Variable Air Volume (VAV) Systems Operations and Maintenance

Table of Contents

Introduction

The primary goal of any heating, ventilation, and air conditioning (HVAC) system is to provide comfort to building occupants and maintain healthy and safe air quality and space temperatures. 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. Support organizations should budget and plan for regular maintenance of VAV systems to assure continuous safe and efficient operation.

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.

Figure 1 presents a typical VAV-based air distribution system that consists of an AHU and VAV boxes, typically with 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. In some cases, VAV boxes have auxiliary heat/reheat (electric or hot water) where the zone may require more heat, e.g., a perimeter zone with windows.

Some features of a VAV system include the following:

  • Distribution system 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 with the potential for savings at the chiller or heating source.
Typical VAV-based HVAC distribution system
Figure 1. Typical VAV-based HVAC distribution system.

There are two major classifications of VAV boxes or terminals—pressure dependent and pressure independent.

A VAV box is considered pressure dependent when the flow rate passing through the box varies with the inlet pressure in the supply duct. This form of control is less desirable because the damper in the box is controlled in response to temperature only and can lead to temperature swings and excessive noise.

A pressure-independent VAV box uses a flow controller to maintain a constant flow rate regardless of variations in system inlet pressure. This type of box is more common and allows for more even and comfortable space conditioning. The balance of this guide will focus on pressure-independent VAV boxes.

Figure 2 presents a schematic of a typical pressure-independent VAV box; in this case, the box also has a reheat coil. This VAV box has three modes of operation: a cooling mode with variable flow rates designed to meet a temperature setpoint; a dead-band mode whereby the setpoint is satisfied and flow is at a minimum value to meet ventilation requirements; and a reheating mode when the zone requires heat.

Schematic of pressure-independent VAV box with hydronic reheat
Figure 2. Schematic of pressure-independent VAV box with hydronic reheat.

There are several different types of VAV and terminal boxes. The most common include:

  • Single duct terminal VAV box – the simplest and most common VAV box, shown in Figures 1 and 2, can be configured as cooling-only or with reheating.
  • 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, one hot (or neutral) and one cold to provide space conditioning.
  • 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.

Key Components

This O&M Best Practice focuses on the pressure-independent VAV terminal box and relevant connections for source air, water, electricity, and controls.

Supply ducting system. Each VAV terminal box is connected to a supply air source. This is a ducted connection that provides air from an AHU. Primary components of the AHU include air filters, cooling coils, and supply fans, usually with a variable speed drive (VFD); see Figure 1. A critical element to the air-supply system is the duct pressure sensor. The pressure sensor measures static pressure in the supply duct that is used to control the VFD fan output, thereby saving energy.

VAV terminal box. The VAV terminal box (see Figure 2) consists of a number of individual components, including:

  • 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 regardless of duct pressure fluctuations.
  • 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 coils may be electric or hydronic.
  • 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.

Zone temperature control. The primary control point for any VAV system is the zone temperature. Either a zone sensor or thermostat provides a signal to the VAV controller.

Safety Issues

As with any electromechanical device, all aspects should be powered down to a safety state before any maintenance or diagnostics are performed. As needed, and per manufacturer’s and electrical safety recommendations, VAV system functions can be enabled for testing and verification or performance. Standard electrical and mechanical safety practices apply to these systems.

Maintenance of Technology

Keeping VAV systems properly maintained through preventive maintenance will minimize overall O&M requirements, improve system performance, and protect the asset. Follow the guidelines in the equipment manufacturer’s maintenance manuals.

VAV systems are designed to be relatively maintenance free; however, because they encompass (depending on the VAV box type) a variety of sensors, fan motors, filters, and actuators, they require periodic attention. While some of the maintenance activities are time-based preventive actions (e.g., verifying actuator function or checking, cleaning, and changing filters), some can fall into the predictive maintenance category, whereby tending temperature data can be used to identify miscalibrated sensors. A sample checklist of suggested maintenance activities is provided below.

It is important to keep a written log, preferably in electronic form in a Computerized Maintenance Management System (CMMS), of all services performed. This record should include identifying features of the VAV box (e.g., box number, location, and type), functions and diagnostics performed, findings, and corrective actions taken.

Maintenance Checklist

For all VAV maintenance, it is important to follow the manufacturer’s recommendations. Proper maintenance should only be performed by trained and qualified personnel. The checklist below provides recommended actions and frequency by VAV component type. This checklist does not supersede maintenance recommendations from the equipment manufacturer, nor is it a replacement for contracted O&M or warranty services.

