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.
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.
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.
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:
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.
Step 3: Ensure maximum airflow matches required minimum airflow capacity. Minimum airflow usually is heating airflow. Because velocity can be difficult to measure accurately with varying airflow rates, design minimum airflow may be much lower than the airflow provided by a vendor's product. This is acceptable, as long as the vendor's controls offer 0 cfm as the lowest airflow option. In practical terms, this means a VAV box will cycle on and off as required to maintain space temperature. Although this may not be optimal, it often is the only option.
Minimum airflow usually is the code-required ventilation rate for a controlled zone. It can be calculated using the percentage of outside air of an AHU.
Carbon-dioxide (CO2) monitoring allows minimum ventilation airflow to be set below what would be needed during full design occupancy, saving energy during reduced occupancy. With CO2 concentration corresponding to the number of people inside of a building, this allows the amount of ventilation needed, be it 15 cfm or 20 cfm per person, to be reduced. Because of concerns regarding the accuracy of CO2 monitors, an ongoing maintenance program is highly recommended.1
Step 4: Calculate total pressure drop.2,3 Total pressure drop — not just vendor-listed static-pressure drop — should be used to aid selection. With the exception of critical noise applications (NC 25 or less), for which static-pressure values should be input progressively lower into a selection program until a desirable option is obtained, total pressure drop should not exceed 0.5 in. wc. Total pressure drop can be calculated as follows:
Delta TP = delta SP + delta VP
where:
TP = total pressure drop
SP = static pressure
VP = velocity pressure
Delta TP = delta SP + [(Vin ÷ 4,005)2 - (Vout ÷ 4,005)2]
Step 5: Maintain inlet air temperature at 55°F year-round. Verify that an AHU has a preheat coil set to deliver 55°F air. Under certain climatic conditions and when the amount of return air is consistently high, computed cooling-coil entering-air temperature can be a valuable guide.
Step 6: Use a manufacturer's standard electric heating coil, sizing it to match or exceed calculations and so that the maximum leaving-air temperature (LAT) is 90°F. A LAT above 90°F will result in stratification and/or short-circuiting. To maintain a LAT of 90°F, minimum (heating) airflow can be adjusted upward.
Step 7: For new buildings for which a building-management system (BMS) has yet to be selected, specify field-mounted, vendor-neutral direct digital controls. This approach gives a designer maximum flexibility. BMS selection for new facilities usually is done during the bidding process, after all design documents have been completed. For existing buildings, field-installed controls or factory-installed controls can be specified in design documents.
The selection of a VAV box is not dependent on the controls vendor. Having third-party controls factory-installed on a VAV box is feasible. While it may not save money, it saves field-installation time at the expense of procurement time. Pre-construction coordination is required.
According to ANSI/ASHRAE/IESNA Standard 90.1-2004, Energy Standard for Buildings Except Low-Rise Residential Buildings, “Zone thermostatic controls shall be capable of operating in sequence the supply of heating and cooling energy to the zone,” preventing “reheating; recooling; mixing or simultaneously supplying air that has been previously mechanically heated and air that has been previously cooled, either by mechanical cooling or by economizer; and other simultaneous operation of heating and cooling systems to the same zone.”
While Standard 90.1-2004 prohibits simultaneous heating and cooling, there are three exceptions:
“Zones for which the volume of air that is reheated, recooled, or mixed is no greater than the large(st) of the following:
“The volume of outdoor air required to meet the ventilation requirements of Section 6.1.2 of (ANSI/)ASHRAE Standard 62(.1-2004, Ventilation for Acceptable Indoor Air Quality) for the zone.” A conservative interpretation of Standard 62.1-2004 may lead to extremely high minimum flow rates for the delivery of minimum outside air to zones, regardless of the percentage of outside air brought in at the system level. CO2 control is recommended to reduce outside air and possibly meet this exception. If a building is served by three AHUs, then the combined outside-air quantity is the system-level outside airflow.
A spreadsheet (Table 1) is recommended for tracking values and simplifying comparisons. Note that the “Ventilation-air cfm” column essentially represents the amount of air that needs to be recooled or reheated. The “System OA cfm” column, meanwhile, lists the building's entire outside-air requirement.
“0.4 cfm per square foot of the zone conditioned floor area.” Despite a lack of confirmed studies and other empirical evidence, many HVAC designers feel that a minimum circulation rate of supply air must be maintained for comfort. Whether or not it does, because it is part of the standard, it needs to be evaluated.
“30 percent of the zone design peak supply rate.” Typically, an air outlet's performance will drop off at low flows, reducing the outlet's ability to mix supply air and room air effectively. This is particularly true with reheat boxes, as the temperature of supply air at minimum volume will be warm, while the buoyancy of the supply air will further decrease mixing — unless outlet velocity is maintained.
“300 cfm (… for zones whose peak flow rate totals no more than 10 percent of the total fan-system flow rate).” Large glass areas facing north and windows shaded by overhangs have low peak supply-air volumes, but relatively high heating loads, often needing heating volumes higher than those required by Section 6.1.2 of Standard 62.1, 0.4 cfm per square foot of zone conditioned floor area, and 30 percent of zone design peak supply rate at reasonable air-supply temperatures. This exception allows 300 cfm for heating, provided system outside air does not exceed 3,000 cfm.
“Any higher rate that can be demonstrated to the authority having jurisdiction to reduce overall system annual energy usage by offsetting reheat/recool energy losses through the reduction in outdoor-air intake in accordance with the multiple space requirements defined in ASHRAE Standard 62(.1).” This is self-explanatory. Good luck dealing with local code officials!
“Zones where special pressurization relationships, cross-contamination requirements, or code-required minimum circulation rates are such that variable-air-volume systems are impractical.” This will apply only in special design scenarios.
“Zones where at least 75 percent of the energy for reheating or for providing warm air in mixing systems is provided from a site-recovered (including condenser heat) or site solar-energy source.” This, too, will apply only in special design scenarios.
In Table 1, both VAV boxes are shown to be exempt and, thus, allowed to reheat air. This type of analysis should be performed for each VAV box on a project.
This article avoided discussion of the cost implications of selecting oversized boxes, life-cycle-cost analysis, VAV-box control strategies, and VAV-box sizing for conference rooms, which is a design topic unto itself.
Readers' comments and design tips are highly encouraged, as our body of engineering knowledge rests on cumulative contributions from many sources.
Fisk, W.J., Faulkner, D., & Sullivan, D.P. (2006). Accuracy of CO2 sensors in commercial buildings: A pilot study. Paper LBNL-61862. Available at http://repositories.cdlib.org/lbnl/LBNL-61862
Taylor, S.T., & Stein, J. (2004). Sizing VAV boxes. ASHRAE Journal, 46, 30-35.
California Energy Commission. (2003). Advanced variable air volume system design guide. Available at http://www.energy.ca.gov/reports/2003-11-17_500-03-082_A-11.PDF
For past HPAC Engineering feature articles, visit www.hpac.com.
A senior mechanical engineer with R.E. Lamb Inc., Edward Liwerant, PE, has more than 30 years of plumbing, fire-protection, and mechanical-engineering experience in several industries. He specializes in designing systems for clients who must maintain operational capability during construction. A graduate of Drexel University, he is a member of the American Society of Heating, Refrigerating and Air-Conditioning Engineers and the American Society of Mechanical Engineers.