Coanda (Surface or Ceiling) Effect. Via restricting secondary air entrainment, a jet moving adjacent to or in contact with an impermeable wall or ceiling creates a low-pressure area immediately adjacent to that surface, causing the air to remain in contact with the surface substantially throughout the length of throw. This Coanda effect, also referred to as the surface or ceiling effect, counteracts, to varying extents, the negative-buoyancy-driven the drop of a horizontally projected cool airstream.

Round and four-way horizontal-throw ceiling outlets exhibit a high Coanda effect because the discharge air pattern blankets the ceiling area surrounding each outlet. This effect diminishes with a directional discharge that does not blanket the full ceiling surface surrounding the outlet due to entrainment into the exposed sides of the jet. Sidewall outlets exhibit varying degrees of Coanda effect, depending on the spread of the particular air pattern, the angle of the jet, and the distance from the outlet’s discharge to the surface.

When outlets are mounted such that they discharge into a free space not near an adjacent surface (e.g., exposed duct-mounted supply air grille applications), the airstream entrains air around the entire perimeter of the jet. As a result, the rate of entrainment is higher and the throw is shortened. More information on jet behavior can be found in Chapter 20 of the 2021 ASHRAE Handbook—Fundamentals.

Temperature Differential. The greater the temperature differential between the supply air projected into a space and the air in the space, the greater the buoyancy effect on the path of the supply airstream. Heated, horizontally projected air rises and cooled air falls, so this effect should be considered during outlet selection. Chapter 20 of the 2021 ASHRAE Handbook—Fundamentals has further discussion of the buoyancy effect.

Low-temperature supply air or cold building start-up in a humid environment may cause condensation. Consider the effect of condensation on outlet and space surfaces during outlet selection.

Sound Level. The sound level from an outlet is largely a function of its discharge velocity and transmission of HVAC system noise, such as from fans and air turbulence, and may be desirable for masking other sounds, or, if too high, may be considered noise by occupants. For a given air capacity, a larger outlet face area has a lower discharge velocity and correspondingly lower generated sound. However, a larger outlet also allows more system-generated sound to pass through the outlet, which may appear as outlet-generated noise to occupants. High-frequency noise can result from excessive outlet velocity but may also be generated in the duct by the moving airstream. Low-frequency noise is usually generated by mechanical equipment and transmitted through the duct and outlet to the room.

The cause of a noise complaint can usually be pinpointed as outlet or system sounds by removing the outlet’s core during operation. If the noise remains essentially unchanged, the system is the source. If the noise is significantly reduced, the outlet is likely the source. The noise may be caused by a highly irregular velocity profile at the entrance to the outlet. ASHRAE research project RP-1335 (Landsberger 2011) and Chapter 58 of the 2023 ASHRAE Handbook—HVAC Applications have more details on the effects of inlet conditions on air outlets.

Variable-Air-Volume (VAV) Outlet. A VAV outlet is an air outlet that self-modulates its primary air flow rate with an integral air damper. Select outlets based on the total range of airflow needed for the space served. Outlet performance characteristics should be evaluated at both minimum and maximum flow. More information regarding selection of outlets can be found in Chapter 58 of the 2023 ASHRAE Handbook—HVAC Applications.

The following procedure is generally used in selecting and locating an outlet in a fully mixed system. More details and examples are available in Rock and Zhu (2002) and Chapter 58 of the 2019 ASHRAE Handbook—HVAC Applications.

Space Considerations. To maximize system efficiency, supply outlets are typically located below the occupant breathing level, so installed low in the sidewall or in-floor. These supply outlets often take up considerably more surface area than outlets used in fully mixed systems. Wall recessed outlets are common but can be configured as quarter-round, semicircular, rectangular, or cylindrical to increase face area. Cylindrical air outlets are generally mounted in a room’s open space, whereas the other configurations are mounted in corners, adjacent to the side wall, or flush with the wall.

Space Heating Considerations. Skistad (2002) reported that displacement ventilation can be combined successfully with radiators and convectors at exterior walls to offset space heat losses. Radiant heating panels and heated floors also can be used with displacement ventilation. When a secondary heating system is used, displacement outlets should supply air at least 1 K cooler than room temperature to maintain displacement airflow to the space.

System and Terminal Considerations. Low-velocity supply air outlets used in fully stratified systems can function properly with either constant- or variable-air-volume (CAV or VAV) supply air systems. For VAV applications, the supply air flow rate should be varied via a thermostat located in the space at a height of 1.2 to 1.5 m.

