Pressure-Dependent VAV Terminal Units

Operating Principle

Pressure-dependent VAV terminal units represent the simplest form of variable air volume control, utilizing only a modulating damper actuated by a zone thermostat. Airflow through the unit varies as a function of both damper position and upstream static pressure according to the fundamental relationship:

Q = K × √(ΔP)

Where:

• Q = volumetric airflow rate (CFM)
• K = flow coefficient (varies with damper position)
• ΔP = pressure differential across damper (in. w.g.)

The absence of airflow measurement or flow control mechanisms means the terminal cannot maintain constant airflow independent of system pressure fluctuations.

System Configuration

Basic Components

Damper Assembly:

• Single-blade or multi-blade opposed-blade damper
• 0-10 VDC or 2-10 VDC modulating control signal
• Spring return or fail-safe positioning

Actuator:

• Electric modulating actuator (typically 24 VAC)
• Proportional control responding to thermostat signal
• 2-5 minute stroke time for stability

Sensing and Control:

• Zone thermostat provides control signal
• No airflow measurement device
• Direct damper modulation based on temperature deviation

Inlet Configuration

Pressure-dependent terminals are available in standard inlet configurations:

• Round connections: 4" through 24" diameter
• Rectangular connections: up to 2000 CFM capacity
• Standard upstream straight duct requirement: 3-5 diameters minimum

Performance Characteristics

Pressure Sensitivity

The fundamental limitation of pressure-dependent terminals is the direct relationship between airflow and static pressure. As duct static pressure varies due to system loading changes, airflow at each terminal changes proportionally.

Flow Characteristics

The flow response to damper position is non-linear and pressure-dependent:

At 0.5 in. w.g. upstream pressure:

At 1.0 in. w.g. upstream pressure: All flow rates increase by approximately 40% at equivalent damper positions, demonstrating pressure dependency.

Control Limitations

Static Pressure Interaction

When one or more zones reduce demand and their dampers close partially, static pressure at other terminals increases. This creates cascading effects:

1. Reduced Load Zone: Damper closes to 30% position
2. Supply Fan Response: Duct static pressure increases (if VFD lag exists)
3. Other Zones: Airflow increases unintentionally
4. System Result: Simultaneous overcooling and undercooling in different zones

Temperature Control Accuracy

Pressure-dependent terminals exhibit poor temperature control under varying system conditions:

Typical control accuracy:

• Best case (stable pressure): ±2°F
• Normal operation: ±3-4°F
• High pressure variation: ±5-7°F
• Worst case: Unable to maintain setpoint

Turndown Limitations

Effective turndown ratio (maximum flow to minimum controllable flow) is limited by:

• Damper authority at low positions: typically 3:1 to 4:1
• Pressure fluctuation effects: reduces effective turndown
• Practical turndown with pressure variation: 2:1 to 3:1

Application Considerations

Appropriate Applications

Pressure-dependent VAV terminals can function acceptably in limited applications:

Building Types:

• Small single-zone-per-floor offices with minimal load diversity
• Spaces with consistent occupancy and load patterns
• Retrofit applications where budget severely constrains options
• Non-critical spaces with relaxed comfort requirements

System Requirements:

• Total system capacity under 10,000 CFM
• Limited number of terminals (typically under 10 zones)
• Zones with similar load characteristics
• Applications with continuous fan operation

Design Constraints

Static Pressure Management:

• Critical requirement for stable duct static pressure
• Fast-responding static pressure reset recommended
• Multiple static pressure sensors improve stability
• Supply fan VFD with rapid response essential

Duct Design:

• Lower design velocities reduce pressure variation
• Oversized ductwork improves pressure stability
• Avoid excessive fitting losses near terminals
• Design static pressure: 0.75-1.0 in. w.g. maximum

Zone Sizing:

• Minimize number of zones per system
• Size zones for similar peak loads
• Avoid mixing perimeter and interior zones
• Consider dedicated systems for high-diversity areas

Comparison with Pressure-Independent Systems

Installation Requirements

Upstream Conditions

Proper installation requires attention to approach conditions:

• Minimum 3 duct diameters straight upstream duct
• 5 diameters preferred for accurate damper control
• Avoid elbows or transitions immediately upstream
• Remove construction debris before startup

Damper Orientation

Blade orientation affects control characteristics:

• Parallel blade: higher flow at partial closure (rarely used)
• Opposed blade: better control authority (standard)
• Airfoil blade: lowest pressure drop, best control

Actuator Mounting

Proper actuator installation ensures reliable operation:

• Mount per manufacturer orientation requirements
• Provide adequate clearance for maintenance
• Protect from water exposure or condensation
• Verify proper damper shaft coupling

