Once simple, variable-air-volume- (VAV-) box selection has become an iterative process complicated by energy- and ventilation-code requirements. What's more, most of the articles regarding the application of VAV boxes are highly technical, leaving us simple HVAC engineers somewhat befuddled, still looking for an easy, reliable, and expedient method of doing our jobs.

Although VAV-box selection should be performed as part of total-air-distribution-system (air handler[s], heating and cooling coils, filters, supply and return fans, diffusers, ducts, controls) design, this article will focus primarily on VAV-box selection and meeting the latest energy-code requirements.

VAV boxes come in many varieties, including cooling only, cooling with reheat, dual-duct, and fan-powered. This article will focus on cooling-with-electric-reheat VAV boxes.

AVOIDING ROGUE ZONES

At the VAV-box-selection step in the design process, all air-distribution zones, the required rate of cooling and heating supply airflow per zone, and the number of VAV boxes per zone should be established. The preferred way of designing air-distribution coverage is to assign perimeter zones and linked VAV boxes to one air-handling unit (AHU) and, when feasible, keep interior zones on another air-distribution system. “Rogue” zones — zones characterized by higher static-pressure demand — should be avoided.

Rogue zones often are the result of design mistakes (e.g., calculating a lower-than-actual rate of airflow), higher-than-anticipated pressure losses in poorly constructed ducts, poor construction supervision, and changes in room layout after VAV boxes are installed. If you think sizing may be an issue late in the construction process, perhaps the best course of action is to oversize a VAV box.

PRESSURE-INDEPENDENT VAV BOXES

In this engineer's opinion, pressure-dependent VAV boxes should be avoided. They modulate damper position in proportion to upstream air-supply pressure and, thus, do not necessarily supply the required rate of zone airflow.

Pressure-independent VAV boxes, on the other hand, modulate air volume in proportion to cooling load. Unlike their pressure-dependent counterparts, they can measure the amount of air passing through them. Also, they offer better control.

VAV-BOX SELECTION

Using vendor-furnished software for VAV-box selection streamlines the design process and provides answers to many sizing questions.

The process for initial VAV-box selection is:

Step 1: Refer to Noise Criteria (NC) values. Select a VAV box using the following NC values:

• General offices: NC 30 to NC 35.
• Open-plan-office areas: NC 40.
• Conference rooms: NC 30.
• Auditoriums: NC 30.
• Lobbies and corridors: NC 40.
• Executive offices: NC 25 to NC 30.
• Computer/equipment rooms: NC 40 to NC 45.
• Classrooms: NC 25 to NC 30.
• Judge's chambers: NC 35.
• Courtrooms: NC 35.
• Jury assembly rooms: NC 40.

Discharge noise rarely is an issue when a VAV box has hard duct at its inlet, a lined outlet plenum, and flexible duct between its plenum and diffusers. Generally, in accounting for radiated noise, the maximum rating of a VAV box located above a standard acoustical ceiling should be no more than 5-NC higher than the desired room rating. For example, the maximum rating of a VAV box for a general office with a desired NC level of 30 should be no more than NC 35.

Usually, inlet diameter determines VAV-box size. If one VAV box cannot meet noise criteria, consider using two. Two or more VAV boxes can be controlled with one temperature sensor.

Step 2: Allow spare airflow capacity. This may help meet noise requirements, as well as deal with unpredictability related to construction and clients making changes at the last minute.