«Health and Productivity Gains from Better Indoor Environments and Their Implications for the U.S. Department of Energy William J. Fisk1 Staff ...»
Better Measurement And Control of Ventilation Rates In U.S. residences, rates of ventilation depend on the quantity of accidental cracks and holes in building envelopes and ducts, on weather conditions, and on window and exhaust fan use. Even in mechanically-ventilated commercial buildings, HVAC systems very rarely include integral systems for measuring and controlling minimum rates of outside air supply; thus, ventilation rates are poorly controlled. The minimum ventilation rates measured in surveys of such buildings often differ substantially from the minimum ventilation rates specified in the applicable codes (Seppanen et al. 1999, Fisk et al. 1992, Lagus Applied Technologies 1995, Teijonsalo et al. 1996, Turk et al. 1989). While the problems associated with measurement and control of outside air ventilation rates have been recognized for many years, there has been little progress toward overcoming the problems. The large range of ventilation rates among buildings suggests an opportunity to improve health and satisfaction with air quality by increasing ventilation rates in buildings with low ventilation rates and decreasing ventilation rates in buildings with high ventilation rates. Due to the dose-response relationships between ventilation rates and health outcomes (Seppanen et al.
Heat Recovery from Ventilation Air
Heat recovery systems that transfer heat (and sometimes moisture) between ventilation exhaust airstreams and the incoming outside air can diminish the energy required for ventilation. These systems are used commonly in northern Europe but rarely in the regions of the US with similar climates. Increasing ventilation rates will make heat recovery more cost effective. The technologies required for heat recovery from exhaust ventilation air are already available, but there is a need for demonstrations and guidelines on how and when to properly implement and operate these systems. By quantifying and demonstrating the benefits and costs of increased ventilation with heat recovery DOE can stimulate the market for these strategies.
A ventilation technology used commonly in Europe, but very rarely in the U.S., is displacement ventilation. This technology supplies air near the floor and produces an upward airflow pattern that is more effective in limiting pollutant exposures than an equivalent amount of well-mixed ventilation. Relative to conventional mixing ventilation, displacement ventilation also removes warm air more effectively. Displacement systems usually supply 100% outside air, increasing ventilation rates relative to conventional systems that supply predominately recirculated air; consequently, heat-recovery systems are often combined with displacement ventilation for energy efficiency. Increased use of displacement ventilation, where appropriate, could reduce health effects and, in some cases, save energy. Research and technology transfer is needed to identify and demonstrate the best opportunities for displacement ventilation.
Breathing rates are about 0.1 L s-1 per person, only 1% of the rate of the rate of outside air supply to buildings (Fanger 2000). Task ventilation (sometimes called personal ventilation) systems that supply outside air preferentially to the breathing zone may be able to substantially reduce pollutant exposures and improve health while maintaining or even reducing quantities of outside air.
These systems supply air near each occupant’s breathing zone. Moderate, 20% to 50%, exposure reductions have been demonstrated for some commerciallyavailable air supply technologies (Faulkner et al. 1993,1998); however, optimization of the ventilation performance of these systems should bring even larger reductions in exposure.
In some climates, direct or indirect evaporative cooling systems can replace compressor-based cooling. These evaporative systems often supply 100% outside air; consequently, they increase ventilation rates and will reduce indoor concentrations of many indoor-generated pollutants. Energy savings relative to compressor-based cooling can be large (e.g., 50%). Research and technology transfer is needed to develop and optimize systems, quantify and demonstrate IAQ and energy performance gains, and evaluate and address concerns about maintenance and increased indoor humidities.
Moisture and Humidity Problems
Figure 3 illustrates the strong relationship of adverse respiratory and asthma symptoms with moisture problems or the mold contamination commonly associated with moisture problems. Many of these moisture problems are a consequence of water leaks in building envelopes, particularly roofs. Other moisture problems result from condensation of water vapor in walls or from inadequate humidity control by HVAC systems in humid climates. The extent of mold contamination resulting from a moisture problem appears to depend on the selection of building materials. In addition to adversely affecting health, moisture problems degrade the thermal performance of building envelopes, increase energy use, and cause extensive materials damage requiring costly repairs. The prevalence and severity of moisture problems are not fully understood, but a very significant number of buildings are affected. For example, in the U.S.
Census data about 15% of houses report water leakage from outdoors (Committee on the Assessment of Asthma and Indoor Air 2000). DOE has a broad range of relevant expertise on building envelope performance (including roofs and foundations), on air and moisture transport through envelopes, and on HVAC performance. Expanded DOE research and technology transfer in this field could help to improve health of the U.S. population, save energy, and prevent costly damage to U.S. buildings.
Higher indoor humidities are associated with increased levels of house dust mites (Chapter 8, Committee on the Assessment of Asthma and Indoor Air 2000).
The allergens from dust mites, arguably the most important of allergens for humans, are associated with both the development and exacerbation of asthma (Chapter 5, Committee on the Assessment of Asthma and Indoor Air 2000).
Particularly elevated indoor humidities, e.g., above 80% RH, can also facilitate growth of molds indoors; however, the influence of more moderate humidities on indoor mold growth is uncertain (Chapter 8, Committee on the Assessment of Asthma and Indoor Air 2000). Again, there is a link to energy -- maintaining low humidities during air conditioning increases energy use. Many associated research questions remain. The relationships of humidity to dust mite and mold contamination are still inadequately understood. Additionally, research, technology development, and technology transfer efforts are needed to improve humidity control by HVAC systems.
