By Tarik, Senior HVAC Specialist at Air Climatisation VS
Quick Takeaway
Your home’s square footage is a useful starting point when comparing central air conditioners, but it cannot determine the correct unit size by itself. Insulation, windows, ceiling height, sun exposure, home layout, ductwork and airflow all affect how much cooling capacity a Lachine home actually needs.
The right central AC should be selected according to the home’s calculated cooling load and the existing system’s ability to distribute air, not simply by choosing the largest unit available.
What Central AC Size Actually Means
Central air conditioners are commonly described using BTUs per hour or tons of cooling capacity.
A BTU, or British thermal unit, is a measurement of heat. In air conditioning, BTUs per hour describe how much heat the system can remove from the home during operation.
One ton of cooling capacity equals 12,000 BTUs per hour.
| Nominal AC capacity | Cooling capacity |
|---|---|
| 1.5 tons | 18,000 BTU/h |
| 2 tons | 24,000 BTU/h |
| 2.5 tons | 30,000 BTU/h |
| 3 tons | 36,000 BTU/h |
| 3.5 tons | 42,000 BTU/h |
| 4 tons | 48,000 BTU/h |
| 5 tons | 60,000 BTU/h |
The word “ton” does not refer to the physical weight of the air conditioner. It is simply another way of describing cooling capacity.
Under rated operating conditions, a higher nominal tonnage represents greater heat-removal capacity per hour. That does not automatically provide better comfort because actual performance also depends on the home’s cooling load, indoor coil, blower, airflow and duct system.
Square Footage Is Only the Starting Point
Homeowners often begin by searching for central AC size by square footage because floor area is easy to measure. A larger home generally has more interior space and building surface to cool, so square footage clearly matters.
The problem is that square footage does not describe how quickly heat enters the building.
Consider two 1,800-square-foot homes in Lachine.
The first is a shaded bungalow with updated windows, improved attic insulation and moderate ceiling heights. The second is a two-storey home with large west-facing windows, an open staircase, a finished basement and an upper floor exposed to afternoon sun.
Their floor areas may be similar, but their cooling loads can be substantially different.
Lachine also includes many older detached and semi-detached homes that have been renovated over time. Additions, finished basements, converted rooms, replacement windows and modified ductwork can all change how the home gains and distributes heat.
That is why square-footage charts should be treated as preliminary screening tools. They may help a homeowner recognize the general class of equipment being discussed, but they cannot produce a final installation recommendation.
Why Fixed Square-Footage Charts Can Be Misleading
Many online charts assign a specific air conditioner size to a range of square footage. They can provide a preliminary reference, but they may create false confidence when they do not account for the home’s construction, sun exposure, layout and airflow conditions.
A chart cannot tell whether the home has:
- Poor or upgraded insulation
- Large south-facing or west-facing windows
- Significant air leakage
- High ceilings
- An open stairwell
- A finished basement
- A rear extension
- Limited return-air capacity
- Restrictive ductwork
- Persistent upper-floor comfort problems
These conditions can change the cooling requirement even when the home’s total floor area remains the same.
Natural Resources Canada recommends determining a home’s cooling load using a recognized sizing method instead of relying on simple rules of thumb. The selected system should closely match that calculated load rather than substantially exceeding it.
Homeowners can review the federal guidance in Air Conditioning Your Home.
Factors That Change Your Home’s Cooling Load
A proper central air conditioner recommendation considers how the building collects, retains and distributes heat.
Insulation and Air Leakage
Insulation slows heat transfer through the attic, walls and other parts of the building envelope. Air sealing reduces the amount of warm, humid outdoor air entering through gaps and penetrations.
An older Lachine home with limited attic insulation or noticeable air leakage may require more cooling than a similarly sized home that has received insulation and air-sealing improvements.
Recent upgrades can also mean that the capacity of the old air conditioner is no longer the best reference for its replacement.
