by Jim Aswegan and David Pich, PE, LEED AP
The goal of a room air distribution system is to provide thermal comfort and a healthy living environment for occupants in the space.
American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 55-2013, Thermal Environmental Conditions for Human Occupancy, and ASHRAE 62.1-2013, Ventilation for Acceptable Indoor Air Quality, provide designers with the guidance to optimize health and comfort in building spaces. Many codes, along with the U.S. Green Building Council’s (USGBC’s) Leadership in Energy and Environmental Design (LEED) 2009 program, require compliance with these ASHRAE Standards.
ASHRAE 55-2013 defines the occupied zone as:
the region normally occupied by people within a space, generally considered to be between the floor and [2 m] 6 ft level above the floor and more than [1 m] 3.3 ft from outside walls/windows or fixed heating, ventilation, or air-conditioning equipment and [0.3 m] 1 ft from internal walls.
The space from the interior walls inward 0.3 m serves as a mixing zone where room air is entrained into the supply air jet and mixes to provide thermal comfort in the occupied space. When designing under-floor air distribution (UFAD) or thermal displacement ventilation (TDV) systems, the occupied area around the outlets may be excluded to a boundary where the total air jet from the outlet contains velocities greater than 0.25 m/second (50 fpm). These areas may also be known as the ‘clear,’ ‘adjacent,’ or ‘near’ zone.
Any design must also include an adequate supply of ventilation air to the breathing zone of the space. ASHRAE 62.1-2013 defines ventilation air as:
that portion of supply air that is outdoor air plus any re-circulated air that has been treated for the purpose of maintaining acceptable indoor air quality.
The breathing zone, on the other hand, is the region within the occupied space between 76 and 1830 mm (3 and 72 in.) above the floor.
Determining thermal comfort
The primary factors to be considered when determining conditions for thermal comfort in the occupied space are:
- air velocity;
- clothing insulation; and
- activity level of the occupants.
All these factors are inter-connected when determining the general occupant comfort of a space. The ideal temperature (operative temperature) is where the occupant will feel neutral to his or her surrounding—feeling neither heat loss to, nor heat gain from, the space.
While the range of acceptable operative temperature may vary depending on other conditions, ASHRAE 55 requires the allowable vertical air temperature difference between head (1702 mm [67 in.]) and ankles (100 mm [4 in.]) be limited to 3 C (5.4 F). Ideal air velocity in the space can vary with other factors, but the goal is generally to keep spatial velocities less than 0.25 m/second during the cooling mode and less than 0.15 m/second (30 fpm) during the heating mode.
For many years, the authors’ company has recommended maintaining the relative humidity level in the space between 25 and 60 percent. ASHRAE 55 does not define a lower limit, and requires the dewpoint temperature be less than 16.8 C (62.2 F).
Another factor affecting comfort is the occupant’s clothing insulation level—the clo. In most office environments, occupants’ clo level is between 0.5 and 1.1, where 0.5 would be a person wearing no socks, sandals, short sleeve shirt or blouse, and shorts or skirt. The 1.1 clo level would include long pants, socks, long sleeve shirt, and dress coat or sweater. The range of operative temperature where both a 0.5 and 1.1 occupants are in the same space is very narrow. The final item of consideration for design comfort is the intended activity level of the occupant in the space. In most office environments, the metabolic (met. rate) is between 1.0 and 1.3. This includes sedentary occupants to casual movement about the space.
The three common methods of room air distribution used in commercial buildings are:
- fully mixed (e.g. overhead distribution);
- partially mixed (e.g. most UFADs); and
- fully stratified (e.g. TDV).
Since interior zones usually have adequate heat loads from occupants and equipment and few heat losses, the discussion for interior spaces will solely concern cooling. For the perimeter spaces, this article discusses how to meet the requirements for heating and cooling from the same overhead outlet. Design methods for cooling an interior zone and heating a perimeter zone vary with each method.
For fully mixed systems, the pattern of the air delivered to the space must be considered when selecting an air outlet. Ceiling diffusers typically exhibit flow in a circular (i.e. radial) or cross-flow (i.e. directional) discharge air pattern. The circular pattern usually provides shorter throw and higher mixing; it tends to maintain ceiling effect to low velocity before turning back on itself. This pattern is ideal for variable air volume (VAV) cooling by providing less drop and more uniform temperatures in the space.
The cross-flow air pattern has longer throw, but less induction means it may lose ceiling effect, creating drafts in the occupied zone. Plenum slot diffusers typically discharge air in a directional air pattern, but some are available with ‘spreaders’ to produce a more radial discharge air pattern. Sidewall grilles equipped with vertical deflectors can be adjusted from zero degree (directional pattern) to 45 degree spread (radial pattern). Regardless of the desired type of outlet, the air pattern can be either radial or directional to best meet the space’s comfort requirements.
For perimeter applications where the same outlet is being used for both heating and cooling, a linear slot diffuser or plenum slot diffuser is typically employed. When a fixed air pattern diffuser is used, it is typical to supply half of the air across the ceiling for cooling and half down the glass for heating.
