Providing thermal comfort for occupants is a primary goal of any air-distribution system. Industry guidelines offer designers a roadmap on how to attain those goals along with meeting codes such as LEED. ASHRAE Standard 55-2013 Thermal Environmental Conditions for Human Occupancy and ASHRAE Standard 62.1-2010 Ventilation for Acceptable Indoor Air Quality are two such guidelines. These standards can help optimize the health, comfort and energy efficiency in buildings.
Defining ASHRAE Standards
The occupied zone is defined by ASHRAE 55-2013 as: The region normally occupied by people within a space, in absence of known occupants, generally considered to be between the floor and 6 ft. level above the floor and more than 3.3. ft. from outside walls/windows or fixed heating, ventilation, or air-conditioning equipment and 1 ft. from internal walls.
An adequate supply of ventilation air to the space’s breathing zone is also a design requirement. Ventilation air is defined by ASHRAE 62.1 2016 as: That portion of supply air that is outdoor air plus any recirculated air that has been treated for the purpose of maintaining acceptable indoor air quality. And the breathing zone is the region within the occupied space between planes, 3 and 72 inches above the floor.
Thermal Comfort: Not One-Size-Fits-All
Temperature: ASHRAE 55 requires allowable vertical air temperature difference between head and ankles to be no more than 5.4F (3.0 C).
Humidity: There is no defined range of humidity level but the dew-point temperature is required to be less than 62.2 F.
Clothing insulation: Keep in mind the range of operative temperatures where people wearing lighter clothing (shorts, skirts, short-sleeve shirts, etc.) and heavier clothing (pants, long-sleeve shirts, etc.) is narrow.
Air velocity: Spatial velocities should be less than 50 feet per minute (fpm) during cooling mode and less than 30 fpm during heating mode.
Activity level of the occupants: An office's metabolic rate is typically between 1.0 (sedentary) to 1.3 (casual movement)
Partially mixed (most underfloor air distribution systems)
Fully mixed (overhead distribution)
Fully stratified (displacement ventilation)
Conserving energy by comfort-conditioning a space’s lower occupied level and stratifying its upper level is the goal of partially mixed systems. Swirl diffusers or rectangular-shaped outlets that deliver conditioned air from the plenum under the floor help enable occupant comfort.
A challenge for these systems are perimeter zones for partially mixed systems. For one, the loads are dynamically changing due to outdoor solar and air temperature changes. And two, choosing outlets limit the throw of the air pattern present a design hurdle. Placing a low-profile fan-powered terminal unit below the floor near the perimeter is one way of designing for perimeter zone control.
Partially mixed systems have a number of advantages. They are ideal for situations where cabling is provided to each work stations. They can also have a lower first cost than fully mixed systems, depending on the design. And because these systems are designed with low supply air pressure, they help save fan energy.
When selecting an air outlet consider the air’s pattern of delivery to the space. For example, a ceiling diffuser typically has either a circular (radial) or cross-flow (directional) discharge air pattern. By providing less drop and more uniform temperatures, a circular pattern is ideal for variable air volume (VAV) cooling. The cross-flow air pattern has longer throw, but its reduced induction means it may lose ceiling effect, which creates drafts in the occupied zone.
Perimeter heating is another factor. ASHRAE Standard 62.1-2016, which ensures ventilation air supplied to a space also be delivered to the breathing zone, has a list of requirements that must be accounted for (Table 6-2). For ceiling supply of warm air with a ceiling return, the requirements for heated air are to reach a terminal air velocity of 150 feet-per-minute to within 4.5 ft. of the floor. The differential temperature between warm supply air and space temperature with a ceiling return must be 15 degrees or less. When the heating supply-air temperature exceeds the 15 degree limit, the ventilation air volume must be increased by 25%.
Thanks to their flexibility, fully mixed systems can meet most applications’ air distribution challenges. They also can be very economical, since they typically have the lowest first cost.
Fully stratified system shown at left
Through an outlet placed at floor level that’s centrally located or near or in walls, these systems condition spaces via discharged cool supply air. Low velocity air (<80 fpm) is discharged horizontally across the floor; until it hits a heat source this air moves with little mixing across the floor. This cooled air will mix with radiant heat, form a source, then stratify toward the ceiling.
Thermal displacement ventilation (TDV) systems offer energy savings and efficiency that other systems can’t match. They require less ventilation air to comply with ASHRAE 62.1, and they can use air side economizers and warmer temperatures to match supply air temperatures. And while TDV systems of the past typically required a heating system that was separate, but new systems are able to heat and cool using a single DV unit, simplifying their installation and maintenance.
Designing for Comfort Pays Dividends
There are many ways to establish and maintain occupant comfort. Which system best accomplishes this depends on what your space requires, but the important thing is to keep people comfortable, period. After all, studies have shown that occupants whom are comfortable are more productive, which will pay dividends for years to come.
For information on this topic, please contact Jim Aswegan at email@example.com or Titus Communications at communications