Make Up Air Unit Calculator for Sizing & Cost
Most contractors guess when sizing makeup air units. They eyeball the exhaust CFM, add a buffer, and hope it passes inspection.
That guesswork costs you money twice.
- Undersized units fail code and create dangerous negative pressure.
- Oversized units waste 10% or more on energy bills every year due to short cycling.
Plug your exhaust CFM into the calculator below. You’ll get the exact BTU heating load and estimated annual operating cost based on your climate zone.
Make Up Air Unit Calculator
Get CFM and MBH specs for your quote.
*Estimate only. Actual requirements vary based on altitude, humidity, ductwork, and specific application needs. Our team will verify specs when you request a quote.
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Heating Load Formula for Makeup Air Units
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How to Calculate Heating Load for Makeup Air
To calculate the heating load for a makeup air unit, multiply your airflow volume by the temperature rise needed and a constant that accounts for air properties. The result tells you how many BTUs per hour your unit must produce to deliver properly tempered air.
The BTU Formula for Sizing Makeup Air Systems
The standard formula for calculating makeup air heating load is:
Q = 1.08 × CFM × ΔT
This formula calculates sensible heating load in BTU per hour. It applies to any application where outdoor air must be heated before entering a building, including commercial kitchens, warehouses, and manufacturing facilities.
| Variable | What It Means |
|---|---|
| Q | Heating load in BTU per hour. This is the heat output your makeup air unit must deliver. |
| CFM | Cubic feet per minute of airflow. This should match your exhaust volume to maintain proper building pressure. |
| ΔT | Temperature difference in degrees Fahrenheit between your desired supply air temperature and your local winter design temperature. |
| 1.08 | A constant derived from air density (0.075 lb/ft³) and specific heat (0.24 BTU/lb·°F) at sea level, multiplied by 60 minutes per hour. |
The 1.08 constant and temperature differential methodology align with ASHRAE psychrometric principles for calculating sensible heating loads in ventilation applications.
Sample BTU Calculation for a Commercial Kitchen
A restaurant in Chicago has a Type I hood exhausting 3,000 CFM. The owner wants makeup air delivered at 60°F. Chicago’s winter design temperature is approximately 0°F.
| Step | Value |
|---|---|
| Exhaust airflow (CFM) | 3,000 |
| Desired supply temperature | 60°F |
| Winter design temperature | 0°F |
| Temperature difference (ΔT) | 60°F |
| Heating load (Q) | 1.08 × 3,000 × 60 = 194,400 BTU/hr |
Divide by 1,000 to convert to MBH (thousand BTU per hour). This commercial kitchen needs approximately 194 MBH of heating capacity from a tempered heated makeup air unit.
Makeup Air Code Requirements for Commercial Buildings
Building codes make makeup air mandatory once your exhaust system exceeds 400 CFM. Here’s what you need to know before requesting a quote.
| Code Requirement | What It Means |
|---|---|
| 400 CFM threshold | Exhaust systems above this require automatic makeup air per IMC and NFPA 96 |
| Pressure limit | Negative pressure cannot exceed 0.02 inches water column |
| Type I hood replacement | Makeup air must equal 80-100% of exhaust volume for grease hoods |
| Tempered air | ASHRAE recommends supply air within 10°F of space temperature; preheat limited to 60°F max |
Without adequate makeup air, you risk hood capture failure, backdrafting from gas appliances, and doors that won’t open properly. A NIOSH investigation found that inadequate ventilation in one industrial kitchen caused carbon monoxide buildup affecting 45 workers.
Your contractor will handle the code details. The calculator above helps you arrive at that conversation with the right CFM and BTU specs.
Why Proper Sizing Saves You Money
Getting the BTU number right isn’t just about passing inspection. It directly affects your makeup air unit cost over the life of the equipment.
How Oversized Makeup Air Units Waste Money
Oversized units short cycle. The burner fires, heats the air too quickly, shuts off, then fires again. This constant on-off pattern wastes fuel and wears out components faster.
Research shows oversized HVAC systems lose roughly 10% efficiency compared to properly sized equipment. On a unit running 12 hours a day through winter, that adds up to hundreds in extra operating costs each year.
How Undersized Makeup Air Units Drive Up Operating Costs
Undersized units can’t keep up with exhaust demand. Building pressure turns negative, pulling unconditioned outdoor air through every gap and crack in the envelope.
That air infiltration increases your heating and cooling load. Instead of bringing in tempered supply air at a controlled rate, your HVAC system fights to condition random outdoor air leaking in from all directions. The initial savings on equipment cost disappears into higher monthly utility bills.
Know Your Numbers Before You Request a Quote
This make up air calculator gives you a ballpark estimate based on your CFM and climate zone. It’s not a final specification. Your contractor will confirm exact sizing based on your building, ductwork, and local codes.
But walking into that conversation with approximate BTU and heating load numbers helps you ask better questions, compare quotes more confidently, and avoid sticker shock when proposals come back.
Ready to Get a Quote on Your Makeup Air Unit
You’ve got your CFM. You’ve got your estimated BTU heating load. Now you can request a quote with the specs contractors need to give you an accurate price.
Contact us with your calculator results and project details. We’ll help you find the right makeup air unit for your application, whether that’s a commercial kitchen, warehouse, or industrial facility.
Not sure which unit type fits your project? Browse our selection of heated makeup air units, heated and cooled units, or commercial makeup air units to see what’s available.
Scott Williamson
Scott Williamson is an engineer specializing in industrial ventilation, including make-up air units.
His expertise has been recognized by leading business publications like HubSpot and Tech Bullion. Scott helps business owners find high-performance solutions that balance energy savings with daily operational needs.