For perimeter heating, the requirements for Table 6-2 of ASHRAE 62.1-2013 must be considered. The intent of table 6-2 is to ensure the ventilation air supplied to the space be delivered to the breathing zone as well. For ceiling supply of warm air with a ceiling return, the requirements for heated air are to reach a terminal air velocity of 0.76 m/second (150 fpm) to within 1.4 m (4 ½ ft) of the floor.
To a terminal velocity of 0.76 m/s or more, air is temperature-independent. This means the distance air travels will be the same for isothermal (catalog values), warm, and cool air. In other words, during heating, ventilation air will be pushed down into the breathing zone with enough heat energy to meet ASHRAE 55’s requirement for a temperature gradient of less than 3 C (5.4 F). Additionally, the differential temperature between warm supply air and space temperature with a ceiling return must be 8 C (15 F) or less. Thus, the maximum supply air temperature for a 24-C (75-F) room would be 32 C (90 F). When the heating supply air temperature exceeds the 8-C limit, the ventilation air volume for heating must be increased by 25 percent.
Choosing an auto-changeover diffuser does not change the ASHRAE 62.1 requirements, but will lower energy cost and improve comfort in the space. Delivering all the warm air down the glass during heating will save energy. With a fixed pattern diffuser, half the warm air will be discharged across the ceiling and, with a ceiling return, can be short-circuited without reaching the occupied space level. Additionally, higher comfort will be realized in the space as the heated air can be designed to deliver warm air all the way to the floor. Comfort may be increased during cooling as well—the cool air will be projected across the ceiling, eliminating potential for drafts from the jet projected down the glass with a fixed pattern diffuser.
For fixed pattern outlets supplying cool and warm air to the perimeter, ceiling heights of less than 3.7 m (12 ft) is desirable so outlets can be selected to provide adequate heating without excessive drafts on the floor during cooling conditions. A floor-to-ceiling wall located within 4.6 m (15 ft) of the perimeter wall will help contain the warm air distributed across the ceiling. Auto-changeover outlets have a bit more flexibility, as long as the throw for the heated air down the glass is long enough to comfort condition the space at acceptable noise levels from the outlet.
Fully mixed systems are flexible enough to accommodate air distribution challenges for most applications by providing adequate thermal comfort for cooling and heating. The systems described in this article may provide some advantages for specific applications, which will be discussed.
Fully mixed systems will provide the lowest first cost for comfort control. Architectural features can add to the cost without improving performance. For perimeter spaces, a single-duct terminal with a reheat device will be a lower first cost, but higher operating expense, than a fan-powered unit with heat. The additional operating cost may be inconsequential for moderate climate zones, but cost-prohibitive for climate zones with more severe winter conditions. Providing a separate heating source, such as baseboard radiation, is common for severe winter climate zones as well.
For partially mixed air distribution systems (typically UFAD), the core area usually experiences even loading throughout the occupied area. The goal of partially mixed systems is to save energy by comfort conditioning the lower occupied level in the space and allowing the upper level to stratify. Occupant comfort is achieved by delivering cool, conditioned air from the plenum under the floor through swirl diffusers or rectangular-shaped outlets near the occupants work area.
Individuals can enhance their personal comfort by adjusting the damper at the outlet near their workspace. For common areas such as hallways and break rooms, outlets can be equipped with actuators controlled by a common thermostat located in the space.
Perimeter zones for partially mixed systems create a greater challenge as the loads are dynamically changing due to outdoor solar and air temperature changes. A common method for perimeter zone control is locating a low-profile, fan-powered terminal unit under the floor near the perimeter supplying air to linear bar grilles. The fan-powered terminal can be equipped with an electric or hydronic coil. Cool plenum air can be supplied to the outlets when cooling is required and the coil can be employed to warm the air as required during heating conditions. The design challenge is selecting outlets that will limit the throw of the air pattern so that air will not bounce off the ceiling and create drafts in the adjacent occupied area.
Energy to operate the fan terminals can be eliminated, and higher comfort can be achieved on the perimeter, by using a passive system of VAV cooling and heating perimeter distribution outlets. With a 150-mm (6-in.) wide custom design bar grille located along the perimeter of the space, a modular 1.2-m (4-ft) long sliding damper with transverse apertures (cooling) can be attached to provide up to 106 L/s (225 cfm) at 17.5 Pa (0.07-in.) plenum pressure.
The damper is controlled by a space thermostat to provide cooling as required. The special arrangement of bars in the grille is designed to limit the throw from the outlet during cooling. A 1.2-m long plenum with fin-tube hydronic or electric heating elements can be attached to the grill to provide up to 0.9 kW (3000 Btu) heat to the perimeter.
The heating units operate by combining the cool convection currents from the glass with the warm currents on the floor. The mixture is induced through the heat exchanger with warm air being discharged through the grille and up the glass. Space temperature is controlled by a room thermostat controlling the water flow or electric current flow to the electric heating element. The modular design allows the system to be customized for use in multiple climate regions.
