit, this part is essential: it pushes air into the room. When dirty, it moves less air, becomes noisier, and can cause vibrations.
In this case, the turbine's dirt buildup directly explained the low airflow and intermittent noise. The system worked longer to achieve a less satisfactory result.
Inspection of the condensate tray and drain
Since a damp smell was present at startup, we inspected the condensate tray. It contained deposits and the drain flow was slowed. It was not a complete blockage, but the risk of water leakage was real, especially during hot and humid days when the device produces more condensation.
A partially clogged drain can cause several problems: odors, stagnant humidity, dripping under the unit, wall stains, and in some cases, a safety shutdown if the system is equipped with a protection device.
Service performed by AirGreen
After the diagnosis, we proceeded with a repair focused on the complete restoration of the indoor unit.
Complete cleaning of the filters and front panel
The filters were removed, cleaned, and inspected. The unit's front panel was also carefully cleaned, especially around the intake and outlet grilles. This step not only improves the appearance of the device but also reduces the amount of particles recirculated.
Indoor coil cleaning
We then cleaned the evaporator coil. This operation requires precision, especially on a unit installed high up in a tight space. The fins must be cleaned without being bent, and the cleaning water must be controlled to avoid damaging the wall or surrounding surfaces.
Once the coil was cleared, air could flow much more freely through the device. This improved the blowing temperature and reduced the time needed to heat or cool the space.
Thorough turbine cleaning
The blower turbine was deeply cleaned. This is often the step that most changes the behavior of an old wall unit. After cleaning, the airflow was noticeably stronger and more consistent. The fan noise also decreased because the wheel was spinning more evenly.
Drain rinsing and condensate tray cleaning
We cleaned the condensate tray, then rinsed the drain line to restore smoother flow. This step helped reduce the musty smell and limit the risk of future leaks. We also checked the drain’s behavior during operation to confirm that water was draining properly.
Final operational tests
After the intervention, we restarted the Mitsubishi Mr. Slim wall-mounted heat pump and tested several fan speeds. The airflow was more stable, the blowing temperature more consistent, the noise reduced, and the client noticed a quick improvement in comfort.
What this repair in Old Port reveals about wall units in tight spaces
This intervention shows that an effective HVAC repair is not just about replacing a part. In many cases, the real problem is related to the installation environment and the internal maintenance of the device.
Mistakes to avoid with an old wall-mounted heat pump
Waiting for the device to stop completely
A drop in airflow, a musty smell, or a turbine noise are already important warning signs. Waiting for a complete breakdown can turn a simple fix into a much more expensive repair.
Clean only the filters
Filters are important, but they are not enough. The fan, coil, and drain must also be maintained. A unit can have relatively clean filters and still be ineffective if the fan is clogged.
Immediately thinking of refrigerant shortage
Less cold air does not automatically indicate a refrigerant leak. In this case, the system was still producing, but the air circulation was poor. A methodical diagnosis helps avoid unnecessary interventions.
Neglecting access around the unit
When a unit is installed very close to the ceiling or in a recess, maintenance becomes more difficult. Therefore, more thorough cleanings should be planned to compensate for these constraints.
Recommendations given to the client
At the end of the intervention, we recommended periodic maintenance suited to the unit’s age and location. For a unit installed in a tight space, a thorough cleaning should be done regularly, especially if the unit is used for both cooling and heating.
We also advised the client to watch for certain signs:
- Return of a musty odor.
- Reduced airflow.
- Rubbing noise in the unit.
- Water drops under the unit.
- Damper that moves with difficulty.
- Unstable blowing temperature.
- Cycles too long to reach the setpoint.
AirGreen, HVAC service specialist in Greater Montreal
At AirGreen, we service wall-mounted heat pumps, wall air conditioners, central heat pumps, MultiZone systems, electric furnaces, and other HVAC equipment in Montreal, Laval, Longueuil, on the North Shore and the South Shore. Every service call is handled with a comprehensive approach: inspection, diagnosis, intervention, final testing, and practical recommendations.
This repair of a Mitsubishi Mr. Slim wall-mounted heat pump at the Old Port restored effective airflow, reduced odors, fixed drainage, and improved comfort without immediate replacement of the unit.