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Sizing Local Exhaust Fans for Canadian Homes
A local exhaust fan pulls moisture, odour and pollutants out at the source, in the bathroom, over the stove or in the laundry room, before they spread through the house. That is a different job from whole-house ventilation, which supplies general fresh air. Codes treat them separately, and each room type here follows its own rule, so the calculator asks different questions for a bathroom, a kitchen and a laundry room.
Bathrooms
Two numbers set the answer. The floor is the code rate: Part 9 of the National Building Code asks for 25 L/s (about 53 CFM) from a fan that runs on demand and 10 L/s (about 21 CFM) from one that runs continuously, and ASHRAE 62.2 rounds those to 50 and 20 CFM. Above the floor, the Home Ventilating Institute suggests 1 CFM per square foot for bathrooms up to 100 square feet. Past 100 square feet it switches to fixtures: 50 CFM for each toilet, shower or tub. A 5 by 8 ft bathroom is 40 square feet, so the area rule says 40 CFM and the 53 CFM floor governs. The bathroom fan calculator goes further on continuous operation and sone ceilings.
Kitchens
ASHRAE 62.2 accepts 100 CFM on demand or 5 air changes per hour continuously. A 150 square foot kitchen with 8 ft ceilings is 1,200 cubic feet, and 5 changes an hour of that is exactly 100 CFM, so the two options agree there. Those figures keep the house ventilated and say little about catching smoke from the pan. Lawrence Berkeley National Laboratory's kitchen ventilation presentation reports that roughly 200 CFM was needed to capture over 80% of cooking pollutants in testing, and it recommends 100 CFM per linear foot of range width, which is 250 CFM for a 30 inch range. The calculator uses that capture rate whenever it exceeds the 62.2 minimum. Strong hoods also pull on the house. The NBC ties makeup air to spillage-susceptible appliances (Article 9.32.3.8), many authorities set their own trigger, and about 400 CFM is a common rule of thumb for when to plan for it, so the makeup air calculator is the next stop for a large hood.
Laundry and Utility Rooms
Neither ASHRAE 62.2 nor the NBC sets an exhaust rate for a laundry room, so any figure here is judgment, not code. The calculator applies the bathroom area rule as a moisture control guide, 1 CFM per square foot with a 50 CFM minimum, which suits a room with a washer and a drying rack. Dryer exhaust is a separate matter: it needs its own duct to the outside and should never be tied into a bathroom or utility fan. If walls or windows in the room stay damp, the condensation risk calculator shows whether a surface is cold enough to wet.
Duct Length, Diameter and Static Pressure
A fan's CFM label is measured at 0.1 in. w.g. (25 Pa), which is a short, straight duct. The calculator picks the smallest round duct that holds velocity under 1,000 fpm for bathrooms and laundry rooms and 1,500 fpm for range hoods, then estimates friction with the standard smooth round duct relation, 0.109136 × CFM1.9 ÷ diameter5.02 in. w.g. per 100 ft, and adds 0.05 in. w.g. for the wall cap and damper. At 53 CFM in 4 inch duct, a 15 ft run comes to about 0.08 in. w.g., so a 53 CFM fan is enough. At 100 ft equivalent length it is about 0.25 in. w.g., and the fan has to be rated near 75 CFM to deliver 53. Flexible duct has more friction than smooth metal, so treat the estimate as a lower bound, and count every elbow with the equivalent length calculator.
Noise and Efficacy
A fan that is too loud gets switched off, which defeats the sizing. ENERGY STAR's ventilating fan criteria cap bathroom and utility room fans at 2.0 sones up to 200 CFM and 3.0 sones from 201 to 500 CFM, and require at least 70% of rated airflow at 0.25 in. w.g. Minimum efficacy is 2.8 CFM per watt for fans from 10 to 89 CFM and 3.5 for 90 to 200 CFM. Near a bedroom, 1.0 to 1.5 sones is a sensible design target, which is a preference and not a standard.
Frequently Asked Questions
Part 9 of the National Building Code sets 25 L/s (about 53 CFM) for a bathroom fan that runs on demand and 10 L/s (about 21 CFM) for one that runs continuously. ASHRAE 62.2 rounds these to 50 and 20 CFM. For sizing above that floor, the Home Ventilating Institute suggests 1 CFM per square foot up to 100 square feet. A typical 5 by 8 ft bathroom (40 square feet) therefore lands on the 53 CFM code floor, not on 40 CFM.
ENERGY STAR caps certified bathroom and utility room fans at 2.0 sones up to 200 CFM and 3.0 sones from 201 to 500 CFM. For a bathroom next to a bedroom, 1.0 to 1.5 sones is a practical target, because occupants switch off a fan that annoys them. One sone is about the loudness of a quiet refrigerator. Fans over 80 CFM tend to run louder unless the motor is remote-mounted inline, away from the room.
ASHRAE 62.2 accepts 100 CFM on demand or 5 air changes per hour continuously, and for a 150 square foot kitchen with 8 ft ceilings both come to 100 CFM. That is a minimum, not a capture target. Lawrence Berkeley National Laboratory testing found roughly 200 CFM needed to capture over 80% of cooking pollutants, and it cites 100 CFM per linear foot of range width as a recommended rate, which is 250 CFM for a 30 inch range.
Fan ratings are measured at 0.1 in. w.g. (25 Pa) of static pressure, which is a short, straight duct. Every foot of duct, elbow and wall cap adds resistance, and a fan delivers less air as that resistance rises. ENERGY STAR requires a bathroom fan to still deliver at least 70% of its rated airflow at 0.25 in. w.g. This calculator estimates the static pressure of your duct run and sets a minimum fan rating that makes up the difference.