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The 400 CFM per ton standard and why it matters in Canada
Every residential central AC manufacturer in Canada rates their equipment at 400 CFM per ton. That is the airflow at which the published SEER rating, sensible heat ratio, and latent capacity are measured in the lab. When your system runs at a different airflow, everything changes. The equipment delivers less cooling than rated, uses more electricity than expected, and in extreme cases damages itself.
In practice, Canadian homes rarely hit exactly 400 CFM per ton. A 3-ton system designed for 1,200 CFM might actually move 1,050 CFM because the installer used undersized flex duct, ran a filter with too high a MERV rating, or left the balancing dampers from the original rough-in in a partially closed position. That puts the system at 350 CFM per ton, which still works but shifts performance toward higher latent removal and slightly reduced sensible capacity.
Low airflow and coil freeze: the most common summer service call in Ontario
When airflow drops below 325 CFM per ton, the evaporator coil surface temperature drops below the freezing point of water. Moisture that should drain off the coil as condensate instead freezes onto the fins. Ice builds up over several hours until it blocks all airflow and the system shuts down on high discharge pressure. The homeowner calls for service, the technician defrosts the coil, and everyone concludes there's a refrigerant leak. But if the charge was correct before, it's still correct. Low airflow was the culprit.
Check the filter first. A 1-inch MERV 8 filter loaded with a winter's worth of dust can cut airflow by 200 to 300 CFM on a residential system. If the filter is clean, measure static pressure across the indoor coil using the static pressure calculator and compare it against the manufacturer's coil pressure drop curve. If the coil itself is dirty, the pressure drop will be 0.1 to 0.2 in wc higher than the clean coil spec, confirming a coil cleaning is needed before any refrigerant work.
High airflow and humidity problems in humid Canadian summers
Running above 500 CFM per ton has the opposite problem. Air moves through the coil so fast that it does not have time to cool to the dew point and give up its moisture. The system satisfies the thermostat on sensible temperature alone, shuts off, and leaves the space at 65 to 70 percent relative humidity. Occupants feel clammy even though the thermostat reads 22 degrees Celsius. This is common in oversized systems installed in Ontario and Quebec homes where the contractor used rule-of-thumb sizing instead of a proper room-by-room load calculation. The fix is either reducing the blower speed to bring airflow down to 400 CFM per ton, or right-sizing the equipment with the cooling load calculator.
Frequently Asked Questions
A frozen evaporator coil is almost always caused by insufficient airflow. When airflow drops below roughly 300 CFM per ton, the coil surface drops below 0 degrees Celsius and moisture freezes onto the fins instead of draining away. The most common causes in Canadian homes are a clogged furnace filter, a blocked return air grille behind furniture, a partially closed supply damper, or a dust-coated blower wheel. Check the filter first. If it is clean and the coil still freezes, measure total system airflow and use this calculator to check your CFM per ton. If you are below 325 CFM per ton, the duct system or blower needs attention before refrigerant charge is even considered. Use the evaporator freeze calculator to diagnose the cause from suction pressure and superheat data.
The industry standard target is 350 to 450 CFM per ton. Most equipment is rated at 400 CFM per ton. Running at 350 to 400 CFM per ton maximizes latent capacity and humidity removal, which matters in humid Ontario and Quebec summers. Running at 400 to 450 CFM per ton maximizes sensible capacity and temperature drop, preferred in drier Alberta and interior BC climates. Below 325 CFM per ton risks coil freeze and compressor damage. Above 500 CFM per ton impairs dehumidification and causes short-cycling. For heat pumps in heating mode, manufacturers typically specify 400 to 500 CFM per ton for optimal heating capacity. Always cross-reference with the available static pressure calculator to confirm the blower can deliver target airflow against your system's duct resistance.