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Why Makeup Air Balance Matters in Canadian Building Design
Every exhaust system removes conditioned air from a building, and that air has to be replaced from somewhere. If it is not deliberately supplied through a properly sized makeup air system, the building draws outdoor air through whatever path offers the least resistance: door gaps, window cracks, chimney flues or combustion appliance vents. That uncontrolled infiltration causes comfort problems and energy waste, and in the worst cases a life-safety hazard.
The serious risk is backdrafting. Natural Resources Canada explains that exhaust devices pump indoor air outside and can depressurize a building until air reverses through a chimney or vent, which is most likely with natural draft equipment. Combustion gases, including carbon monoxide, then spill into the occupied space. A commercial kitchen hood that moves thousands of CFM cannot be left to draw its replacement air through doors and flues.
How Much of the Exhaust to Replace
The International Code Council's guidance on the 2024 International Mechanical Code describes the rule for commercial kitchens: makeup air from all sources must be approximately equal to the exhaust removed, and it must not weaken the hood's capture. All sources includes transfer air, which is why common practice splits the job, with a dedicated unit supplying roughly 80 to 90 percent and transfer air from the dining room or corridor supplying the rest. Kitchens are usually left slightly negative so odours stay in, and compensating hoods that deliver part of the air at the hood face reduce the share the separate unit must carry. For a dryer bank or process exhaust with no transfer path, size the dedicated air at or near 100 percent. The code is a US model code, and Canadian authorities set their own rules, so confirm what applies to your project.
Reading the Pressure Balance
The balance is dedicated makeup air plus transfer air, divided by exhaust. Take 2,400 CFM of exhaust, a makeup air unit at 85 percent (2,040 CFM) and 240 CFM of transfer air. Supply is 2,280 CFM, which is 95 percent balanced, a slight negative that is normal for a kitchen. Transfer air is not free, because it pulls conditioned air out of the neighbouring room, and that room then draws its own replacement through its envelope. Check the whole-building result with the pressurization calculator, and when you are ready to select equipment, the makeup air unit sizing calculator carries the airflow into a unit selection.
Winter Tempering Is the Main Operating Cost
The heat needed to warm the dedicated air is 1.2 kg/m³ × 1.006 kJ/kg·K × airflow × temperature rise. For 2,040 CFM (0.963 m³/s) heated from -18.9 °C (-2 °F) to a 21.1 °C (70 °F) setpoint, the rise is 40 K and the load is about 46.5 kW, or 159,000 BTU/h. For commercial kitchens the IMC wants the air conditioned to within 6 °C (10 °F) of the room setpoint unless the building HVAC offsets the load or a compensating hood supplies it, so the partial tempering option to 13 °C falls outside that limit in a 21 °C kitchen. This is a design-day figure. The annual cost depends on degree days, and the annual energy cost calculator turns a load into dollars, while the ventilation heat load calculator handles the ventilation side.
Frequently Asked Questions
The International Mechanical Code asks for makeup air from all sources to be approximately equal to the exhaust. In practice a dedicated makeup air unit carries most of it, around 80 to 90 percent, and the rest arrives as transfer air from adjacent spaces such as the dining room. Compensating hoods supply part of it right at the hood face, which cuts the separate unit's share. A kitchen is normally left slightly negative so odours stay in, but a large deficit causes door problems and can backdraft combustion appliances. Use the kitchen exhaust calculator first to get the total exhaust.
Untempered makeup air in a Canadian winter sends cold drafts across cooks and staff and can freeze nearby pipes or equipment. For commercial kitchens the IMC requires either the building HVAC to offset the makeup air load or the air to be conditioned so supply stays within 10 °F (6 °C) of the room setpoint, with an exception for compensating hoods. That makes the heating coil a large share of both capital and operating cost. Warming 2,040 CFM from -18.9 °C (-2 °F) to 21 °C takes about 46.5 kW (159,000 BTU/h). Use the ventilation heat load calculator for the annual picture.
Partly. Transfer air counts toward the total air replaced, which is why the balance here is dedicated plus transfer air over exhaust. It is not free, because it removes conditioned air from the neighbouring room, which then draws its own replacement through its envelope. Use transfer paths from spaces that can tolerate some negative pressure, and never draw transfer air from a room that holds combustion appliances.
Natural Resources Canada explains that exhaust devices pump indoor air outside and can depressurize a house until air reverses through a chimney or vent, which is called backdrafting and is most likely with natural draft equipment. Combustion gases then spill into the room. Canadian installation codes tie this to a depressurization test, for example CAN/CGSB-51.71 under CSA B365 for solid fuel appliances, and call for makeup air when the limit is exceeded. Avoid running a range hood and a dryer at the same time in a house with natural draft equipment.