How to address thermal comfort in commercial buildings
- Understand how to achieve thermal comfort in commercial buildings while meeting industry codes and standards.
- Address factors to consider when creating conditions for occupant comfort.
- Learn about the common methods of air distribution that address thermal comfort factors.
Providing thermal comfort for occupants is a primary purpose of any air distribution system. Industry guidelines offer designers a roadmap on how to attain those goals along with meeting codes and standards. ASHRAE Standard 55-2013: Thermal Environmental Conditions for Human Occupancy and ASHRAE Standard 62.1-2016: Ventilation for Acceptable Indoor Air Quality are two such standards that 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 feet level above the floor and more than 3.3 feet from outside walls/windows or fixed HVAC equipment and 1 foot 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. The breathing zone is the region within the occupied space between planes, 3 and 72 in. above the floor.”
Thermal comfort does not come in a one-size-fits-all variety. There are a number of factors to consider when creating conditions for thermal comfort, including:
- Temperature: ASHRAE 55 requires allowable vertical air temperature difference between the head and ankles to be no more than 5.4°F .
- 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 when people are 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 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).
Commercial buildings use three common methods of air distribution, each of which address the above factors differently. The three common methods of air distribution are:
- Partially mixed (most underfloor air distribution systems)
- Fully mixed (overhead distribution)
- Fully stratified (displacement ventilation).
Partially mixed systems
Conserving energy by comfort-conditioning a space’s lower-occupied level and stratifying its upper-occupied 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 one, the loads are dynamically changing due to outdoor solar and air temperature changes. A second challenge is choosing outlets limits the throw of the air pattern presents 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.
Although some challenges are present, partially mixed systems have a number of advantages as well. They are ideal for situations where cabling is provided to each work stations. They also can 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 with energy efficiency.
Fully mixed systems
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 that should be considered. ASHRAE Standard 62.1-2016, which ensures ventilation air that is supplied to a space, also is delivered to the breathing zone. For ceiling supply of warm air with a ceiling return, the requirements for heated air are to reach a terminal air velocity of 150 fpm to within 4.5 feet of the floor. The differential temperature between warm supply-air and space temperature with a ceiling return must be 15°F or less. When the heating supply-air temperature exceeds the 15°F limit, the ventilation air volume must be increased by 25%.
Due to their flexibility, fully mixed systems can meet most applications’ air distribution challenges. They also can be very economical because they typically have the lowest first cost.
Fully stratified systems
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. While TDV systems in the past typically required a heating system that was separate, new systems are able to heat and cool using a single TDV unit, simplifying installation and maintenance.
There are many ways to establish and maintain occupant comfort. Which system best accomplishes this depends on the building’s requirements, but the important thing is to keep occupants comfortable while also adhering to codes and standards.
Jim Aswegan is chief engineer at Titus with more than 50 years of service. Aswegan provides applications support for engineers and participates in industry organizations including ASHRAE, U.S. Green Building Council, and the Air-Conditioning, Heating, and Refrigeration Institute.