Makeup Air Calculator
Calculate replacement-air CFM from exhaust airflow, then estimate the sensible heating capacity needed to temper the outdoor air.
Makeup airflow
1,800 CFM
Heating capacity
145.4 MBH
Heating capacity
42.6 kW
Design ΔT
65 °F
Estimated air density
0.0863 lb/ft³
Sensible heating only. Density is approximately corrected for entering temperature and elevation; final MAU selection still requires manufacturer performance data.
Turn the calculation into a complete design workflow
Use the free result as a quick engineering check, then continue into MEPFlow for connected system design, sizing and project calculations.
How the Makeup Air Calculator Works
Start with the total exhaust airflow and the fraction that will be replaced mechanically. The balance can come from intentional transfer air, infiltration or another outdoor-air system depending on the pressure strategy for the building.
For heating load, MEPFlow uses the fundamental sensible-heat relationship based on air mass flow rather than blindly applying 1.08 × CFM × ΔT. Air density is adjusted approximately for outdoor temperature and site elevation, which is especially useful for very cold makeup-air applications.
The result is the sensible capacity required to raise the selected makeup-air flow from outdoor design temperature to the target supply temperature. Humidification, latent cooling, heat recovery, burner turndown and equipment derating are separate design checks.
Calculation basis
Q = ρ × 60 × cp × CFM × ΔT
When to Use This Calculator
- Kitchen exhaust replacement air
- Washroom exhaust makeup air
- Industrial exhaust systems
- Preliminary MAU heating capacity
Engineering Checks Before Final Design
- Coordinate the replacement fraction with the building pressure strategy.
- Cold-air density can increase required heat above the familiar 1.08 shortcut.
- Final equipment selection requires manufacturer performance data.
Frequently Asked Questions
What percentage of exhaust should be makeup air?
There is no universal percentage. Use the amount required by the project pressure strategy after accounting for transfer air and other outdoor-air systems.
Why include elevation?
Air density decreases with elevation, changing the mass of air moved at the same volumetric CFM and therefore the sensible heating load.
Is 1.08 × CFM × ΔT always accurate?
It is a convenient standard-air approximation. For very cold outdoor air or elevated sites, using density at the design condition is more representative.
Does this calculate cooling and dehumidification?
No. The current tool calculates sensible heating only. Cooling-coil and latent loads require entering-air humidity conditions.
Engineering References
MEPFlow uses public engineering references to document the calculation basis. Always verify the final design against the standard, code edition and manufacturer data applicable to your project.