🔥 Heating Capacity
💰 Estimated Annual Heating Cost
Sizing Breakdown
Full Calculation Table
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How to Use the Makeup Air Unit Sizing Calculator
Enter the total exhaust airflow this unit needs to replace, from a kitchen hood, industrial process, or general exhaust calculation.
Choose full comfort, partial tempering, or minimum freeze protection depending on how the served space is used and what other heat sources exist.
Enter winter and summer design outdoor temperatures, and decide whether the unit needs a cooling coil in addition to heating.
Click Calculate Makeup Air Unit Sizing to see required heating capacity, optional cooling capacity, and estimated annual heating cost.
Understanding Makeup Air Unit Sizing
A makeup air unit conditions 100 percent outdoor air with no return air mixing, purpose-built to replace air removed by exhaust systems. Unlike a conventional AHU sized around zone sensible cooling load, a makeup air unit is sized primarily by exhaust volume, with heating and optional cooling capacity following from that airflow and your delivery temperature target.
Sizing Starts with Exhaust Volume, Not Zone Load
The core sizing driver for a makeup air unit is how much air the exhaust system removes, not the sensible cooling or heating load of the served space. A restaurant kitchen hood, covered by the restaurant HVAC calculator, or an industrial process exhaust system, covered by the industrial HVAC calculator, both drive makeup air unit sizing through their exhaust airflow requirement rather than a conventional load calculation.
Delivery Temperature Target Drives Heating Capacity
Full comfort tempering brings outdoor air all the way to a typical occupied comfort temperature, appropriate where makeup air delivers directly into a comfort-controlled space. Partial tempering brings air to a minimum acceptable temperature, commonly used where the served space has other heat sources or tolerates a wider range, such as a working kitchen. Minimum freeze protection tempers only enough to prevent equipment freeze damage, appropriate for spaces where the makeup air primarily needs to avoid causing operational problems rather than contribute to comfort. Choosing a lower delivery temperature target substantially reduces heating capacity and operating cost.
Heating Fuel and Efficiency
Natural gas-fired heating, whether direct-fired or indirect, is common for Canadian makeup air units given the large heating capacity often required and generally favorable gas pricing at that scale compared to electric resistance heating. Direct-fired units introduce combustion byproducts directly into the airstream and require specific code compliance for that application, while indirect-fired units keep combustion gases separated through a heat exchanger. Confirm which approach is appropriate for your specific application and local code requirements.
Should the Unit Include Cooling?
Many makeup air units provide heating only, relying on the served space's other systems or tolerance for warmer summer conditions. Units serving spaces with a comfort cooling requirement, or high cooling-load kitchens where uncooled makeup air would add unwanted summer load, sometimes include a cooling coil as well. Adding cooling increases capital cost and unit size but avoids makeup air working against the space's cooling system during summer months.
Pairing with the Building's Ventilation and Pressurization Design
Confirm this unit's makeup air percentage against the total exhaust it needs to offset using the building pressurization calculator, since under-sizing makeup air relative to exhaust creates negative pressure problems throughout the building, not just in the immediate area served.
Frequently Asked Questions
A makeup air unit conditions 100 percent outdoor air with no return air mixing, sized specifically to replace air removed by exhaust systems like kitchen hoods or industrial process exhaust. A regular AHU typically mixes return air with a smaller percentage of outdoor air and is sized around zone sensible cooling load rather than exhaust replacement volume. See the AHU sizing calculator for that conventional approach.
Not always. Many makeup air applications, especially serving industrial or kitchen exhaust, only temper outdoor air to a minimum acceptable delivery temperature rather than full occupied comfort temperature, since the served space often tolerates a wider range or has other heat sources like cooking equipment. Full comfort tempering is more common where makeup air is delivered directly into an occupied comfort space.
Natural gas-fired direct or indirect heating is common for makeup air units in Canada given the large heating capacity often required and generally favorable natural gas pricing compared to electric resistance heating at that scale. Electric heating remains an option, particularly for smaller units or locations without gas service, though operating cost is typically higher at large capacities.
It depends on the application. Many makeup air units provide heating only, since the space they serve often has other cooling sources or tolerates warmer summer conditions. Applications delivering makeup air into a comfort-controlled space, or units serving high cooling-load kitchens, sometimes include a cooling coil as well, which adds capacity and cost but keeps the makeup air from adding unwanted cooling load in summer.
Related HVAC Tools
Kitchen exhaust source calculation
Process exhaust source calculation
Confirm building-wide airflow balance
Related 100% outdoor air system
General ventilation baseline
Conventional mixed-air alternative
Corridor makeup air application
City-specific design conditions