SZVAV or VVZT sequence of operation

Below is the contents of the first page of a Trane engineering newsletter explaining in good detail of szvav or vvzt operation. 

providing insights for today's hvac system designer

Engineers Newsletter

volume 42-2

Understanding Single-Zone VAV Systems

Single-zone variable-air-volume (VAV) is not a new concept, but due to new energy code requirements and greater attention to reducing energy use, it is being applied more frequently. This EN will review these new requirements, discuss the benefits and challenges of single-zone VAV systems, and identify common applications for this system.

A conventional, single-zone constant-volume system uses a temperature sensor in the zone to vary cooling or heating capacity, while the supply fan delivers a constant quantity of air whenever the system is operating.

In a single-zone VAV system, however, the temperature sensor in the zone is used to vary the cooling or heating capacity and the airflow delivered by the supply fan to maintain supply-air temperature at a desired setpoint (Figure 1).

Traditionally, single-zone VAV has been used for larger, densely occupied zones that have variable cooling loads. Common examples include gymnasiums, cafeterias, lecture halls, auditoriums, large meeting rooms, churches, and arenas. Therefore, it has been available primarily in larger air-handling units and large, packaged rooftop equipment. This type of equipment has long been available with variable-speed fans and cooling/heating that can be staged or modulated to control discharge-air temperature.
However, due to increased focus on reducing energy use, single-zone VAV is starting to be used more often in K-12 classrooms, retail stores, dormitories, and even offices. As its popularity has increased for these smaller zones, this functionality is beginning to be offered in smaller equipment, such as small packaged rooftop units or direct expansion (DX) split systems, fan-coils, classroom unit ventilators, and water-source heat pumps.

Requirements of ASHRAE Standard 90.1

Requirements for VAV control in single-zone systems were added to the 2010 edition of ANSI/ASHRAE/IESNA Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings. This is especially significant because the U.S. Department of Energy has mandated that each state must update its commercial building code to meet or exceed Standard 90.1-2010 by Oct. 18, 2013.
These requirements are included in the mandatory provisions of Section 6.4. This means that projects must comply with the requirements for single-zone VAV control, regardless of whether the Simplified Approach (by reference from Section 6.3), prescriptive path (Section 6.5), or Energy Cost Budget (Section 11) is used for compliance.

Section 6.4.3.10 (see inset, p.2) includes two parts: the first addresses systems that use chilled water for cooling, while the second part addresses systems that use direct expansion.

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WARNING:

Information in this article is intended for use by individuals possessing adequate backgrounds of electrical and mechanical experience and who comply with all federal, state, and local laws, rules, orders, or regulations related to the installation, service, or repair of a heating or central air conditioning product. Any attempt to install, service, or repair a heating or central air conditioning product may result in personal injury and/or property damage. The manufacturer or seller cannot be responsible for the interpretation of the information contained herein, nor can it assume any liability in connection with its use.