Understanding HVAC Duct Systems: Pressure, Airflow, and Components

Fundamentals of HVAC Duct Systems

• Pressure Measurement: Duct system pressure is measured in inches of water column (in. WC).
• Blower Function: The blower creates pressure to force air through the duct system, conditioning equipment, and into the room.
• Atmospheric Pressure: Standard atmospheric pressure is 14.696 psi.
• Manometer Usage: A manometer measures air velocity pressure in a duct.
• Airflow Measurement Tools: Anemometers or velometers are commonly used to measure airflow in air distribution systems.
• Airflow Check: Airflow can be checked by feeling it 2-3 feet from a register.
• Forced-Air vs. Natural-Draft: Forced-air systems use a blower, unlike natural-draft systems.
• Duct Pressure Components: Duct pressure equals velocity pressure plus static pressure.
• Reducing Plenum Systems: These maintain constant pressure along the trunk duct.
• Extended Plenum Systems: These prolong the main plenum closer to the farthest supply register.
• Temperature Difference & Insulation: A 15°F difference between duct interior and exterior may require insulation.
• Equivalent Duct Length: Fittings' resistance is calculated as equivalent duct length for friction loss estimation.
• Belt Tension Adjustment: Adjusting belt tension alters airflow and motor amperage.
• Duct System Types: Plenum, extended plenum, reducing plenum, and perimeter loop are common types.
• Oversized Ducts: These increase costs and reduce airflow velocity.
• Extended Plenum Drawback: Air velocity decreases towards the end of the plenum.
• Natural-Draft Example: Baseboard hot water heat is a natural-draft system.
• Sheet Metal Gauge: Galvanized sheet metal thickness is measured by gauge.
• Flexible Duct Sizes: Flexible round ducts come in sizes up to 24 inches in diameter.
• Friction Loss: Each foot of duct creates resistance to airflow, known as friction loss.
• Forced-Air System Components: Blower, supply system, return system, grilles, and registers.
• Duct Pressures: Static, total, and velocity pressures.
• Pitot Tube: Measures airflow using concentric tubes inserted into the duct.
• Airflow Calculation: Example: 8" x 20" duct at 700 fpm carries 778 cfm.
• Blower Power: Centrifugal blowers are powered by motors.
• Drive Clips: Used to connect duct sections.
• Friction in Ductwork: Friction from air rubbing against duct walls is unavoidable.
• Supply Duct Function: Distributes air to terminal units, registers, grilles, or diffusers.
• Plenum System Characteristics: Suited for jobs with outlets close to the unit; economical and easy to install.
• Branch Duct Balancing: Dampers balance airflow to individual areas.
• Register Airflow: Airflow from a register may vary depending on installation.