Windows, Orientation and Shading
Windows can create a substantial cooling load, especially when they receive strong afternoon sun.
The cooling requirement may be affected by:
- Total window area
- Window orientation
- Glass type
- Interior blinds or curtains
- Exterior shading
- Nearby trees
- Adjacent buildings
- Patio doors and rear extensions
A home with large west-facing windows may experience a stronger late-day cooling demand than an otherwise similar home with limited direct exposure.
Ceiling Height and Open Interior Spaces
Square footage measures floor area, not the volume of air inside the home.
Rooms with high or vaulted ceilings contain more air and often have more exposed wall and roof area. Open staircases and connected living spaces also allow warm air to move between floors.
A two-storey home may therefore need a different cooling and airflow strategy from a bungalow with the same floor area.
Number of Floors
Upper floors often experience greater solar heat gain because they are closer to the attic and roof. Warm air moving upward through the home can add to the temperature difference between floors.
When the upper floor remains uncomfortable, installing a larger outdoor unit is not necessarily the solution. The problem may involve limited supply airflow, inadequate return air, duct leakage, poor balancing or thermostat placement.
For larger ducted homes, central HVAC planning in Beaconsfield provides a closer look at how home layout, ductwork and airflow affect whole-home comfort.
Basements and Finished Living Areas
An unfinished basement usually has a different cooling load from an above-grade living area. It may remain naturally cooler because much of the foundation is below ground.
A finished basement still affects system planning, especially when it contains bedrooms, a family room, a home office or another frequently used space.
However, basement floor area should not automatically be treated exactly like above-grade living space. The assessment should consider how the basement is insulated, how it is used and whether the duct system was designed to serve it.
Additions and Renovations
Many homes change considerably after the original HVAC system is installed.
Common modifications include:
- Rear additions
- Finished basements
- Converted garages
- Expanded kitchens
- Added bathrooms
- New bedrooms
- Removed interior walls
- Open-concept renovations
- New windows
- Improved insulation
A renovation can change both the cooling load and the way air moves through the home.
Removing walls may improve circulation in one area but create new temperature differences elsewhere. Adding finished space may also increase the demand on ducts that were designed for the original layout.
Occupants, Lighting and Appliances
People, lighting, cooking equipment, computers and other appliances release heat indoors.
These internal heat gains may be particularly relevant in homes with:
- Large families
- Home offices
- Extensive kitchen use
- Entertainment equipment
- Frequent gatherings
- Several heat-producing devices operating together
A home office with multiple monitors and computer equipment can create a noticeably different load from a lightly used bedroom of the same size.
Bigger Central AC Is Not Automatically Better
Choosing a larger unit may seem like a safe way to avoid insufficient cooling. In practice, excessive capacity can create its own comfort problems.
Short Operating Cycles
An oversized air conditioner may lower the temperature near the thermostat quickly and then shut off. These short operating cycles can repeat frequently throughout the day.
The thermostat may reach its setting, but the system may not run long enough to distribute air evenly throughout the house.
Weaker Humidity Removal
Central air conditioning removes moisture while the indoor coil is cold and air is passing across it.
When an oversized system stops too quickly, it may satisfy the thermostat before removing enough humidity. The home can feel cool but damp or clammy.
This is especially relevant during humid Greater Montreal summers, when indoor comfort depends on both temperature and moisture control.
Uneven Temperatures
A fast temperature drop near the thermostat does not prove that distant rooms or upper floors have received enough conditioned air.
The thermostat may end the cooling cycle while bedrooms, closed rooms or the second floor remain warmer.
Higher Airflow Requirements
Larger cooling equipment generally requires more airflow. If the furnace blower, indoor coil, supply ducts or return ducts cannot support that airflow, the system may experience increased static pressure and reduced performance.
Installing a larger outdoor unit on restrictive ductwork is like connecting a larger pump to pipes that cannot carry additional flow. The equipment may offer more nominal capacity, but the distribution system remains the bottleneck.
