Too much power, not enough comfort: the hidden consequences of an oversized wall-mounted heat pump
At AirGreen, we’re often called in to fix mistakes made during rushed or poorly planned HVAC installations. One of the most striking examples is a home in Bois-des-Filion, on the North Shore, where a Gree Extreme 24,000 BTU wall-mounted heat pump had been installed in a single-storey house of just 900 sq. ft.
On paper, more power might seem like a good thing. In reality, an oversized heat pump can seriously reduce comfort, waste energy, and cause massive utility bills. This is the true story of a Quebec family who learned the hard way… before finding relief through our intervention.
Unstable comfort, rising energy use, and constant frustration
Short cycles, lingering humidity, and temperatures that never stabilize
From the very first weeks, the homeowners noticed that the wall-mounted AC was constantly turning on and off, sometimes every five minutes. The air felt cool, but the heavy, muggy sensation didn’t go away. In winter, same issue: the heat would rise quickly, then drop sharply, creating an uncomfortable, even stifling atmosphere.
Plus, the windows were constantly fogged up, even in below-zero weather. This pointed to excess indoor humidity, a typical sign of a system that doesn’t run long enough to properly dehumidify.
A much higher Hydro-Québec bill than expected
After the first winter, the family discovered a 47% spike in their energy consumption, even though they had purchased an Energy Star–certified unit, known for efficiency.
That’s when our team was called in to investigate.
The AirGreen diagnosis: power output mismatched to air volume
A thermal miscalculation right from the start
We quickly realized that no load calculation had been done before the install. The previous contractor had simply recommended “the biggest unit available, just to be safe.” But that logic is not only flawed — it’s counterproductive in residential HVAC.
For a 900 sq. ft. home with standard insulation, a 12,000 to 15,000 BTU wall-mounted heat pump would have been sufficient. The Gree Extreme 24,000 BTU was oversized by 60 to 100%, which led to short and inefficient cycles.
Short cycles = lower efficiency and premature wear
A heat pump that constantly turns on and off never reaches its optimal performance zone. That reduces real energy efficiency, increases wear on the compressor, and wipes out your expected savings.
Our team recorded temperature swings of over 2.5°C every 10 minutes, with humidity levels peaking at 68%. The air remained uncomfortable — despite all that power.
Our AirGreen solution: unit removal, precise recalculation, full reinstall
Step 1: detailed thermal evaluation of the home
We started by performing a complete load calculation, accounting for:
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The home’s orientation
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Insulation (walls, ceiling, windows)
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Air volume per floor
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Occupancy habits (day/night presence)
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Actual heating and cooling needs
The result: the recommended load was 13,800 BTU for the affected areas.
Step 2: replacing with a properly sized unit
We removed the Gree Extreme 24,000 BTU and replaced it with a Gree Lomo 15,000 BTU, better suited to the home’s air volume. We also repositioned the indoor unit to ensure better air distribution, and optimized the settings for auto mode, defrost, and auto-restart.
Step 3: results observed in the very first week
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34% reduction in energy consumption in the first month
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Room temperature stable within ± 0.5°C
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Humidity level maintained around 42%
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Significant improvement in perceived comfort
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40% reduction in operating noise
What to remember to avoid this kind of situation
Never oversize a wall-mounted heat pump
Contrary to popular belief, bigger is not always better. An oversized heat pump:
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Can’t dehumidify properly
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Causes uncomfortable temperature swings
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Wears out its components faster
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Consumes more than a properly sized unit
Always demand a thermal evaluation before installation
At AirGreen, every project starts with a precise thermal load assessment. We don’t just sell machines. We deliver HVAC solutions that are sustainable, tailored, and cost-effective, built for the real needs of your home.
Trust a local, certified, and well-trained team
Our technicians understand the climate-specific challenges of Montréal, Laval, Longueuil, the North Shore, and the South Shore. They’re trained to recommend the right capacity, in the right location, with the right settings, for peak performance.
Conclusion
An oversized wall-mounted heat pump can turn your home into a true energy nightmare. Short cycling, discomfort, humidity, noise, sky-high bills… the consequences are real. Fortunately, they can be avoided.
At AirGreen, we do things differently. We prioritize precision, technical expertise, and client care to ensure long-lasting comfort and controlled energy use.
Before installing a wall-mounted heat pump, talk to our experts. Your comfort depends on it.