For fully stratified systems in humid climates, using series fan-powered terminal units or other mixing zone devices may allow supply air to be delivered from the central HVAC system at conventional temperatures and then blended with return/plenum air in the terminal unit to bring the air to an appropriate discharge temperature. However, this may compromise the space contaminant removal benefits of the displacement system.

Supply outlets used in stratified air systems tend to be mounted low in the sidewall or in-floor. They may also be mounted in the floor or risers beneath seats in public assembly facilities. Both mounting locations can produce adequately low discharge velocities; however, the outlets needed tend to be quite large. Because discharge velocities are very low, the supply airstream produces little momentum, and small obstacles, other than heat sources, in the flow path have little effect on its travel. Selection and application of air outlets for these systems is based primarily on the following considerations:

Supply outlets used in fully stratified systems are typically selected for a maximum face velocity of 0.2 to 0.36 m/s. Limitation of the face velocity is determined by noise requirements and proximity of occupants to the outlet. Where space noise requirements are not so stringent and stationary occupants are far away from diffusers, higher face velocities can be used. It is also important that the supply air outlet and its ductwork be designed to distribute airflow evenly across the outlet’s entire discharge area to avoid excessive velocity deviations.

The area adjacent to the supply outlet where local velocities may exceed 0.25 m/s is defined as the adjacent zone, where local velocity/temperatures may combine to create drafty conditions. Manufacturers of outlets specifically intended for fully stratified systems publish predicted adjacent-zone values that depend on the outlet supply airflow rate and the initial temperature difference between the supply and room air. Stationary occupants should not be present routinely in the adjacent zone.

For applications requiring very low noise criteria, such as broadcast or recording studios or performing arts venues, acoustical performance is an important consideration. Because of their low-velocity discharge, these supply outlets can generally be selected to meet acoustical criteria for these applications.

Space Considerations. Partially mixed air distribution systems often rely on a pressurized plenum to deliver conditioned air to the supply air outlets; therefore, most of these outlets are not individually ducted. For example, underfloor air distribution systems commonly use the cavity beneath a raised access floor as a pressurized plenum. The supply outlets are mounted in the access floor tiles and can easily be relocated in response to space changes and workstation relocation.

Most supply outlets used in pressurized floor plenum applications can be easily adjusted for airflow by the space occupant. In such applications, it is usually effective to provide an adjustable outlet in every office or workstation to afford occupants control of their own environment. Many of these outlets can also be fitted with a thermostatically controlled damper that provides variable air flow rates to the space.

System and Terminal Considerations. Supply air outlets designed specifically for partially mixed room air distribution systems can use either CAV or VAV systems. For VAV applications, the supply air flow rates should be determined via a thermostat located in the space at a height of 1.2 to 1.5 m.

Supply outlets used in stratified air systems tend to be mounted low in the sidewall or in-floor. Because of their high degree of mixing supply and room air together, these outlets can be selected for much higher discharge velocities than those used in fully stratified systems, resulting in significantly smaller outlet discharge areas. Selection and application of air outlets for partially mixed systems is based primarily on the following considerations:

Supply outlets should be selected such that their vertical projection achieves comfort and ventilation objectives. Limiting the vertical projection to a terminal velocity of 0.25 m/s of the supply air to below the respiration level, typically 1.4 m per ASHRAE Standard 62.1, allows convective heat plume formation around occupants to convey respiratory contaminants up and out of the occupied zone. This creates breathing-level CO₂ concentrations like those associated with fully stratified room air distribution systems. Projections that exceed this level discourage formation of such plumes and result in space ventilation like that of fully mixed systems.

The area adjacent to the supply outlet where local velocities may exceed 0.25 m/s is defined as the adjacent zone, where local velocity/temperatures may combine to create drafty conditions. Manufacturers of outlets specifically intended for fully stratified systems publish predicted adjacent-zone values that depend on the outlet supply airflow rate and the initial temperature difference between the supply and room air. Stationary occupants should not be present routinely in the clear zone.

For applications requiring very low noise criteria, such as broadcast or recording studios or performing arts venues, acoustical performance is an important consideration. These supply outlets may be suitable to meet acoustical criteria for these applications.

