Duct Pressure Relief Damper

What is a Duct Pressure Relief Damper in HVAC?

A duct pressure relief damper is a mechanical device (optionally with automatic sensing) installed directly on supply or return air ducts. Its main purpose is to regulate and release excess pressure when abnormal pressure build-up occurs inside the duct system. This helps to:

• Protect ducts from bulging, tearing, or joint failure

• Maintain stable system pressure and avoid airflow imbalance

• Extend the lifespan of the HVAC system and related equipment (fans, motors, VCD dampers, etc.)

For example, when airflow becomes uneven between duct branches or terminal outlets are blocked, pressure can accumulate in certain duct sections. A pressure relief damper automatically releases excess pressure to keep the system stable.

When is a Pressure Relief Damper Required?

Some inexperienced engineers or contractors may overlook pressure relief dampers in HVAC designs. However, omitting this device can cause serious problems when the system operates under fluctuating pressure conditions.

Situations That Cause Dangerous Pressure Build-Up

• Uneven fan operation between air zones → sudden pressure spikes

• Blocked ducts due to dust, debris, or faulty terminal devices (diffusers, grilles)

• All supply outlets closed while the fan is still running

• BMS control failure causing fans to run while dampers remain closed → “air lock”

• System imbalance between supply and return air

Without a pressure relief damper, ducts may be damaged or produce excessive vibration and noise.

Signs You Need a Pressure Relief Damper

• Pressure gauge exceeds design limit (typically > 250 Pa)

• Duct vibration or abnormal noise at elbows or transitions

• Cracks or damage at joints or supports

• Airflow imbalance across zones despite normal equipment operation

High-rise buildings, shopping malls, cleanrooms, and high-pressure ventilation systems typically require pressure relief dampers by design standards.

Structure and Working Principle

Basic Components

A standard duct pressure relief damper typically includes:

• Frame (Housing): Galvanized steel or stainless steel casing, rectangular or square to match duct size

• Blade(s): Single or multi-blade connected by a shaft

• Spring or Pressure Control Mechanism: Triggers opening when pressure exceeds setpoint

• Rubber Seal (Gasket): Ensures airtight closure and reduces noise

• Mechanical Linkage: Lever or spring system for quick response

Advanced models may include pressure sensors or servo motors integrated with BMS for intelligent control.

Operating Principle

• Normal condition: Pressure ≤ setpoint → damper remains closed and sealed

• Pressure rises: When pressure exceeds threshold (e.g., 200–300 Pa), force overcomes spring tension → damper opens → excess air is released

• Pressure drops: Spring returns blade to closed position automatically

Mechanical dampers operate without electricity, saving energy and ensuring automatic protection.

Types of Pressure Relief Dampers

Mechanical vs Automatic

• Mechanical: Suitable for small factories or auxiliary areas

• Automatic: Used in malls, hospitals, large industrial HVAC systems

One-Way vs Two-Way Relief Dampers

• One-Way: Releases air outward only → used in cleanrooms, cold storage, controlled airflow zones

• Two-Way: Allows bidirectional airflow depending on pressure → used in positive/negative pressure rooms (healthcare, labs)

Selection depends on overall HVAC system design and pressure behavior.

Practical Applications

Pressure relief dampers are mandatory in HVAC systems with precise pressure control, such as cleanrooms and fire pressurization systems.

Selection Guidelines

• Airflow type: Bidirectional airflow requires a two-way damper

• Design pressure: Typical opening threshold 150–300 Pa (adjustable for some systems)

• Maintenance accessibility: Choose durable, high-quality seals if maintenance is difficult

• BMS compatibility: Required for smart buildings and remote monitoring

• Fire/GMP requirements: Some applications require fire-resistant, corrosion-resistant, or hygienic materials

For large projects, consulting with manufacturers or HVAC specialists is recommended to ensure correct damper selection.