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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 must be replaced from somewhere. If it isn't deliberately supplied through a properly sized makeup air system, the building draws unconditioned outdoor air through whatever path offers the least resistance: door gaps, window cracks, chimney flues, or combustion appliance vents. This uncontrolled infiltration creates comfort problems, energy waste, and in the worst cases, life-safety hazards.
The most serious risk is backdrafting: when a building runs at excessive negative pressure, combustion appliances like water heaters, furnaces, and fireplaces can fail to draft properly, pulling combustion byproducts including carbon monoxide back into the occupied space instead of venting them outside. This is why building codes and mechanical codes require makeup air for any significant continuous exhaust source, particularly commercial kitchen hoods which can exhaust thousands of CFM continuously.
Calculating the right makeup air percentage
Standard exhaust-only canopy hoods typically need 80-90% of their exhaust airflow replaced by dedicated makeup air, recognizing that some replacement air will naturally come from adjacent spaces through door undercuts and other transfer paths. Compensating (short-circuit) hood designs supply a portion of makeup air directly at the hood face, which can reduce the separately ducted makeup air requirement to 50-70% of total exhaust, though this design requires careful engineering to avoid disrupting hood capture performance.
For non-kitchen applications like large dryer banks or industrial exhaust systems, makeup air is typically sized at or near 100% of exhaust to maintain neutral building pressure, since there's usually no equivalent "transfer air" pathway available.
The Canadian winter tempering cost problem
Tempering makeup air to comfortable temperature in a Canadian winter is energy-intensive. Bringing in 2,000 CFM of -20°C outdoor air and heating it to 21°C requires a substantial heating coil and meaningful ongoing fuel or electricity cost. Some designs use partial tempering (heating only enough to prevent extreme discomfort, perhaps to 13°C) to balance comfort against energy cost, particularly in spaces where some temperature variation is acceptable, like back-of-house kitchen areas as opposed to dining rooms.
This calculator estimates the additional heating load created by tempering makeup air at your specified design conditions, helping you evaluate the energy cost tradeoff between different tempering strategies. Use the ventilation heat load calculator for a complete annual energy cost analysis.
Frequently Asked Questions
Most exhaust hood systems require makeup air equal to 80-90% of the hood's total exhaust CFM, with the remainder from transfer air. Compensating hoods supply some makeup air directly at the hood face, reducing the separate makeup air unit's capacity to 50-70%. Without sufficient makeup air, the kitchen runs at excessive negative pressure, causing door problems and potentially dangerous backdrafting of combustion appliances. Use the kitchen exhaust calculator first to get your total exhaust CFM.
Untempered makeup air at -20°C or colder creates severe drafts and discomfort, and can freeze nearby equipment if introduced without heating. Tempered makeup air units include a heating coil to raise supply temperature to 16-18°C before entering the space. This significantly increases capital and operating cost, which is why right-sizing the system to actual exhaust requirement matters. Use the ventilation heat load calculator for the full annual energy cost analysis.