Adjustable-Blade Grilles. This is the most common type used as a supply outlet. A single-deflection grille includes a set of either vertical or horizontal vanes or blades. Vertical vanes deflect the airstream in the horizontal plane; horizontal vanes deflect the airstream in the vertical plane. A double-deflection grille has a second set of vanes, typically installed behind and at right angles to the face vanes and controls the airstream in both horizontal and vertical planes.

Fixed-Blade Grilles. These are like the adjustable-blade grille except that the vanes or blades are not adjustable and may be straight or angled. For supply grilles, the angle(s) at which air is discharged depends on the deflection of the vanes. Fixed-blade grilles are more commonly used as return or exhaust air inlets. Angled blades reduce visibility into the ductwork.

Linear Bar Grilles. When used for supply, these outlets typically have fixed bars at the face. The bars normally run parallel to the length of the outlet and may be straight or angled. These devices supply air in a constant direction and are usually attached to a separate supply air plenum that has its own inlet. Linear bar grilles can be installed in multiple sections to achieve long, continuous lengths or installed as a discrete length. Also commonly used in underfloor air distribution systems, some linear bar grilles allow the discharge pattern to be changed using removable or adjustable cores. Many also incorporate a damper and actuator for automatic control of the supply air flow rate via a space thermostat. Linear bar grilles are sometimes manufactured with curved shapes for architectural purposes. When linear bar grilles are used for supply, similar grilles are also commonly used as return or exhaust inlets to provide a consistent architectural appearance.

Eggcrate Grilles. These fixed-blade grilles are only recommended for return or exhaust applications and typically have large free areas exceeding 90% of the face.

Transfer Grilles. These fixed-blade grilles are installed in an interior wall or door allowing air transfer from an enclosed room to an adjacent room or plenum to allow normalization of pressure between them.

Security Grilles. Various fixed-blade grille types are available for high-security applications. Security grilles are available with additional features like heavy-duty construction, suicide deterrents, lattice faces, and tamperproof, nonremovable blade designs. Institutional guidelines exist for proper selection and placement of security grilles.

Heavy-Duty Grilles. Robust, heavy-duty designs are available for applications that require greater durability (e.g., industrial, gymnasium, institutional).

Spiral Duct Grilles. Grilles manufactured with a frame designed to be mounted directly onto the surface of a round duct without requiring an additional duct boot.

Round Diffuser. This diffuser is a series of concentric conical rings flush with the ceiling plane or with dropped inner cones, typically installed either in gypsum-board ceilings or on exposed ducts. Round ceiling diffusers are available in a broad range of sizes and capacities, and with or without adjustable inner cones that allow the diffuser to discharge air either horizontally or vertically.

Square Diffuser. This diffuser consists of concentric square, drawn louvers that radiate from the center of the diffuser. They are commonly mounted in suspended or gypsum-board ceilings or are attached to exposed ducts. Their necks can be round or rectangular, and the necks’ sizes should be matched to the ducts’. Available with faces that are flush with the ceiling plane or with dropped inner cones, these diffusers have a fixed horizontal radial discharge pattern or an adjustable discharge pattern that allows the direction to be either horizontal or vertical. Special borders can be selected to accommodate various mounting applications; for example, pans are available so that a desired face size can be smaller than a suspended ceiling’s grid size.

Perforated Diffuser. This diffuser consists of a duct collar and a single perforated plate that forms the diffuser’s face, with typical free area of about 50%. The perforated face tends to create a slightly higher pressure drop and sound than similarly sized square ceiling diffusers. They are available with deflection devices mounted at the neck or on the face plate. The deflectors may be adjustable, to provide horizontal air discharge in one, two, three, or four directions. Special borders can be selected to accommodate various mounting applications.

Louvered-Face Diffuser. This diffuser consists of an outer border, which includes an integral duct collar, and a series of louvers or curved blades. Louvered-face diffusers typically provide a horizontal discharge perpendicular to the louver length. The louvers may be arranged to provide four-way, three-way, two-way opposite, two-way corner, or one-way discharge. Some louvered-face diffusers are available with adjustable louvers that can change the discharge pattern. Special borders are available to accommodate various mounting applications.

Plaque-Face Diffuser. This diffuser is constructed with a duct collar and a single plaque that forms the diffuser’s face. This air outlet typically has a horizontal, radial discharge pattern. Special borders can be selected to accommodate various mounting applications.

