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VAV-PRC011M-EN 151

Application Considerations

pressure levels and ultimately the NC within the space, is from radiated sound. This is readily

known for fan-powered units, but less commonly known for single- and dual-duct units. Radiated

sound emanates from the unit and enters the occupied space via means other than through the

supply ductwork. The most typical path is through the plenum space, then through the ceiling, then

into the occupied space. While discharge sound should never be ignored, radiated sound is the

most dominant and usually the most critical sound source.

When increasing air valve sizes, BE CAREFUL. Oversizing an air valve can adversely impact the

ability to modulate and properly control temperature in the space. In extremely oversized

situations, the air valve will operate like a two-position controlled device, with air either being “on”,

or “off”, and not really much in between. The best way to avoid this is to understand that the

minimum for most air valves is 300 FPM. This is a function of the flow sensing device and the ability

of the pressure transducer and controller to properly read and report flow. This is not manufacturer

specific, as physics applies to all. Therefore, when sizing air valves, regardless of the max cooling

velocity the minimum velocity for proper pressure independent flow is 300 FPM.

Modulation capability and range is vital for proper operation of VAV systems. With oversized units,

the unit will act as a constant volume system eliminating the energy savings and individual zone

control advantages of VAV systems. A good rule of thumb is to size cooling airflow for around 2000

FPM. VAV systems only operate at full flow when there is a maximum call for cooling in the zone.

The greatest portion of the time, an air valve will be operating at partial flows.

When sizing fan-powered units, the fan airflow range can be determined by looking at the fan-

curve. Because parallel and series fan-powered units operate at a constant fan flow, selections can

be made all the way to the lowest flow ranges of the fan curve. A good balance of performance and

cost is to select fans at 70-80% of maximum fan flow.

Insulation types

Insulation is a factor to consider when dealing with the acoustics of terminal units. Most insulation

types will provide similar acoustical results, but there are exceptions. Double-wall and closed-cell

foam insulation will generally increase your sound levels because of the increased reflective

surface area that the solid inner-wall and closed-cell construction provides. This increase in sound

will have to be balanced with the IAQ and cleanability considerations of the dual-wall and closed-

cell construction.

Placement of units

Unit placement in a building can have a significant impact on the acceptable sound levels. Locating

units above non-critical spaces (hallways, closets, and storerooms) will help to contain radiated

sound from entering the critical occupied zones.

Unit Attenuation

Terminal unit-installed attenuators are an option available to provide path sound attenuation.

Manufacturer-provided attenuators on the discharge of a terminal unit are targeted at reducing

discharge path noise and are typically a simple lined piece of ductwork. It would often be easier and

less expensive to design the downstream ductwork to be slightly longer and require the installing

contractor to include lining in it. Attenuators on the plenum inlet of fan-powered terminals are

targeted at reducing radiated path noise since the plenum opening on a fan-powered terminal unit

is typically the critical path sound source. Significant reduction in radiated path noise can result

from a well-designed inlet attenuator. The attenuation from these attenuators is due to simple

absorption from the attenuator lining and occupant line of sight sound path obstruction. Therefore,

longer attenuators and attenuators that require the sound to turn multiple corners before reaching

the occupied space provide superior results, particularly in the lower frequency bands.

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