Table 1. Sample VAV system maintenance checklist.
Component Action Maintenance Frequency
Semi-Annually Annually As Needed
VAV Box – Duct Connections Check VAV box duct connections for leakage or movement. Verify that hangers and mountings are secure.          X  
VAV Box Zone Temperature Sensor (Thermostat) Verify function and accuracy (compared to calibrated value). Check signal to controller to verify corresponding control, damper action, and minimum setting.       X    
VAV Box – Airflow Sensor Verify function of flow sensor (compared to calibrated value) and corresponding control of box damper. Clean sensor per manufacturer’s recommendations.       X    
VAV Box – Controls

Verify function by technology type and per manufacturer’s recommendations:

Pneumatic – check for air leaks in hoses and fittings.

Electronic – check for proper electrical connections.

Direct Digital Control (DDC) – check for proper connections corresponding to damper action.

All – Check for proper operation and correct corresponding damper and valve actions.

       X    
VAV Box – Damper Check seals and alignment in duct.          X  
VAV Box – Damper Linkage and Control Check linkage for tension and position relative to control point. Lubricate per manufacturer’s recommendation. Verify minimum and maximum positions are correct.          X  
VAV Box – Filter (if present) Check, clean, and/or replace filters on all fan-powered VAV boxes. Change per manufacturer’s recommendations.        X          X
VAV Box – Hydronic Reheat (if present) Check and clean reheat coil. Check control valve and fittings for water leaks, and check coil for cleanliness and fin condition.          X        X
VAV Box – Electric Reheat (if present) Check and clean reheat coil. Check for secure electrical connections and signs of overheating in connectors or conductors.          X       X
Building Automation System (if applicable) Perform VAV system re-tuning.         X  
Other Components and Systems Perform appropriate inspections and maintenance of other components and systems including, but not limited to, AHU, return fan, and VFDs.        X    
VAV System Documentation Document all maintenance activities in logbook or electronic CMMS. Upon Activity Completion

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 supply duct and control point for 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.

O&M Cost

Modern VAV systems are designed to be more efficient and have less overall wear due to reduced system fan speed and pressure versus the on/off cycling of a constant volume system. However, at the zone level, the VAV system can have greater maintenance intensity due to the additional components of dampers, sensors, actuators, and filters, depending on the VAV box type. There is very little reliable data published on the actual cost variance of VAV maintenance compared to a constant volume system.

Additional Support

Because VAV systems are part of a larger HVAC system, specific support comes in the form of training opportunities for larger HVAC systems. To encourage quality O&M, building engineers can refer to the American Society of Heating, Refrigerating and Air-Conditioning Engineers/Air Conditioning Contractors of America (ASHRAE/ACCA) Standard 180, Standard Practice for Inspection and Maintenance of Commercial Building HVAC Systems.

Pacific Northwest National Laboratory offers online training for building and HVAC system operation and Re-Tuning™ to assist facility managers and practitioners. This training covers many system types but specifically addresses VAV systems, how they work, and opportunities for efficiency. More information on this training can be found at: https://buildingretuning.pnnl.gov/

Sources of Information

AHRI Standard 880-2017. Standard for Performance Rating of Air Terminals. Air Conditioning, Heating, and Refrigeration Institute, Arlington, VA. http://www.ahrinet.org/App_Content/ahri/files/STANDARDS/AHRI/AHRI_Standard_880_IP_2017.pdf.

ANSI/ASHRAE/ACCA Standard 180-2012. Standard Practice for Inspection and Maintenance of Commercial Building HVAC Systems. American National Standards Institute, New York, NY. https://www.ashrae.org/technical-resources/standards-and-guidelines/read-only-versions-of-ashrae-standards.

ASHRAE Standard 62.1-2016. Ventilation for Acceptable Indoor Air Quality. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Atlanta, GA. https://www.ashrae.org/technical-resources/standards-and-guidelines/read-only-versions-of-ashrae-standards

California Energy Commission. 2003. Advanced Variable Air Volume System Design Guide. Sacramento, CA. https://www.researchgate.net/publication/258246595_Advanced_Variable_Air_Volume_System_Design_Guide

EPA (Environmental Protection Agency). 2008. ENERGY STAR Building Upgrade Manual. U.S. Environmental Protection Agency, Washington, D.C. https://www.energystar.gov/buildings/tools-and-resources/building-upgrade-manual.

FEMP (Federal Energy Management Program). 2010. O&M Best Practices Guide, Release 3.0, Chapter 9, O&M Ideas for Major Equipment Types, Section 9.7, Air Handling Systems. U.S. Department of Energy, Federal Energy Management Program, Washington, D.C. https://www1.eere.energy.gov/femp/pdfs/om_9.pdf.

PNNL (Pacific Northwest National Laboratory). 2011. Self-Correcting Controls for VAV System Faults. PNNL-20452. Pacific Northwest National Laboratory, Richland, WA. https://www.pnnl.gov/main/publications/external/technical_reports/PNNL-20452.pdf

Actions and activities recommended in this Best Practice should only be attempted by trained and certified personnel. If such personnel are not available, the actions recommended here should not be initiated.

Published April 2021