CFM per TR Rule Explained: How Much Airflow is Required for Air Conditioning?
Airflow is one of the most important parameters in HVAC design. Even if the cooling capacity is correctly selected, incorrect airflow can reduce efficiency, increase energy consumption, and create comfort problems.
What is CFM?
CFM (Cubic Feet per Minute) is the volume of air supplied by an HVAC system every minute.
What is TR?
TR (Ton of Refrigeration) is a unit of cooling capacity.
- 1 TR = 12,000 BTU/hr
- 1 TR = 3.517 kW
Standard CFM per TR Rule
| Cooling Capacity | Required Airflow |
|---|---|
| 1 TR | 400 CFM |
| 2 TR | 800 CFM |
| 3 TR | 1200 CFM |
| 5 TR | 2000 CFM |
| 10 TR | 4000 CFM |
HVAC Airflow Calculation Formula
CFM = TR × 400
Example 1
Cooling Load = 8 TR
CFM = 8 × 400 = 3200 CFM
Example 2
Cooling Load = 15 TR
CFM = 15 × 400 = 6000 CFM
Typical Airflow per Ton
| Application | Typical CFM/TR |
|---|---|
| Comfort Air Conditioning | 400 |
| High Sensible Heat | 450–500 |
| High Humidity | 350–380 |
| Computer Rooms | 450–550 |
| Hospitals | 400–500 |
Effects of Incorrect Airflow
Low Airflow
- Coil freezing
- Poor cooling
- Reduced efficiency
- Possible compressor damage
High Airflow
- Poor dehumidification
- Higher fan power
- Noise issues
- Draft complaints
Quick Reference Table
| TR | Airflow (CFM) |
|---|---|
| 1 | 400 |
| 2 | 800 |
| 3 | 1200 |
| 4 | 1600 |
| 5 | 2000 |
| 7.5 | 3000 |
| 10 | 4000 |
| 15 | 6000 |
| 20 | 8000 |
FAQ
Is 400 CFM per TR always correct?
No. It is a design guideline. Actual airflow depends on sensible heat ratio, humidity, and manufacturer recommendations.
Why do some projects use 450 CFM/TR?
High sensible heat applications such as data centers often require higher airflow.
What is the HVAC airflow calculation formula?
CFM = TR × 400
Conclusion
The 400 CFM per TR rule is the standard starting point for HVAC comfort cooling design. Always verify airflow with project-specific requirements and equipment data before finalizing the design.
Prepared for: AskMEP.com