Additional Noise
Higher airflow through restrictive ducts, registers and grilles may create noticeable air noise.
Noise can also occur when the system is trying to move more air than the existing distribution network was designed to handle.
What Happens When Central AC Is Too Small
An undersized air conditioner may operate for long periods and still struggle to maintain the target indoor temperature during design conditions.
Possible signs include:
- The indoor temperature continues rising during hot weather
- The system runs almost continuously without approaching the thermostat setting
- Certain parts of the home remain consistently uncomfortable
- The system cannot recover after a large indoor temperature increase
However, long operating cycles are not automatically evidence that the unit is undersized.
A properly selected air conditioner may run for extended periods during hot and humid weather. Longer, steadier cycles can support more even temperatures and better humidity removal than repeated short cycles.
Before blaming equipment capacity, an assessment should also consider:
- Dirty or restrictive filters
- Closed or obstructed registers
- Duct leakage
- Insufficient return air
- Poor air balancing
- Thermostat location
- Excessive solar heat gain
- Changes to the home since the system was installed
The goal is not to make the air conditioner stop as quickly as possible. The goal is to maintain comfortable temperature and humidity throughout the home without unnecessary cycling.
Ductwork Can Limit the AC Capacity You Can Install
A central air conditioner is only one part of a complete cooling system.
The outdoor unit rejects heat outside. The indoor coil absorbs heat from the air. The blower moves that air through the supply ducts and back through the return system.
All these components must work together.
Supply Duct Capacity
Supply ducts carry conditioned air to individual rooms. If the ducts are too restrictive, poorly arranged or damaged, increasing the nominal AC capacity may not improve comfort.
The additional airflow demand may instead create higher pressure, noise and uneven delivery.
Return-Air Capacity
Return ducts bring air back to the furnace or air handler. Limited return capacity can restrict the entire system, even when the supply side appears adequate.
Bedrooms with closed doors may also need proper return paths so air can continue circulating.
Furnace or Air-Handler Blower Capacity
When central AC is added to an existing furnace, the blower must be able to provide the airflow required by the indoor coil and outdoor unit.
The technician should review:
- Available blower speeds
- Blower performance under actual static pressure
- Indoor coil requirements
- Supply and return restrictions
- Equipment control settings
For homes that already have a ducted system, the guide to central heat pump planning in Pointe-Claire explains why existing ducts should be evaluated rather than automatically assumed to be suitable.
Indoor Coil Compatibility
The indoor evaporator coil must be properly matched with the outdoor air conditioner and compatible with the available airflow.
Replacing only the outdoor unit without considering the coil, blower and controls can create an incomplete equipment combination.
Homeowners planning a replacement can review the broader considerations involved in central HVAC installation and upgrade planning.
Replacing the Old AC With the Same Size Is Not Always Correct
The capacity shown on the old air conditioner’s data plate is useful information, but it is not proof that the replacement should be identical.
The previous system may have been:
- Correctly sized
- Oversized from the beginning
- Undersized for the original home
- Selected using an outdated rule of thumb
- Installed before a major renovation
- Installed before insulation or window upgrades
The building may also have changed since the previous system was selected.
Common changes in Lachine homes can include:
- Finished basements
- Rear extensions
- Converted garages
- Added bedrooms
- New windows
- Improved insulation
- Air-sealing work
- Modified ductwork
- New return-air grilles
- Changes in how rooms are used
Past performance should still be discussed. If the old system maintained temperature, controlled humidity and operated with reasonable cycles, that information is valuable.
If it short-cycled, left the upper floor warm or struggled with humidity, repeating the same nominal capacity may repeat the same problem.
What an HVAC Specialist Should Check
A central AC recommendation should explain more than the number printed on the outdoor unit.
During an on-site assessment, the technician should review the home as a complete system.
