When the Cool Air Doesn’t Come: Understanding the Unexpected Performance of a Wall-Mounted Air Conditioner
At AirGreen, we respond weekly to homeowners and building managers across Montréal, Laval, Longueuil, the North Shore, and the South Shore who are dealing with a wall-mounted air conditioner that just doesn’t seem to be delivering its full potential. Even with a recent installation or a reputable brand like Fujitsu, Gree, Sharp, or Mitsubishi, the unit simply isn’t cooling the way it should. So what’s behind this drop in performance? Is it a manufacturing defect, a poor installation job, or simply a lack of maintenance?
In this article, we uncover the hidden reasons why a wall-mounted air conditioner might not be working as efficiently as expected… and more importantly, how we can resolve the issue thanks to our local HVAC expertise.
A Clogged Filter: The Number One Enemy of Performance
One of the most common issues we encounter during HVAC maintenance visits in Montréal and the South Shore is dust buildup in the filters. A clogged filter restricts airflow, forcing the system to work harder while producing weaker results.
Warning Signs:
-
Weak airflow even at max setting
-
Unusual noises
-
Higher electricity bills
AirGreen Tip: Clean your filters every 3 months. During our preventive maintenance visits, we clean not only the filters, but also the coils, fans, and drainage trays.
Poor Installation: A More Common Problem Than You’d Think
Recently in Laval, we were called in to assess a Gree wall-mounted heat pump installed by another company. The issue? The drainage pipe had a reversed slope, causing moisture to back up into the system. The result? Lower cooling capacity, strange noises, and excessive condensation.
What We Observe on the Job:
-
Excessively long refrigerant lines: affects refrigerant efficiency.
-
Poor or missing insulation on lines: causes cooling loss.
-
Incorrect indoor unit placement: leads to poor air distribution.
At AirGreen, we rigorously follow each manufacturer’s guidelines and tailor every installation to the building’s specific conditions, including height, sunlight exposure, load-bearing walls, and more.
Incorrect or Insufficient Refrigerant Charge
Without the exact right amount of refrigerant (R-32, R-410A, etc.), your system simply can’t perform properly. Too much creates excess pressure in the compressor; too little lowers thermal efficiency.
Real Case:
In Longueuil, a client with a Sharp ZU1 18,000 BTU wall-mounted AC complained of slow cooling. Upon inspection, we detected a micro leak in a weld joint. After pressure testing, we purged and recharged the system. The result: restored performance and noticeable energy savings.
Worn-Out or Poorly Maintained Internal Components
A dirty fan, a faulty variable-speed motor, or a miscalibrated temperature sensor can all keep the unit from properly detecting temperature and adjusting airflow.
At AirGreen, we use advanced diagnostic tools during our HVAC maintenance visits. In the North Shore, for example, we recently repaired a Tosot 24,000 BTU wall-mounted AC whose indoor fan was running at just 50 % capacity due to heavy buildup.
Incorrect Remote Settings
It may sound trivial, but in several cases, the problem lies simply in wrong operating mode selection. The unit is set to "fan" instead of "cool," the temperature is too close to the ambient temperature, or a timer is active without the user realizing it…
Our technicians always take the time, after every HVAC installation, to train customers on how to properly use their wall-mounted heat pump.
A Poorly Insulated or Oversized Room
Choosing the right BTU capacity is essential. A unit that’s too small for the space will never achieve the desired temperature, no matter the quality or brand.
Examples:
-
12,000 BTU for a poorly insulated 900 sq. ft. condo in Montréal = not enough.
-
9,000 BTU in a well-insulated bedroom with little sunlight = sufficient.
Our evaluations always include:
-
Insulation analysis
-
Window orientation
-
Air volume to be cooled
-
Presence of heat-generating appliances
Neglected Maintenance Reduces System Lifespan
We can’t stress it enough: regular HVAC maintenance is crucial. At AirGreen, we offer annual maintenance contracts for homeowners and commercial buildings. Thanks to this, our clients in the South Shore and Montréal avoid costly breakdowns and extend their units’ lifespans.
The AirGreen Advantage: Beyond Diagnosis, Long-Term Solutions
Across hundreds of service calls in Laval, Brossard, Repentigny, Saint-Lambert, and Blainville, we’ve resolved complex cases of underperforming wall-mounted air conditioners. What sets us apart is our ability to combine technical expertise, state-of-the-art tools, and real-world, hands-on experience.
Practical Tips to Ensure Your Wall-Mounted AC Performs Optimally
-
Have the unit inspected annually by a certified technician
-
Clean the filters at least 4 times a year
-
Don’t block the indoor unit with furniture or curtains
-
Make sure the room being cooled is properly sealed
-
Check your remote settings regularly
AirGreen: Your Trusted HVAC Partner in Greater Montréal
Our service is always personalized, transparent, and results-driven. Whether you own a condo in Ville-Marie, a single-family home in Boisbriand, or manage a building in Saint-Hubert, we’ll assist you with diagnosing, repairing, maintaining, or replacing your wall-mounted air conditioner.
We’re here to help you get back the comfort you deserve—all year long.