Hemispherical Flow Diffuser. This diffuser provides a vertically radial air discharge pattern. The discharge penetrates the conditioned space perpendicular to the mounting surface. These diffusers are typically used for applications needing high air change rates, and/or low local velocities. Some outlet models are flush to the mounting surface; others protrude into the space below the ceiling. Most function similarly with or without an adjacent ceiling surface.

Laminar Diffuser. Sometimes called nonaspirating, this diffuser provides a unidirectional discharge perpendicular to the mounting surface. The free area of the perforated face is typically less than 35%. Most outlets are designed to develop a uniform velocity profile over the full face of the diffuser. This minimizes mixing with surrounding ambient air and reduces entrainment of any surrounding contaminants. These diffusers are typically used in hospital operating rooms, cleanrooms, or laboratories.

Linear Slot Diffuser. Linear slot diffusers may be installed in multiple sections to achieve long, continuous lengths, or installed as a discrete length. They can consist of a single slot or multiple slots and are available in configurations that provide vertical to horizontal airflow. Typically, a supply air plenum is provided separately and attached between the diffuser and the duct during installation. Other applications include field mounting the linear slot diffuser directly into a supply duct.

T-Bar Slot Diffuser. This diffuser is manufactured with an integral plenum and normally is installed in modular suspended (T-bar) ceilings. Available with fixed-deflection or adjustable-pattern controllers, these devices can discharge air from vertical to horizontal.

Light Troffer Diffuser. A light troffer diffuser serves as the combined plenum, inlet, and attachment device to an air-handling light fixture, which has a slot to receive the diffuser at or near the face of the lighting device, to discharge supply air into the space. Only the air-handling slot is visible from the occupied space.

Swirl Diffuser. This diffuser features a series of openings arranged in a radial pattern around the center of the diffuser face. This promotes a high degree of entrainment of room air, resulting in very high induction ratios, which maximize mixing in the area adjacent to the diffuser face. Swirl diffusers may be mounted in ceiling, sidewall, or floor locations. They are commonly used for UFAD applications where it is necessary to locate the supply outlets close to the space occupants. When mounted in floor locations, care should be taken to ensure that the diffuser can meet the floor loading requirements.

Displacement Diffuser. Typically located in floor or low-sidewall locations, this diffuser is designed to limit discharge velocities to 0.2 to 0.36 m/s to minimize mixing between supply and room air. These outlets tend be large to generate the low velocities required for thermal displacement ventilation: low-velocity discharge allows the cooler supply air to fall to the floor and remain there because its buoyancy is lower than that of the ambient room air above it. These outlets are available in various shapes and configurations that facilitate flush or adjacent mounting to the sidewalls or floor of the space.

Textile Air Dispersion System. This outlet system is designed to both convey and disperse air within the space being conditioned. Diffusion options include outlets selected for a full range of entrainment. The supply air is delivered through (1) the weave of the fabric, (2) microperforations, (3) a long and narrow pattern of small jets, (4) orifices, and/or (5) nozzles. Typically, these systems are made of fabric, sheet metal, or plastic film.

Variable-Air-Volume (VAV) Diffuser. A VAV diffuser is an air outlet that self-modulates its primary air flow rate with an integral air damper. Select outlets based on the total range of airflow for the space served. Outlet performance characteristics should be evaluated at both minimum and maximum flow. More information regarding selection of outlets can be found in Chapter 58 of the 2023 ASHRAE Handbook—HVAC Applications.

Variable Geometry Diffuser. One type of VAV outlet is a variable geometry diffuser. These diffuser assemblies vary their discharge area or air pattern in response to changes in space or supply air temperature.

Dampers can be attached to grilles (the combination of a grille and a damper is called a register), diffusers, or installed as separate units in the duct, to regulate the device’s airflow rate:

Other miscellaneous accessories, such as remote control devices to operate the dampers, and travel limit stops, are also available.

Figure 4. Accessory Controls for Ceiling Diffusers

When these dampers are located on or near the air outlet, they may cause nonuniform airflow discharge and increased pressure drop and sound. In the 2023 ASHRAE Handbook—HVAC Applications, Chapter 49 describes the effects of damper location on sound level and Chapter 58 addresses the effects of damper location on sound, throw performance, and pressure drop.

