Example Data Table
| Use Case | Total Exhaust CFM | Replacement Percent | Safety Factor | Typical Design Note |
|---|---|---|---|---|
| Small kitchen hood | 1200 | 85% | 10% | Check comfort near cooking line. |
| Workshop exhaust | 2500 | 90% | 10% | Review dust collector start cycles. |
| Paint booth support | 8000 | 100% | 15% | Confirm code and equipment ratings. |
| Mechanical room | 3500 | 100% | 10% | Check combustion air requirements. |
Formula Used
Total Exhaust CFM = Main Exhaust CFM + Extra Exhaust CFM
Replacement CFM = Total Exhaust CFM × Replacement Percentage ÷ 100
Room Volume = Length × Width × Height
ACH Based CFM = Room Volume × Minimum ACH ÷ 60
Required Make Up Air = Max(Replacement CFM, ACH CFM) − Infiltration Credit, then apply safety factor.
Free Area = Required CFM ÷ Target Opening Velocity
Heating Load = 1.08 × Required CFM × Temperature Difference
Metric Flow = Required CFM × 1.69901082
How to Use This Calculator
Enter the exhaust airflow from hoods, fans, booths, or process equipment. Add any extra exhaust that runs with the main system. Choose the replacement percentage. Many designs use less than full replacement when some transfer air is acceptable.
Enter room dimensions and minimum air changes if you want a room volume check. Add an infiltration credit only when that air path is reliable. Add a safety factor for filter loading, future equipment, dirty grilles, and balancing changes.
Use the opening velocity and free area factor to size louvers, grilles, or ducts. Lower velocity can reduce noise and pressure drop. Finally, enter indoor and outdoor temperatures to estimate heating load.
Make Up Air Planning Guide
Why Make Up Air Matters
Make up air replaces air removed by exhaust systems. It supports stable building pressure. It also helps fans work near their intended rating. Without replacement air, a room can become too negative. Doors may become hard to open. Exhaust flow can drop. Drafts can become uncomfortable. Combustion equipment may also face unsafe venting conditions.
Airflow Balance Basics
The first step is adding all exhaust airflow. This includes kitchen hoods, restroom fans, process exhaust, dust collectors, and other active systems. The calculator then applies a replacement percentage. A value near one hundred percent is common when strong neutral balance is needed. A lower value can work when transfer air is planned.
Room Volume Check
Some spaces also need a minimum air change rate. The room volume method converts length, width, height, and air changes into CFM. This gives a second design target. The calculator uses the larger value. That approach helps avoid undersizing when the space is large or highly ventilated.
Opening and Duct Sizing
Air must enter through an opening, duct, grille, or louver. The required free area depends on airflow and velocity. The gross opening is larger because louvers and screens block part of the area. A lower velocity usually means quieter flow. It may also reduce pressure drop. Large openings can improve comfort near doors and work areas.
Heating Load Review
Outdoor air often needs heating. The sensible heat formula estimates the load in BTU per hour. The formula uses airflow and temperature difference. Cold climates can create large heating loads. That load may require a dedicated heater, tempering coil, or energy recovery system.
Design Notes
This tool gives planning results, not final engineering approval. Real systems also need code review, hood rules, combustion air checks, filtration, dampers, fan curves, and balancing. Use the result as a strong estimate. Then compare it with local rules and equipment documents before installation.
FAQs
What is make up air?
Make up air is outdoor or transfer air supplied to replace exhausted air. It helps maintain pressure balance, comfort, and fan performance.
Why does exhaust need replacement air?
Exhaust removes indoor air. Without replacement air, the room can become negative. This can reduce exhaust flow and cause drafts.
What replacement percentage should I use?
Many systems use 80% to 100%. Critical systems may need full replacement. Always check local codes and equipment requirements.
Can infiltration count as make up air?
It can be counted only when it is reliable and acceptable. Random leakage is usually not a good design source.
How is heating load calculated?
The calculator uses 1.08 × CFM × temperature difference. This estimates sensible heating load for standard air conditions.
What is free area factor?
Free area factor shows how much of a grille or louver is open. A 0.65 factor means 65% is open area.
Why include a safety factor?
A safety factor helps cover filter loading, future equipment, measurement error, grille losses, and balancing adjustments.
Is this calculator suitable for final design?
It is best for planning and comparison. Final design should follow codes, fan data, hood rules, and professional review.