The Right Place For Displacement
Poor environments in schools influence the health, performance, and attendance of students. Many existing space conditioning systems in schools using conventional mixed ventilation systems fail to provide the IAQ, acoustics, and comfort that can produce optimal student and teacher performance.
Displacement ventilation (DV) is a cost-effective means of providing an excellent indoor environment by delivering cool supply air directly to the occupants in a space. The air is heated or cooled so that it enters the room at about 65°F, considerably warmer than with a conventional air conditioning system, that delivers air at around 55°. The fresh air, supplied near the floor at a very low velocity, falls toward the floor due to gravity, and spreads across the room until it comes into contact with heat sources. The cool supply air slowly rises as it picks up heat from occupants and equipment. The warm, stale air rises toward the ceiling where it is exhausted from the space.
This vertical airflow pattern near each occupant, often referred to as a thermal plume, makes it less likely that germs will spread. The air distribution system provides for effective ventilation, since the fresh supply air is delivered directly to each occupant. All this can be provided at a first cost that is comparable to that of conventional mixed ventilation systems that rely on creating fully mixed air in the room.
WHAT ARE THE BENEFITS?The learning environment will be improved in a way that delivers more fresh air to the students and teacher, while controlling noise and providing comfort. These healthy surroundings should result in improved health (lowered absentee rates) and better productivity (higher test scores).
HOW IMPORTANT IS IAQ?Evidence strongly suggests that poor environments in schools, primarily due to the effects of indoor pollutants, adversely influence the health, performance, and attendance of students and teachers. This evidence links high concentrations of several air pollutants to reduced attendance levels. There is also persuasive evidence that microbiological pollutants are associated with increases in asthma effects and respiratory infections, which are both related to reduced school performance and attendance.1 DV systems offer an effective, energy-efficient means of delivering fresh air and removing airborne pollutants to improve classroom air quality.
HOW CAN DV IMPROVE CLASSROOM ACOUSTICS?The low-velocity air leaving the diffuser is very quiet compared to the inrush of air that may be experienced with mixed ventilation system diffusers.
As a result, building acoustic standards are more easily achieved with DV systems. It will no longer be necessary to turn off the air conditioner in the classroom to be able to hear the students and teachers.
HOW WILL MAINTENANCE BE IMPROVED?DV systems use remote central or semi-central cooling systems, reducing the number of fans and compressors compared to decentralized systems that may use wall-mounted units or unit ventilators or dedicated packaged rooftop units for each classroom. The presence of these many fans and compressors increases the potential frequency of service calls and their proximity to the teaching space increase the chance of classroom disruptions. (Mixed ventilation systems that use remote or semi-central cooling systems will have similar advantages.)
Other issues such as poor IAQ, drafts, and equipment noise that may result in frequent service calls with mixed ventilation systems should not frequently occur with DV systems.
HOW DOES IT SAVE ENERGY?There are several reasons behind the cooling energy savings with DV. First and foremost, the higher supply air temperature (SAT) of 65° greatly increases potential for free cooling. In some California climates, there are 2,000 hrs annually when the outside temperature is between 55° and 65°. This benefit is especially evident in coastal California climates.
Secondly, the higher SAT also increases the efficiency of mechanical cooling equipment. There is less "lift," less work required for the compressor to raise the refrigerant pressure and temperature before it reaches the condenser.
Finally, since warmer air is exhausted from the ceiling return, DV systems reduce cooling coil loads. There are two zones in the classroom, a stratified occupied zone and a mixed upper room zone. Much of the heat from the upper part of the room never enters the occupied zone and thus does not have to be removed by the cooling system.
Each of these savings opportunities contribute to a large cooling energy savings.
HOW DOES THIS TECHNOLOGY FIT IN WITH LEED® AND CHPS?Displacement ventilation can be directly responsible for additional points under the LEED® rating system for superior energy performance. Cooling energy savings potential can result in additional points for exceeding energy code requirements. Credit can also be captured for improvements in ventilation effectiveness that LEED recognizes can be achieved with DV.