Extractors are installed in duct-collar connections to the outlet and are used to improve the flow distribution into a supply outlet. The device shown in Figure 3C has vanes that pivot such that the supply airflow to the grille or register remains perpendicular to the face of the outlet. The device shown in Figure 3D has fixed vanes. Both devices restrict the area of the duct in which they are installed and should be used only when the duct is large enough to allow the device to open to its maximum position without unduly restricting airflow to the downstream duct. These devices may increase the system pressure requirement, thereby limiting downstream airflow and increasing the sound level. An extractor mounted directly to the grille, as shown in Figure 3F, is sometimes called a scoop.

Adjustable vanes, as shown in Figure 3E, are individually adjustable. Typically, two sets of vanes are used. One set equalizes flow across the collar, and the other set turns the air. The vanes should not be adjusted to act as a damper because balancing requires removing the grille to gain access and then reinstalling it to measure airflow.

Equalizing or flow-straightening devices or grids allow adjustment of the airstream to obtain more uniform flow to the diffuser.

Quadrant blank-off flat plates restrict airflow in one or more quadrant(s) of the diffuser inlet.

Other balancing devices are available. Consult manufacturers’ literature as a source of information for other air-balancing devices.

Single-duct terminal units can be cooling only, heating only, or cooling and heating. Terminal units for cooling modes often have reheat coils that add sensible heat to the supply air as part of the overall humidity control. Hot-water or steam coils, or electric resistance heaters, are placed in or attached directly to the air discharge of the terminal unit. This type of equipment can provide varying reheat, customized to the thermal zone being served.

The basic single-duct unit consists of an insulated casing, airflow regulator, and possibly also an actuator, airflow-measuring device, and selected controls. Accessory discharge attenuators, silencers, and multiple outlet attenuators are also common. In the lower and middle latitudes, typical applications include those with

A single-duct unit with a reheat can also be used in zones where heating-mode needs are low so that the reheat coil can serve dual purposes. The terminal unit usually reheats at the minimum airflow setting. An auxiliary higher heating setting may be available as an option with additional controls.

Interior zones, where ventilation requirements may be larger than the desired heating airflow, may require additional reheat capacity.

A single-duct exhaust valve is a type of single-duct ATU (air terminal unit) designed for exhaust air and consists of an insulated casing, airflow regulator, and possibly also an actuator, airflow-measuring device, and selected controls. Accessory inlet attenuators or silencers are sometimes used. Specialty materials may be needed for corrosion resistance. The designer should consider pressure drop and system effects.

A venturi valve is a type of single-duct ATU and can be used for either supply or exhaust. Traditionally, the device consisted of a round venturi valve body and an internal shaft that supports a cone assembly. Newer technologies remove the internal cone assembly and use damper blade assemblies to control airflow.

Cone Assembly Type. The cone assembly is spring-loaded on the shaft. As pressure in the primary duct increases, the cone compresses the internal spring and moves closer to the venturi throat, increasing resistance to the airstream. As pressure in the primary duct decreases, the spring retracts and the cone assembly moves farther from the venturi throat, decreasing resistance to the airstream. This is called mechanical pressure independence. If the venturi valve is provided with an actuator to allow for variable airflow, the actuator and positioning arm reposition the shaft, which in turn moves the cone assembly relative to the throat, changing the airflow.

Damper Assembly Type. The damper assembly is controlled by an actuator that responds to differential pressure between two different points along the venturi valve body or by other means. The damper modulates to control airflow as pressure fluctuates, yielding a pressure-independent system that directly measures airflow through the valve body. A controller is used to modulate the valve damper actuator and control the overall system sequence of operation, whether constant volume or variable volume.

Dual-duct terminal units are typically controlled by a room thermostat or sensor. They are each connected to two supply air ducts that deliver two of warm, cold, recirculated, mixed, or outside air to provide desired room control. Volume-regulated units have individual airflow control devices to regulate the amount of air from the two supply ducts. Dual-duct units can have reheat coils too if mixing of the hot and cold ducts’ air is insufficient for humidity control or space heating. When a single temperature control device regulates the relative amounts of both warm and cold air to control temperature, a separate airflow control device may function to control and limit total airflow from the unit. Specially designed baffles may be required inside the unit or at its discharge to mix varying amounts of warm and cold air and/or to provide uniform temperature distribution downstream. Dual-duct units can be equipped with constant- or variable-flow control. These are typically pressure independent, to provide consistent flow rate and temperature control. Dual-duct terminals may also be used with outdoor air conditioned and supplied by a primary air-handling unit and ducted to the terminal units; such systems’ AHUs maintain the required volumetric flow of ventilation air into the spaces served. Dual-duct units with cooling and outdoor air may need a local heating device; these primary-secondary air distribution systems thus need careful analysis and design.