Efficient Air Filtration Air filtration (or other particle air cleaning systems) show some promise in moderately reducing allergy and asthma symptoms (Chapter 10, Committee on the Assessment of Asthma and Indoor Air 1999) and portable air cleaners are commonly used by allergic and asthmatic individuals. In addition, more efficient air filtration systems in HVAC systems can dramatically reduce indoor concentrations of fine particles from outdoors (Fisk et al. 2000c). There is persuasive evidence that death rates, hospital admissions, and respiratory symptoms increase with higher outdoor particle concentrations (EPA 1996). Since people are indoors 90% of the time, the exposures to these outdoor particles occur predominately indoors. Consequently, one would expect that the adverse health effects associated with outdoor particles could be substantially reduced through the use of more efficient filtration systems; however, these benefits have not been demonstrated. Once again, there are strong ties with building energy use. A 200 W portable air cleaner, operated continuously, would consume $170 of electricity per year. More efficient filters in HVAC systems also tend to increase fan energy requirements unless the filter is designed for a low airflow resistance.
Research is needed to determine when particle air cleaning is (or is not) effective in improving health and to evaluate and demonstrate energy and cost effective efficient methods of particle air cleaning.
Better Indoor Temperature Control
Despite the significant attention placed on thermal comfort by building professionals, dissatisfaction with indoor thermal conditions is the most common source of occupant complaints in office buildings (Federspiel 1998). In a large field study (Schiller et al. 1988), less than 25% of the subjects were moderately satisfied or very satisfied with air temperature. Also, 22% of the measured thermal conditions in the winter, and almost 50% of measured thermal conditions in the summer, were outside of the boundaries of the 1988 version of the ASHRAE thermal comfort zone. Temperatures are also linked to health. In several studies, increased air temperatures are associated with increases in SBS symptoms (Mendell 1993, Mendell et al. 1999) and with reduced satisfaction with indoor air quality (Fang et al. 1998a, 1998b). These findings indicate that greater effort should be placed on HVAC system designs or controls that do a better job than current systems of maintaining thermal conditions within the prescribed comfort zones. Because indoor air temperatures influence occupant health symptoms as well as comfort, the recommended range of indoor temperatures may also need to be reexamined.
HVAC System Maintenance and Operation
Improved maintenance and operation of HVAC systems is another practice with the potential to simultaneously save energy and improve IEQ and health. As discussed above, Sieber et al. (1996) found that large increases in SBS symptom prevalences were associated with evidence of poorer ventilation system maintenance or cleanliness. Many common problems with HVAC system performance (some discussed previously) are reported anecdotally and in published literature. Examples of these problems include: fouling of cooling coils and drain pans by deposited particles and microbial growth; large indoor air temperature oscillations or temperatures maintained outside of the thermal comfort envelope; dirty duct systems; deterioration of HVAC insulation; missing air filters; poor control of indoor-outdoor or inter-room air pressure differences;
closed fire dampers; poor air distribution leading to excessive noise, drafts, and thermal comfort problems; insufficient or excessive outside air ventilation;
improper damper operation (sometimes the damper linkage is disconnected from the dampers or actuators); fans running backwards, not operating or operating at the wrong times; sensors that are far out of calibration or disconnected; and water leaks. Each of these problems may be due substantially to maintenance and operation problems, although design and construction limitations and errors also play an important role. Research and technology transfer programs are needed to determine the prevalence and underlying causes of these problems and to quantify and demonstrate the energy and IEQ benefits of problem prevention and remediation.
Rethinking HVAC Architectures
Many of the HVAC system problems (mentioned in the previous text) which increase energy use and deteriorate IEQ, have been recognized for many years;
however, progress in resolving these problems has been very limited.
Improvements to HVAC technologies tend to be incremental and to occur slowly.
In parallel with efforts to incrementally-improve existing HVAC architectures, DOE, working in partnership with industry, could rethink HVAC from the ground up with simultaneous goals of improved IEQ, energy efficiency, and maintainability. Innovative HVAC architectures might include many of the following features: outside air supply separated from the system used for thermal conditioning; water used to transport energy around the building (pumping water is more energy- and space-efficient than blowing air through long ducts); individual control of thermal comfort at each workstation; outside air supply near the breathing zone of each workstation with airflow controlled by occupancy sensors; high efficiency particle filters; a modular design with easily removable and replaceable components so that maintenance occurs in the shop;
and advanced sensors and controls. The initial step in this program would be to assemble a highly multidisciplinary panel of experts who will define objectives and work together on innovate HVAC architectures, unfettered by current product designs.
Numerous cross-sectional studies have compared the prevalence of SBS health symptoms experienced in air-conditioned buildings with the prevalence experienced in naturally-ventilated buildings. A large majority of these studies have found that the occupants of the air-conditioned buildings report significantly more symptoms after controlling for other factors (Seppanen and Fisk 2000). The reason for these rather consistent findings is not known. One of the hypothesized explanations is that HVAC systems are sometimes contaminated, for example with microorganisms, deposited particles, and residual oils from the manufacturing process, and become a source of indoor air pollutants. These findings suggest that health symptoms might be reduced through increased use of natural ventilation within commercial buildings located in suitable climates. Naturally-ventilated buildings also tend to use less energy, consequently, simultaneous energy savings and improvements in health may be possible. However, additional research is needed before promoting a shift toward natural ventilation. Within the U.S., there has been only one modest-size study that compared symptom prevalences between naturally-ventilated and airconditioned buildings (Mendell et al. 1996). Also, until the cause of the increased symptoms in naturally-ventilated buildings is known, it is premature to conclude that symptom prevalences will be lower in new naturally-ventilated buildings.
Indoor Pollutant Source Reduction