The Collaborative for High Performance Schools (CHPS) encourages schools to be more efficient than minimum California Title 24 requirements. If the school is already designed for energy efficiency, the use of DV, in conjunction with other energy-efficiency measures, will allow additional CHPS energy credit points in achieving superior energy performance. CHPS also recognizes the importance of acoustics; and while acoustic design involves many issues, DV can help the designer receive credit for superior acoustic performance.
To accurately predict energy savings with DV, the engineers on the design team must recognize the differences in system operation possible when using DV systems in employing building simulation programs such as EnergyPro. The models used should account for the facts that DV systems reduce cooling coil loads and with DV systems, warmer air is exhausted from the ceiling return, permitting less airflow to be used than with a fully mixed, non-stratified room air distribution.
CAN THIS TECHNOLOGY HELP INCREASE PROPERTY VALUES IN THE NEIGHBORHOOD?Better energy utilization, IAQ, and acoustics add up to higher school performance that can provide LEED and CHPS recognition for the neighborhood school that will attract the attention of potential residents. Better attendance due to an improved environment will increase revenue to the school, providing increased teaching resources. An improved classroom environment and increased attendance will result in improved learning and higher test scores. Higher test scores will provide positive recognition for the school, helping to create the accurate impression that the neighborhood has good schools. The result will likely be an increase in property values in the neighborhood served by the high-performance school.
WHAT ARE THE BUILDING REQUIREMENTS?The primary requirement for DV is a high ceiling. This allows heat and contaminants to be carried away effectively to the ceiling exhaust. A 9-ft ceiling is adequate, but a high ceiling (12 ft) will enhance the benefits of stratification. Air is typically delivered through two sidewall diffusers installed at the interior corners of the classroom. Each diffuser takes up approximately 6 sq ft of wall space and about 1 sq ft of floor space. Shapes and designs are available that integrate seamlessly into the space with diffusers mounted in a corner, positioned under casework or recessed into the wall.
DV allows for simplification of ductwork. For multi-story structures, the floor-to-floor ceiling height does not need to increase for DV. If the suspended ceiling is eliminated, this may allow for a reduced floor-to-floor height.
IS A RETROFIT A SUITABLE CANDIDATE?Any school that meets the minimum requirements - a ceiling height of at least 9 ft - is a suitable candidate for a DV system. One unique requirement is a steady supply of cool 65° air to remove heat and contaminants from the space. Smaller packaged rooftop units typically do not provide the cooling capacity control to maintain a steady supply air temperature. Central systems using either a central chiller or packaged VAV cooling system may be more suitable for a retrofit, since these systems often can be used with only adjustments to the controls.
WHAT ABOUT RELOCATABLES?Relocatables often have issues with noise, and could well benefit from DV. However, since they are usually conditioned by individual packaged units, they are less suitable for a DV installation. As DV technology becomes more commonplace, manufacturers may offer a DV/compatible cooling system as an option for these installations.
WHAT KINDS OF HVAC SYSTEMS CAN I USE?The primary requirement that is unique to DV is the need for a steady flow of warmer 65° supply air. A central plant system and hydronic coil is the easiest means of controlling supply air temperature. A three-way valve controls the flow to the coil at the air handler or terminal unit to maintain a steady SAT. A packaged VAV system with multiple cooling stages will also provide good control of SAT.
Packaged rooftop units serving single classrooms generally do not have the temperature or airflow control required for DV. However, a system with multiple compressors or a variable-capacity compressor should provide the necessary control.
WHAT ABOUT HEATING?Heating can be provided through the low-velocity displacement diffusers, but in heating, the goal is a well-mixed space. A morning warm-up sequence that heats the space to the setpoint prior to occupancy will minimize heating requirements during occupied hours. For colder mountain climates such as Lake Tahoe, CA, a supplemental perimeter heating system may be required to maintain comfort during the colder months.
WHAT ABOUT INDOOR HUMIDITY?Some designers point out that with a higher SAT, the system will not provide for dehumidification. This is correct, but in most California climates, the outside air humidity level (in lb of moisture per lb of dry air) is lower than indoor design conditions throughout most of the year. Thus, providing outside air ventilation will also serve to dehumidify the space during the cooling season. For some coastal climates, a psychrometric analysis should be performed at design wb conditions, to see if dehumidification is required.
