Duct Systems Pressure Classification

Overview

Engineering reference data for Duct Systems Pressure Classification in HVAC systems.

Key Formulas

Sensible Heat

$Q = \dot{m} c_p \Delta T$

Heat causing temperature change.

Latent Heat

$Q = \dot{m} h_{fg} \Delta\omega$

Heat causing moisture change.

COP (Cooling)

$COP = Q_c / W$

Coefficient of performance.

Variables

Symbol	Description	Unit
$Q$	Heat transfer	W
$\dot{m}$	Mass flow rate	kg/s
$c_p$	Specific heat of air	J/(kg·K)
$\Delta T$	Temperature difference	K

Pressure Classifications

Duct systems are commonly divided into three pressure classifications based on fan static pressure and duct velocity requirements:

Application Guidelines

Selecting the appropriate pressure classification depends on the duct's location within the distribution system:

• Medium pressure classification is commonly used for primary main ductwork, including fan connections, risers, and main distribution trunks. These sections carry the highest volume and are closest to the air handler.

• Low pressure classification should be used for secondary ductwork, including runouts and branches from main ducts to end terminals (diffusers, grilles, registers). These sections operate at reduced pressure and velocity as air is distributed to individual zones.

High pressure systems are typically reserved for specialized applications requiring compact ductwork or long-distance transport with minimal duct size.

References

• Original Source