A nonmixing dual-duct unit is effectively two single-duct terminal units side by side. The basic unit incorporates separate cold and hot air inlets that are to be attached to the two primary-air ducts, with sometimes neutral-temperature air in one of the ducts (e.g., during mild weather). The units are usually applied in exterior zones in buildings where overhead heating and cooling are desired but not auxiliary heating, and when zero minimum airflow is acceptable during changeover between heating and cooling. Proper discharge configurations are required to reduce the likelihood of improper mixing.

A mixing dual-duct unit is the same, with an integral mixing/attenuator section on the downstream end of the terminal unit to minimize temperature stratification in the discharge airstream. They are often used in interior and exterior zones in buildings (e.g., hospitals) where overhead heating and cooling are desired, but an auxiliary heating coil is not, and zero minimum airflow is unacceptable during changeover between heating and cooling. Evaluate mixing performance to fit the application, and consider the additional pressure drop of mixing/attenuators sections.

Induction terminals supply primary air or a mixture of primary and recirculated air to the conditioned space. They achieve this function with a primary air jet that induces recirculated air, known as secondary air in this application, via grilles to the ceiling plenum or from individual rooms via a return duct. Typically, the primary air is of higher design velocity than that delivered to other types of terminal units, and the extra momentum is used as the inducing energy source. The induction unit contains devices that are actuated in response to a thermostat to modulate the mixture of primary air and induced secondary air. Reheat coils may be required to meet interior load requirements. Historically, induction units were used with high-velocity ducts, and often produced much noise. Modern versions often work well with much lower-velocity air but require careful selection. Generally, induction units are considered where there is a desire for zone-level air distribution equipment, installed in or over occupied spaces, that have no fans or other components that may require frequent access for maintenance. Any terminal unit that uses recirculated air must have an air filter, and that filter should be readily accessible for inspection and periodic replacement.

Fan-powered terminal units are used as secondary-level air handlers in primary-secondary air distribution systems and are typically installed in return air plenums. They can be specified with either heating, reheating, or cooling coils, or a combination. They differ from air-to-air induction units in that they each include a blower, driven by a small motor, which moves air from the conditioned space, ceiling plenum, or floor plenum, that may be mixed with the primary air from the main air handler. The characteristics of fan-powered units are

Some units are equipped with special insulation and a vapor retarder to reduce condensation with below-dewpoint supply temperatures.

Fan-powered terminal units can be divided into two categories: (1) series, with all primary and induced air passing through a blower in the unit that operates continuously during the occupied mode; and (2) parallel, in which the blower operates only on demand when induced air or heat is required.

Fan-powered terminal units typically use fractional horsepower alternating current (ac) electric motors for their included blowers. Increasingly, high-efficiency electrically commutated motors (ECMs) are used instead of older, less efficient motor designs. The efficiency of the motor becomes more important as the units operate for more hours of each year.

A series unit typically has two inlets, one for primary air from the central air handler and one for secondary air recirculated from the spaces served, often via a ceiling or floor plenum. All air delivered to the zone passes through the blower. The blower operates continuously whenever the primary air fan is on and can be cycled as required when the primary fan is off. As the zone’s load decreases, an airflow control device throttles the amount of primary air delivered to the blower. The blower makes up for this reduced amount of primary air by acquiring more air from the conditioned space or ceiling plenum through its secondary air inlet. Sometimes a series unit has two ducted inlets, like a dual-duct terminal unit, in addition to the induction air inlet, which may or may not have ducting to it. The second primary air duct is typically used for dedicated outdoor air systems. Fan airflow and primary air can be varied when the units are in part-load condition, but fan airflow should never be less than the total amount of air supplied to the unit by the ducted inlets.

Fan-Powered Series Flow. The basic unit consists of a single-duct unit, blower/motor, and selected controls where the motor and primary damper are arranged such that mixing occurs upstream of the blower; this unit is depicted in Figure 5. They also have been called constant-volume or constant-fan units. Supplemental heating coils, such as steam, hot water, or electric, are generally required. Electric-resistance heating coils are typically located on the discharge of the unit; water coils may be on the discharge or the induction port. Heating coils on the induction port, and thus upstream of the unit’s blower, increase ambient temperature at its motor and decrease motor life. Supplemental cooling coils are sometimes located on the induction port for some applications like when utilized in conjunction with dedicated outdoor air systems. A low-profile version of the fan-powered series flow unit is typically less than 300 mm tall and is commonly used where zoning requirements limit building height and the architect wishes to maximize the number of floors, because these units fit in a shallow ceiling plenum. Unlike standard fan-powered terminals, the fan/motor assembly is installed flat on its side with the wheel rotating in a horizontal plane.

