AHU Sizing Calculator – Accurate Air Handling Unit Calculation Made Simple
Determine the correct air handling unit capacity for your building. Ensure proper airflow, cooling, and comfort while improving energy efficiency.
AHU Capacity Calculator
How to Use an AHU Sizing Calculator
Follow these simple steps to use an ahu sizing calculator effectively:
Step-by-Step Instructions
- 1Enter Room Area: Input the total area in square feet or square meters.
- 2Input Ceiling Height: Provide the height to calculate room volume.
- 3Add Occupancy Details: Enter the number of people in the space (for advanced load factoring).
- 4Include Heat Load Sources: Add equipment, lighting, and external heat gains if applicable.
- 5Select Air Changes per Hour (ACH): Choose based on room type (e.g., office, hospital, cleanroom).
- 6Click Calculate: The calculator will output required airflow (CFM or CMH).
Tip: Always double-check units before calculation. Use standard ACH values for better accuracy.
How to Calculate AHU Sizing Manually
You can calculate AHU size using airflow requirements based on the following relationship:
Basic Formula
Where:
- Room Volume = Area × Height
- ACH = Air Changes per Hour
Step-by-Step Example
Example:
- Room Area = 500 sq ft
- Ceiling Height = 10 ft
- ACH = 6
Step 1: Calculate Volume
Volume = 500 × 10 = 5000 cubic feet
Step 2: Apply Formula
CFM = (5000 × 6) / 60
Step 3: Final Result
CFM = 30000 / 60 = 500 CFM
Result: You need an AHU with at least 500 CFM airflow capacity.
Additional Considerations
- Add 10–20% safety margin
- Consider duct losses
- Adjust for climate conditions
AHU Sizing Conversion Chart
Reference table for common AHU sizing requirements at 10 ft height and 6 ACH:
| Room Area (sq ft) | Height (ft) | ACH | Airflow (CFM) |
|---|---|---|---|
| 200 | 10 | 6 | 200 |
| 300 | 10 | 6 | 300 |
| 500 | 10 | 6 | 500 |
| 800 | 10 | 6 | 800 |
| 1000 | 10 | 6 | 1000 |
| 1500 | 10 | 6 | 1500 |
Quick Tip: 1 Ton of cooling ≈ 400 CFM. Use this for quick estimation.
Frequently Asked Questions (AHU Sizing Calculator)
Air handling unit size is primarily determined by calculating the total cooling and heating loads of the building to establish the required airflow in CFM. Engineers then select a physical unit size that can accommodate the necessary blower, cooling coils, heating elements, and filtration systems.
An undersized air handling unit will run continuously while failing to maintain the desired indoor temperature and humidity levels. This constant operation leads to excessive energy consumption, accelerated wear on system components, shortened equipment lifespan, and poor overall occupant comfort.
While you can technically oversize an air handling unit, it is highly discouraged. An oversized system will cool the space too rapidly and shut off before it can adequately remove indoor humidity. This short cycling results in a cold but clammy environment and increases long-term operational costs.
AHU capacity refers to the maximum volume of air the unit can process and distribute, typically measured in cubic feet per minute. It also encompasses the system's thermal capacity, rated in tons of refrigeration or BTUs, which indicates its ability to remove or add heat to the conditioned air.
Selecting the right AHU involves analyzing building heat loads, determining the required airflow rates, and considering static pressure requirements. You must also evaluate spatial constraints within the mechanical room, energy efficiency goals, and specific air filtration or acoustic requirements.