Building Conditions
The assessment should consider:
- Conditioned floor area
- Number of storeys
- Ceiling heights
- Insulation levels
- Air leakage
- Window area and orientation
- Exterior shading
- Basement use
- Additions or renovations
Existing Mechanical Equipment
The technician should identify:
- Furnace or air-handler model
- Blower capacity
- Existing indoor coil
- Current outdoor unit
- Thermostat and controls
- Electrical requirements
- Available equipment space
- Drainage requirements
Air Distribution
The inspection should also consider:
- Supply duct condition
- Return-air capacity
- Register placement
- Airflow to problem rooms
- Closed-door pressure issues
- Duct leakage
- Static pressure
- Air balancing
Existing Comfort Problems
Homeowner observations provide useful diagnostic information.
The technician should ask whether:
- The upper floor becomes too warm
- Certain rooms receive weak airflow
- The basement becomes too cold
- Humidity remains high
- The old unit starts and stops frequently
- The system runs continuously without maintaining temperature
- Airflow is noisy
- Comfort changes when bedroom doors are closed
With more than 9,500 successful installations, Air Climatisation VS evaluates equipment capacity and airflow conditions before recommending a central AC size. The goal is to select equipment that matches the home and can operate properly with the existing or proposed duct system.
Questions to Ask When Comparing Recommendations
When reviewing central AC proposals, ask how the recommended capacity was determined.
Useful questions include:
- Was the recommendation based only on square footage?
- Were windows, insulation and sun exposure considered?
- Was the existing ductwork inspected?
- Can the furnace blower support the required airflow?
- Is the indoor coil properly matched to the outdoor unit?
- Were existing humidity or comfort problems considered?
- Is the recommendation based on the old equipment size or the current cooling load?
- Will any duct or return-air changes be required?
- How will the system serve the upper floor and distant rooms?
A contractor should be able to explain why the proposed capacity fits the home. A reliable answer should include building and airflow factors, not merely a square-footage rule copied from a chart.
The Practical Way to Use Square Footage
Square footage remains useful when choosing a central air conditioner, but it should be used in the correct order.
First, measure the conditioned areas of the home and identify how each space is used.
Next, document the conditions that affect heat gain:
- Floors and ceiling heights
- Windows and exposure
- Insulation
- Basement configuration
- Renovations
- Occupancy
- Existing comfort problems
Then, have the cooling load and existing air-distribution system evaluated.
Only after those steps should a specific tonnage be selected.
The objective is not to install the biggest central AC that the house can physically accommodate. It is to choose equipment that closely matches the home’s cooling demand and can operate properly with the indoor coil, blower and ductwork.
Frequently Asked Questions
Can square footage tell me exactly what central AC size I need?
No. Square footage can provide an initial reference, but it does not account for insulation, air leakage, windows, sun exposure, ceiling height, layout, humidity, ductwork or airflow. A final recommendation should be based on the home’s cooling load and mechanical system.
Is an oversized air conditioner better during very hot weather?
Not necessarily. An oversized unit may cool the area near the thermostat quickly but shut off before removing enough humidity or distributing air evenly. The correct objective is capacity that closely matches the design cooling load, not maximum available capacity.
Should my replacement central AC have the same capacity as the old one?
The old unit’s capacity should be considered, but it should not automatically determine the replacement. The previous system may have been incorrectly sized, and renovations, insulation improvements, new windows or duct modifications may have changed the home’s cooling requirements.
Choose Capacity Based on the Home, Not Just the Floor Area
For a detached, semi-detached or townhouse property in Lachine, square footage can help begin the comparison between central air conditioner capacities, but the final decision requires a closer look at the building and its air-distribution system.
Insulation, windows, sun exposure, ceiling height, home layout, indoor equipment and duct capacity should all be considered before selecting a specific tonnage.
Book a FREE HVAC Estimation Visit to review your Lachine home’s cooling load, existing equipment, ductwork and airflow before choosing a central AC capacity.