Fan-powered series-flow units are generally used for

Dual Air Inlet Fan-Powered Series Flow. These units are similar in construction to the standard series-flow terminals but have an additional air inlet that provides a direct connection to the terminal unit for ventilation air. They are commonly used in buildings where ventilation air is supplied via a dedicated ventilation duct system to each terminal unit; this is generally done where it is desirable to monitor ventilation air quantities to each zone.

Low-Temperature Fan-Powered Series Flow. These units are the same as fan-powered series flow but have special construction to minimize the potential for condensation. They can be used with cold-water/ice storage systems that provide below-dewpoint temperature primary air to the zone terminals, or where standard terminals may be exposed to high humidity.

Underfloor Fan-Powered Series Flow. This type is a fan-powered series flow terminal unit designed to fit between the pedestal supports of raised- or access-floor systems. They are available in several unit sizes, but with limited height and width.

Primary and induction ports, if any, to such units may or may not be ducted. Typically, in cooling mode, cool primary air is supplied to the well-sealed floor plenum, and heated air may be ducted to the terminal unit that is then mixed with the cool air in varying amounts. In these cases, the control system must perform the proper damper sequence to control room air conditions.

Parallel fan-powered terminal units, in cooling mode, supply the primary air directly to the space, bypassing its fan. In heating mode, the blower section induces plenum air and is mounted in parallel with the primary airflow control device, this unit is depicted in Figure 6. A backdraft damper is included to limit the amount of primary air flowing through the blower section when the blower is not energized. The blower in these units is generally energized after the primary airflow has reached the minimum flow rate. Typically, the parallel unit provides constant-air flow rate heating and variable air flow rate cooling. Although it is not recommended due to control instability, these units are sometimes applied in CAV mode with electronically commutated motors programmed to maintain constant airflow rate discharge. The motor control algorithm gradually increases the unit’s fan speed as the primary airflow is reduced. Parallel units are typically limited to one ducted supply inlet.

One weakness in parallel fan-powered terminals is leakage of primary air through the fan’s backdraft damper and the mixing chamber housing. ASHRAE research project (RP-1292 (David 2007) demonstrated that much of the energy saving associated with intermittent fan cycling is offset, and in fact may be significantly less than the energy loss caused by this leakage.

Figure 5. Series Fan-Powered Terminal Unit

Fan-Powered Chilled Water Series Flow. This unit type takes all the features of a basic fan-powered series flow unit and incorporates a supplemental cooling coil on the induction port. Typically, this cooling coil is designed for sensible cooling only, to avoid condensation issues and in many cases, is used in conjunction with dedicated outdoor air systems.

Fan-Powered Parallel Flow. Sometimes called variable-volume or intermittent-fan units, these consist of a single-duct unit, blower/motor, backdraft damper, and selected controls; the motor and primary damper are arranged such that mixing occurs downstream of the blower. Supplemental heating coils, either steam, hot water, or electric, are generally required. Electric-resistance heating coils are typically located on the discharge of the unit. Water coils may be on the discharge or the induction port, although the discharge location adds to the supply air system’s static pressure requirements and increases leakage through the backdraft damper, as shown in ASHRAE RP-1292 (Davis 2007). Heating coils on the induction port increase the air temperature at the unit’s fan motor and decrease its life.

Fan-powered parallel-flow units are used in exterior zones where heating and cooling loads may vary considerably, and in buildings where heating is needed when the central air handler is shut down during unoccupied hours.

Figure 6. Parallel Fan-Powered Terminal Unit

Bypass Terminal Units. A bypass terminal unit handles primary air through its inlet. The unit bypasses primary air to the ceiling plenum or to ducts leading to the air handler’s return to meet the needs of the conditioned space. Primary air, diverted into the ceiling plenum, returns to the central air handler as return air or building exhaust. This method provides a low first cost with minimum controls, but it is energy inefficient compared to other systems.

The basic bypass terminal consists of a diverter-type damper, actuator, bypass port, and selected pressure-dependent controls. A balancing damper is recommended ahead of the inlet.

