The Case for TDV in California Schools

California is on the verge of spending $50 to $75 billion dollars on the construction of new schools and the modernization of existing schools, according to the state architect, Stephen Castellanos, FAIA. We have an opportunity to spend this money wisely or to continue with conventional practice, which is less than ideal with regard to indoor environmental quality (IEQ) and energy efficiency. Conventional classroom HVAC systems have poor ventilation effectiveness, many are noisy, and they are not particularly energy efficient. Problems are worse with relocatable classrooms which represent 30% or more of school classroom housing. Opportunities exist with both new construction and modernization of existing building to make quite significant improvements. Many older schools are being air conditioned through state matching funds and poor IEQ conditions are often created through the process. Teachers complain about the noise of HVAC systems, and when they have a choice, often turn them off while class is in session.

Thermal displacement ventilation (DV) is an excellent HVAC design option for California classrooms. It has many advantages over conventional systems including better acoustics, greater energy efficiency, excellent ventilation effectiveness and cleaner and more healthy air in general. With TDV, about 500 to 800 cfm of 100% outside air is delivered at about 65 ºF to each classroom. Air is delivered near the floor and exhausted near the ceiling. Air is delivered at a low face velocity so that it does not mix. The cool fresh air at the floor rises as it is warmed by occupants or other heat generating objects in the space. Each occupant is provided with his/her own private supply of ventilation air. Cross contamination from colds or other personal pollutants is minimized as air rises straight up and out of the space. No air is recirculated. The amount of air that is brought into each classroom is about equal to the amount of fresh air that is required by codes to be provided by conventional systems. There are no problems of bringing in the right amount of OA since all air is OA.

Underfloor air distribution (UFAD) systems, such as that used for the East End Complex, are similar to DV, but there are some important distinctions. In large open areas like offices, this is necessary to deliver the air through an under floor plenum, but in schools effective displacement ventilation can be provided by delivering air in two or more corners of the space. This saves the cost of a raised floor and provides ventilation and cooling which is just as effective. Another distinction is that most UFAD systems in offices are actually mixing systems, at least for the first five to six ft of space. Often, they are not true TDV systems.

When classrooms are well insulated and windows are shaded, the space has a balance point temperature of about 40 ºF, which means that for most hours of school operation, the space is in a cooling mode, once it is occupied. Heating is required, but generally only for early morning warm-up and during particularly cold conditions. For early morning warm-up, the displacement system can deliver warm air instead of neutral air (65 ºF) until the space is occupied and the lights are turned on, at which time it will go into a cooling mode.

Energy savings are significant with DV. Fan energy is significantly reduced since the total air delivery rate is about ½ to 1/3 that of conventional systems, but in addition, static pressure is lower. The combination of less volume and lower pressure results in quite significant savings. Another big benefit is that the hours during a year when economizers can operate is extended due to the higher delivery temperature. Since DV systems are 100% OA systems economizer dampers are eliminated, making the systems more reliable. While conventional systems deliver air at about 55 ºF, TDV systems deliver air at 65 ºF. For most California climates, the time when an economizer can be effective is extended for 2,000 to 2,500 hours, e.g. the number of hours when the outside air is between 55 ºF and 65 ºF. During economizer operation, there is no need for mechanical refrigeration. The third energy benefit is related to compression cooling or mechanical refrigeration. A higher delivery temperature means that chilled water systems can operate at a higher temperature. The range of water economizers is extended and mechanical cooling operates at a higher suction temperature, resulting in significant energy savings.

TDV systems are whisper quiet. There is virtually no noise from air delivery because of the low face velocity from the diffusers, generally less than 100 feet per minute (fpm). At this speed, noise at the diffuser is virtually eliminated. Most DV systems also have a central fan or air handler which is remote from the classroom. With both the fan and the refrigeration equipment being located in a remote location, noise at the classroom is very low. The CHPS minimum requirement for high performance classrooms is a maximum of 45 dBA, while ADA and ANSI recommendations are 35 dBA[1]. Conventional rooftop packages located over the classroom can contribute 50 dBA or more, while wall mounted HVAC systems typical of relocatable classrooms and many AC retrofits have noise levels as high as 60 to 70 dBA. Acoustics is a critical aspect of indoor environmental quality (IEQ) in classrooms and HVAC system design is a critical feature in achieving acceptable conditions. Classroom acoustics is expected to be the next accessibility issue and may soon be mandated by federal or state regulations. 

Effective ventilation in classrooms is strongly linked to student and teacher productivity, according to the USEPA and other sources. We all know that students are drowsy when the air is stale and the space is stuffy. Overhead air delivery systems do a poor job of delivering fresh are to all parts of a classroom. Students near a diffuser may get fresh air (or at least mixed air since some is returned), but students located away from the diffusers may get little or no fresh air. Ventilation effectiveness is a measure of how well fresh air is distributed to occupants in a space. DV systems provide the best ventilation effectiveness and use natural processes to achieve this goal, e.g. cool air rises as it is warmed by human bodies. Displacement systems assure that all occupants receive 100% fresh air. Cross contamination of internally generated pollutants (like sneezes, perfumes, etc) are also minimized since air rises straight up over each occupant and is exhausted at the ceiling.

In spite of its advantages, DV is not used in California schools. There are case studies in New Hampshire, Minnesota, Massachusetts and other areas where DV has been used effectively in schools, but not in California. In this project we will investigate the market barriers and develop strategies to overcome these barriers.

Conventional engineering calculations are based on HVAC systems that mix the air in the space. The assumption is that the temperature and air quality from top to bottom and from side to side is relatively uniform. Because of the mixing assumption, systems are typically designed to remove 100% of the heat from lights, equipment, occupants and solar gains. With DV, air within the space is deliberately not mixed. DV works by providing a temperature gradient between the floor and the ceiling. Air temperature at the floor is maintained at about 65 ºF while the temperature at the ceiling is of no real concern and is allowed to drift to perhaps 85 ºF. Heat generated by the lights can typically be ignored, except for the radiation component and only a fraction of the other load components needs to be considered in the load calculations.

Conventional engineering would indicate that DV will not maintain thermal comfort. Since conventional tools are all that most engineers have to work with, this is a big barrier in the implementation of DV systems in schools. School districts are looking for cost effective and reliable systems, but non want to be the first to try a new system.

Table 1 – Halton Schools with Thermal Displacement Ventilation