HVAC-Kanalrechner
Technical Guide

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:

  1. Duct size — Higher friction rates allow smaller ducts
  2. Energy efficiency — Higher friction rates mean the blower works harder
  3. Noise levels — Higher friction rates typically mean higher velocities and more noise
  4. 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:

  1. Find the total available static pressure from the equipment specifications
  2. Subtract pressure losses for coils, filters, grilles, and other components
  3. Divide the remaining pressure by the total equivalent length of the longest duct run
  4. 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
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