There are several options for removing moisture without lowering the supply air temperature. Return air bypass lowers the SAT off the coil by having a portion of the return air bypass the cooling coil. The warmer bypassed air is mixed with the dehumidified air to achieve the desired 65° supply. Another option is to use a "run-around" coil, to capture waste heat rejected from the condenser and heat the air downstream of the cooling coil.
HAS IT BEEN DONE BEFORE?Yes. DV has been used in schools in the Northeast and Midwest since the 1990s, and has been installed in demonstration schools in California. Several school examples have been documented where DV systems have made a significant difference in changing the culture, providing fresher air, relief from asthma and other disorders, and creating a fresher environment with fewer absences. A teacher in Boscawen, NH suffering from asthma and other problems with the previous systems had perfect attendance after the DV systems were installed.2 Student absentee rates also fell.
In a school in Overland Park, KS, the culture has changed as result of installation of DV systems. Where teachers had complained of poor IAQ, they now cannot believe the difference that the new systems have made.3
In Elk River, MN, visitors to a school with DV often note that the air seems noticeably fresher than in traditional buildings.4 In Howell Township, NJ, 60% lower absentee rates were obtained as a result of use of DV systems.5 Several school districts that have used DV systems now specify them as a requirement for new schools and major remodels.
At the Kinoshita Elementary School in the Capistrano Unified School District in Orange County, CA, DV and a standard mixed ventilation system were installed, instrumented, and operated in adjacent classrooms. The DV system used a packaged rooftop unit with a variable capacity scroll compressor capable of delivering a continuous flow of 62° to 65° supply air. The mixed ventilation system used a standard packaged unit delivering 55° air to the classroom. The DV conditioned classroom had consistently lower CO2 levels at the return than in the occupied zone, illustrating that a stratified room air distribution was created that effectively swept pollutants out of the occupied zone.
In the Capistrano, CA, study, sponsored by the California Energy Commission, the acoustics were consistently better for the DV classroom (40 to 44 dBA) compared to the control classroom (48 to 50 dBA) as well. After a calibration period that corrected faulty economizer and room thermostat settings, the DV system cooling savings were 39% for the month of November 2005. Savings were principally due to the extended economizer range of the DV system and the reduction in cooling load in the occupied zone. Teacher feedback has been positive with the teacher in the DV system room saying; "It's like walking in fresh air, like being outside all the time. During open house all the other teachers wanted to know when they're getting one in their classrooms."
The technology, used in Europe since the 1970s, has also found its way into libraries, casinos, auditoriums, lobbies, atria, and other open spaces with high ceilings.
WHAT DOES IT COST?The only additional costs are for the low-velocity displacement diffusers and for the enhanced compressor capacity control needed to maintain the flow rate and temperature. The displacement diffusers carry a slight cost premium of about $1/sq ft over a conventional set of four ceiling diffusers per classroom.6 However, some of this is offset by the simplification of ductwork. In some cases, using DV provides an opportunity to downsize cooling equipment, which will offset some of the added diffuser and capacity control cost. Practitioners of these systems have found that construction costs of schools using displacement ventilation are quite comparable to first costs of an average school construction project using mixed ventilation. ES
AcknowledgementsThe information presented in these pages is based on work performed for the California Energy Commission under its Public Interest Energy Research program. The work is part of an Indoor Environmental Quality effort that is described on the California Energy Commission's PIER IEQ Program website.
Arent is an associate engineer with Architectural Energy Corporation (San Francisco), and has acted as technical lead for Public Interest Energy Research (PIER) research on displacement ventilation. His work on DV includes performance monitoring of two demonstration projects, and publication of a design brief on the subject for Energy Design Resources. He can be reached at firstname.lastname@example.org.
Blatt is an energy utilization consultant based in Mountain View, CA, and has over 30 years experience in commercial building energy efficiency research. One of his current projects involves connecting California Energy Commission's IEAQ research findings to the market. He is an ASHRAE fellow and is active on ASHRAE TC5.3. He can be reached at email@example.com.
Meister, a senior mechanical engineer, has been with the California Energy Commission in Sacramento for 13 years. He manages research in school HVAC, consumer and office electronics, and demand response. He is a licensed mechanical engineer in California, a CHPS Technical Committee member, and a member of ASHRAE. He can be reached at firstname.lastname@example.org .