A Complete Guide to Friction Rate in HVAC Design
HVAC Duct Calculator Team ·
What Is Friction Rate?
Friction rate is the pressure drop that occurs as air flows through ductwork, measured in inches of water gauge per 100 feet of duct (in/wg per 100 ft). It quantifies the resistance the duct system creates against airflow.
Why Friction Rate Matters
Selecting the correct friction rate is critical because it directly affects:
- Duct size — Higher friction rates allow smaller ducts
- Energy efficiency — Higher friction rates mean the blower works harder
- Noise levels — Higher friction rates typically mean higher velocities and more noise
- System balance — Incorrect friction rates lead to uneven air distribution
Common Friction Rates by Application
Different applications have different requirements:
- Residential supply ducts: 0.08 in/wg per 100 ft
- Residential return ducts: 0.06 in/wg per 100 ft
- Commercial low-pressure: 0.08 – 0.10 in/wg per 100 ft
- Commercial medium-pressure: 0.10 – 0.20 in/wg per 100 ft
- Commercial high-pressure: 0.20 – 0.60 in/wg per 100 ft
- Industrial: 0.30 – 1.00 in/wg per 100 ft
Calculating Available Friction Rate
To determine the available friction rate for your system:
- Find the total available static pressure from the equipment specifications
- Subtract pressure losses for coils, filters, grilles, and other components
- Divide the remaining pressure by the total equivalent length of the longest duct run
- Multiply by 100 to get the rate per 100 feet
Formula: Available Friction Rate = (Available Static Pressure ÷ TEL) × 100
Best Practices
- Always design for the longest run in the system
- Include equivalent lengths for all fittings
- Leave a safety margin of 10-15% for unforeseen losses
- Verify velocities are within acceptable ranges for the application
- Use our HVAC Duct Calculator to quickly find the right friction rate for your project