Commissioning and Balancing

Initial Setup Challenges

Pressure-dependent systems require extensive balancing:

1. Proportional balancing required: Adjust all dampers to achieve design flows at one operating condition
2. Iterative process: Multiple rounds of adjustment as system settles
3. Pressure-dependent results: Balance valid only at specific system loading
4. Seasonal variation: May require rebalancing between seasons

Test and Balance Procedures

Critical measurements:

• Inlet static pressure at each terminal
• Actual airflow at design conditions
• Supply air temperature
• Zone temperature at various loads

Adjustment methodology:

• Balance at design cooling conditions
• Document inlet static pressures
• Set minimum damper positions
• Verify control response through full range

Performance Optimization

Static Pressure Reset

Implementing aggressive static pressure reset partially compensates for pressure-dependent behavior:

Reset strategies:

• Trim and respond: Reset based on most open damper position
• Multiple sensor averaging: Reduce localized pressure spikes
• Fast response required: Update setpoint every 30-60 seconds
• Setpoint limits: Maintain 0.5-1.0 in. w.g. range

Damper Minimum Position

Setting minimum damper positions addresses ventilation and control:

• Typically 20-30% open position minimum
• Ensures minimum ventilation airflow
• Maintains some control authority
• Reduces extreme pressure variations

Zone Grouping

Strategic zone grouping improves system performance:

• Group zones with similar load patterns
• Separate perimeter and interior zones
• Consider dedicated systems for problem areas
• Limit zones per system to 6-8 maximum

Economic Analysis

First Cost Advantage

The primary advantage of pressure-dependent terminals is low first cost:

Installed cost comparison (per terminal):

• Pressure-dependent: $400-600 installed
• Pressure-independent: $900-1,400 installed
• Cost differential: $500-800 per terminal
• Typical system savings: $5,000-15,000 for 10-20 zone system

Operating Cost Penalties

Lower first cost is offset by higher operating costs:

Energy penalties:

• Overcooling/overheating: 10-20% increase in energy use
• Damper hunting and cycling: 5-10% fan energy increase
• Poor humidity control: Possible reheat energy waste
• Estimated annual penalty: $0.50-1.50 per square foot

Comfort and productivity costs:

• Temperature complaints and adjustments
• Maintenance calls for “system not working”
• Reduced occupant satisfaction
• Difficult to quantify but significant

Life-Cycle Cost

Over typical 20-year equipment life:

• Higher energy costs typically exceed first cost savings
• Increased service calls add maintenance costs
• Tenant complaints create management costs
• Pressure-independent systems usually more cost-effective

Troubleshooting Common Issues

Insufficient Airflow

Symptoms: Zone cannot achieve setpoint despite damper fully open

Causes:

• System static pressure too low
• Excessive pressure drop in ductwork
• Undersized terminal selection
• Supply air temperature inadequate

Solutions:

• Verify design static pressure at terminal inlet
• Check for duct obstructions or damage
• Increase supply fan speed if margin available
• Consider terminal replacement if severely undersized

Excessive Airflow/Noise

Symptoms: Zone overcools, damper nearly closed, noise complaints

Causes:

• Excessive static pressure at terminal
• Poor system pressure control
• Oversized terminal for actual load
• Ductwork design velocity too high

Solutions:

• Implement or improve static pressure reset
• Add manual volume damper upstream to limit maximum flow
• Verify supply fan VFD operation
• Consider terminal replacement if severe

Poor Temperature Control

Symptoms: Zone temperature varies widely, cannot maintain setpoint

Causes:

• Static pressure fluctuation in duct system
• Inadequate damper control authority
• Thermostat calibration or location problems
• Simultaneous heating/cooling from adjacent zones

Solutions:

• Improve static pressure control and reset
• Verify minimum damper position settings
• Check thermostat calibration and location
• Consider upgrade to pressure-independent terminals

Conclusion

Pressure-dependent VAV terminal units represent the most basic form of VAV control, suitable only for small systems with limited zone diversity and relaxed comfort requirements. The fundamental limitation—airflow variation with system pressure—creates control challenges that require careful system design, aggressive pressure control, and extensive commissioning.

For most commercial applications, the modest first cost savings do not justify the performance limitations, higher energy use, and increased maintenance requirements. Pressure-independent terminals provide superior performance and typically better life-cycle costs, making them the preferred choice for systems with more than 6-8 zones or any application requiring precise temperature control.

Modern building requirements for energy efficiency, indoor air quality, and occupant comfort make pressure-dependent systems increasingly obsolete except in the smallest and simplest applications.