VARIABLE AIR VOLUME (VAV)

by | Mar 1, 2023 | Air Conditioning, Uncategorized

OBJECTIVE:

  • Introduction.
  • Description of Technology. 
  • Individual components of the VAV box        
  • Performance Monitoring
  • Advantages of VAV.
  • Disadvantages OF VAV.
  • Applications O VAV

INTRODUCTION:

  • Variable air volume (VAV) systems enable energy-efficient HVAC system distribution by optimizing the amount and temperature of distributed air. 
  • Appropriate operations and maintenance (O&M) of VAV systems is necessary to optimize system performance and achieve high efficiency.
  • The purpose of this equipment O&M Best Practice is to provide an overview of system components and maintenance activities to keep VAV systems operating safely and efficiently. 
  • Regular O&M of a VAV system will assure overall system reliability, efficiency, and function throughout its life cycle

Description of Technology

  • VAV systems supply air at a variable temperature and airflow rate from an air handling unit (AHU).
  • Because VAV systems can meet varying heating and cooling needs of different building zones, these systems are found in many commercial buildings. 
  • Unlike most other air distribution systems, VAV systems use flow control to efficiently condition each building zone while maintaining required minimum flow rates. 

In this distribution technology two types 

  1. VAV-based air distribution system that consists of an AHU and VAV boxes
  2.  Schematic of a typical pressure-independent VAV box

1. VAV-based air distribution system that consists of an AHU and VAV boxes

Typical VAV-based HVAC distribution system
  • One VAV box per zone.
  • Each VAV box can open or close an integral damper to modulate airflow to satisfy each zone’s temperature setpoints. 

Some features of a VAV system include the following:

  • Provides conditioned air to spaces to meet varied zonal temperature and airflow requirements.
  • Variable frequency drive-based air distribution system can reduce supply fan energy use.
  • Supply-air temperature reset capability allows adjustment and reset of the primary delivery temperature.

There are two major classifications of VAV boxes or terminals

  1. Pressure dependent 
  2. Pressure independent

2. Schematic of typical pressure-independent VAV box

Schematic of pressure-independent VAV box with hydronic reheat

In this case, the box also has a reheat coil. 

This VAV box has three modes of operation:

  1. A cooling mode with variable flow rates designed to meet a temperature setpoint.
  2. A dead-band mode whereby the setpoint is satisfied and flow is at a minimum value to meet ventilation requirements.
  3. A reheating mode when the zone requires heat. 

Types of VAV and terminal boxes

  • Single duct terminal VAV box – the simplest and most common VAV box
  • Fan-powered terminal VAV box – employs a fan that can cycle on to pull warmer plenum air/return air into the zone and displace/offset required reheat energy.
  • Dual ducted terminal VAV box – takes advantage of two ducts to the unit.
  • Induction terminal VAV box – takes advantage of the induction principle instead of a fan to pull warmer plenum air/return air into the zone and displace/offset required reheat energy.

Individual components of VAV box

Which Terminal Unit (VAV Box) is Right for Me? - WGI
  • Airflow sensor – measures the airflow at the inlet to the box and adjusts the damper position to maintain a maximum, minimum, or constant flow rate.
  • Damper – modulates the airflow based on airflow sensor and zone temperature requirements.
  • Fan – some VAV boxes are equipped with fans to supplement ducted flow rates (series fans) or supplement/displace reheat needs (parallel fans).
  • Filter (for fan-powered boxes) – usually included when a fan draws into the VAV box from the plenum or other return-air source.
  • Reheat coil – optional accessory that warms the air leaving the box.
  • The system controls – Depending on the age of the system, VAV box controls may be pneumatic, electronic, or direct digital. An airflow sensor in the box measures airflow. Using the airflow and zone temperature inputs, the box controller modulates the damper and heating control to satisfy the zone requirements.

Performance Monitoring

The most common option for VAV performance monitoring is using the structure’s building automation system (BAS). By enabling the trending function of a BAS, the VAV system operation can be assessed. Key points to trend include:

  • Static pressure in the supply duct and control point for the system VFD fan to assure modulation with changing VAV box flow rates.
  • VAV box damper position versus zone temperature and reheat status to assure damper minimum setting before reheat application.
  • Reheat valve position versus call for heat.
  • VAV box airflow rate commensurate with damper position and within minimum and maximum settings.
  • VAV box delivered air temperature appropriate for zone conditions.
  • VAV box reheat call appropriate for conditions and corresponding chiller operating point and reset status.
  • Zone temperature.
  • Zone occupancy status.

Advantages of VAV

The advantages of VAV systems over constant-volume systems include 

1.More precise temperature control.

2. Reduced compressor wear.

3. Lower energy consumption by system fans.

4. Less fan noise.

5. Additional passive dehumidification

Disadvantages of VAV

1.  Imbalance Of Building Pressures.

2. Partial Load Comfort Problems.

3.  Reduction In Outdoor Air.

4. Supply Air Dumping.

APPLICATIONS OF VAV

VAV systems achieve their full potential in applications where there multiple zones with variable loads. Some examples are malls, hotels, and office buildings. VAV boxes have a role similar to the duct heaters used in a CAV system Supermarkets,

  • Commercial buildings,
  • Office buildings,
  • Banks
  • Movie theaters.