UFAD systems are ideal for applications with cabling being provided to each individual work station. Additional monies can be saved by reducing the cost to reconfigure the footprint of the work area to accommodate changes in space work requirements.
The engineer must consider supply air temperature rise in the raised floor of a UFAD system. Depending on construction, supply temperature rise can exceed 0.5 C (1 F) per 3 m (10 ft), so large floor plates generally require multiple supply air injects points, either covered by multiple air handlers per floor or under floor ductwork. Multiple floor air-handling units need more usable floor space and underfloor ductwork can limit the floor plate’s flexibility. Therefore, buildings with large floor plates are not ideal for this type of system.
UFAD systems are generally designed with low supply air pressure, which allows for fan energy savings over a more conventional fully mixed overhead system. To maintain the low-pressure design, the floor plenum needs to be clear of obstruction. Full-height walls—deck-to-deck or deck-to-ceiling—will degrade UFAD system performance. (It can be overcome, but it is not ideal.)
There are many variables in a building with a UFAD system. The floor is an additional expense but, depending on the design of the building system, there is the opportunity to shorten the floor-to-floor cost, which can provide considerable savings. Depending on the UFAD system design, first cost can be lower compared with a fully mixed system with manual diffusers, limited ductwork, and minimal testing and balancing compared to a comparable overhead system.
Maintenance should be similar to a fully mixed overhead system, if a fan-powered perimeter is employed. When a passive VAV perimeter system is used, maintenance should be less, with fewer fan-powered components to maintain.
In a fully stratified design, which typically involves TDV systems, a space is conditioned by discharging cool supply air through an outlet either positioned at floor level near or in a wall or centrally located in the open space. Low-velocity air (i.e. < 0.4 m/s [80 fpm]) is discharged horizontally across the floor. Air moves with little mixing across the floor until it contacts a heat source such as an occupant or piece of warm equipment in the space. Cool air will mix with the radiant heat from the source and stratify toward the ceiling. The return is usually located at or near the ceiling.
The area between the outlet and where the air speed reaches 0.2 m/s (40 fpm) is the ‘clear zone,’ and should not be included in the occupied area. Some manufacturers provide units with adjustable air patterns so the clear zone can be controlled to meet project requirements for space occupancy.
ASHRAE 62.1’s Table 6-2 provides the information required to calculate the minimum ventilation rates for outdoor air in the breathing zone. It offers an Ez factor for various air distribution configurations to be divided into the Table 6-1 value to ensure the minimum prescribed outdoor air reaches the breathing zone. For TDV systems, the Ez factor is 1.2—this means ventilation air can be reduced by 17 percent, or the 17 percent can be used toward the 30 percent LEED requires for an additional Indoor Environmental Quality (EQ) Credit 2, Increase Ventilation Effectiveness.
While TDV systems typically require a separate system for heating, new systems now have the ability to heat and cool using one displacement ventilation unit. A standard rectangular outlet is located near or mounted in a wall discharging cool air from the upper chamber. When heating is required, an internal baffle moves to change the flow of air from the upper chamber to the lower chamber where it flows through a linear bar grille to satisfy heating requirements.
A TDV requires similar ductwork and distribution as a fully mixed system. First cost is generally higher because TDV is large and covers a greater area than diffusers in mixed systems. Maintenance cost should be comparable to a fully mixed system. However, since TDV generally requires floor space to place the diffuser, it is not ideal for spaces with high churn.
TDV and low-mixing UFAD system will require less ventilation air to comply with ASHRAE 62.1. These systems can utilize air-side economizers and warmer temperatures to match the warmer supply air temperatures. These combined energy-saving features help increase overall system efficiency and decrease long-term payback over a fully mixed system.
Regardless of which type of system is being specified on a project, studies have shown occupants who are comfortable are more productive.1 In other words, designing for comfort pays back dividends forever.
1 Examples include “Productivity and Indoor Environment,” by Derek Clements-Croome and Li Baizhan (University of Reading, Department of Construction Management and Engineering), which appeared in volume one of the 2000 Proceedings of Healthy Buildings. Another example can be found in Susan S. Lang’s “Warm Offices Linked to Higher Productivity,” from the March 2005 issue of Human Ecology.
Jim Aswegan is chief engineer at Titus, with 48 years of service. He provides applications support for engineers and participates in industry organizations including the American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE), the U.S. Green Building Council (USGBC), and the Air-conditioning, Heating, and Refrigeration Institute (AHRI). Aswegan’s expertise includes grilles, registers, and diffusers (GRDs), along with terminal units and acoustics. He can be contacted via e-mail at firstname.lastname@example.org.
David Pich, PE, LEED AP, is the director of HVAC technology for Titus, and has more than 18 years of experience in consulting engineering. He provides application support for engineers and, as a LEED AP, offers technical training in Titus’ consulting Engineer Seminars. Additionally, Pich supports and participates in industry organizations such as ASHRAE, USGBC, and AHRI. He can be contacted via e-mail at email@example.com.