Bypass terminals are used primarily with packaged rooftop air conditioning equipment with a direct-expansion (DX) coil where zoning is desired, but relatively constant airflows across the system components (e.g., coils, fans) are required to minimize the potential for freeze-up. The system offers a lower initial cost design. It does not provide the energy-saving advantages of a VAV system with fan-speed control; almost all the energy savings are from the reduced motor speeds that occur except at full design-load conditions.

Active beams, as shown in Figure 10A are induction terminals that consist of a primary air duct connection, a series of induction nozzles, hydronic heat transfer coil(s), supply outlet(s), and induced air inlet section(s). Primary air, discharged through the induction nozzles, creates a lower-pressure region that induces room air through the chilled- and/or hot-water coil(s). The room air is conditioned before being mixed with primary air, and then the mixture of primary and conditioned induced air is then discharged to the space. This mixture’s temperature and/or flow rate is modulated in accordance with the space demand and control strategy. Active chilled beams are available in the following configurations:

Active beams, when properly applied, generally produce fully mixed room air distribution, although displacement ventilation versions (cabinet style) are available as well. Their air distribution characteristics depend on the beam’s supply outlet configuration, like those described in Table 1. Most active beams use linear slots on their discharge, and room air distribution is therefore characteristic of that outlet type because the induced air produces significant supply air velocity. The following piping configurations are generally used:

Cooling-only active beams use two pipes (supply and return) to their coils. Active beams can also be used for overhead heating; when applied as such, the beams are typically provided with four-pipe combined coils. These beams’ coils have a common fin stack, but the tubing that passes through the stack is dedicated to either cooling or heating. In most cases, 75% of the tube passes are dedicated to cooling, and 25% are supplied hot water. The water flow rate through either circuit is varied by zone chilled- and/or hot-water control valves that are modulated the zone temperature sensor.

Where both heating and cooling are needed but two-pipe beams are used, six-port control valves can be effective. These valves incorporate four supply and return ports for both chilled and hot water. They then determine the water source that is required and modulate the flow to and from one or more two-pipe beams through a shared local two-pipe system. This can considerably reduce zone piping costs and allow the use of more efficient two-pipe circuitry in the beam heat transfer coils; however, it loses the ability to heat or cool each beam individually, if jointly connected on a local two-pipe system.

Overhead beams, if so designed, may be used in suspended or finished ceilings, or independently mounted below. Although active beams are usually installed in the upper part of the space, they can also be located in sidewalls or at floor level, with their supply air delivered at high or low velocity, depending on the air distribution strategy (mixing or displacement) that is desired. Both active and passive chilled beams can be integrated with additional services (e.g., lighting or fire protection) as required.

Passive beams, as shown in Figure 10B, consist of a sensible cooling coil and a casing, and can be fitted into an architectural enclosure when exposed. They provide space sensible cooling mainly by negative-buoyancy-driven natural convection: the coil provides heat exchange between the chilled water and the induced room air. When cool water is circulated through the coil, room air between the fins is cooled and falls naturally into the room. Displaced room air then pushes air through the beam’s coil.

Figure 10. (A) Active and (B) Passive Beams

There are two basic types of passive beams: exposed and recessed. Exposed passive beams are commonly used in spaces without suspended ceilings (i.e., in rooms with either finished or no ceilings).

Recessed passive beams are generally integrated into a suspended ceiling. When passive beams are installed in such a ceiling, a minimum clearance between the top of the beam and any surface above it should be provided to ensure a sufficient return air path. Generous shadow gaps, dummy beam sections, and/or return or transfer grilles are recommended for creating the near-zero pressure loss return air path from the room to the plenum.

Passive beams cannot be used effectively for heating spaces below them due to positive air buoyancy. Instead, a decoupled heating system (e.g., low-level finned tube or radiant panels or floors) is commonly used. However, the passive beams may be used at times to intentionally warm the plenum or floor above them somewhat, for comfort or freeze protection.

Space ventilation and dehumidification must be provided by a complementary air delivery system. Air delivery systems that promote fully stratified or partially mixed air distribution when in cooling mode are often preferred, because passive beams mounted high operate more efficiently in thermally stratified environments.

Additional information on active and passive beams can be found in Woollett and Rimmer (2014) and Chapter 58 of the 2023 ASHRAE Handbook—HVAC Applications.