1、4700 Indoor Air Quality of an Energy-Efficient, Healthy House with Mechanically Induced R. Wendt M. Khan, Ph.D. H. Aglan, Ph.D., P.E. ABSTRACT Issues associated with indoor air quality (IAQ) and its impact on occupant health have prompted research into the design and construction of “healthy houses.
2、” Most of the houses constructed have been “upscale housing. ” An afford- able, energy-ejcient, healthy house was built at Tuskegee University with features that improve IAQ, reduce energy consumption, and do not increase the cost of the house beyond the means of the targeted homeowners. Tests were
3、conducted on the “healthiness” of the house using ajltered fresh air ventilation system in the heating, cooling, and swing seasons. Initial tests of the Tuskegee healthy house do indicute that meeting the sometimes competing priorities of afordability, energy ejciency, and IAQ will require a more ba
4、lanced combi- nation of system operation than simply keeping the ventilation turned on. Future tests of the house will help tojnd a balance that best optimizes the combination of affordability, energy ejciency, and IAQ. INTRODUCTION According to the Environmental Protection Agency (EPA), the majorit
5、y of people in the U.S. spend 90% of their time indoors, and 65% of that time is spent at home (EPA 1995). Issues associated with indoor air quality (IAQ) and its impact on occupant health have prompted research into the design and construction of “healthy houses.” In a report to Congress in 1991, t
6、he EPA reported that indoor pollution levels can be from 2 to 5 times (and occasionally 100 times) higher than pollution levels found outdoors (EPA 1991). The EPA considers indoor pollution among the top five environ- mental risks to public health and has estimated that it costs $2 billion in medica
7、l costs and lost productivity every year (EPA Fresh Air S. Livengood E. Ibrahim, Ph.D., P.E. 1995). A challenge facing the housing industry is to design and build “healthy homes” that address these IAQ issues. Designs and standards are being developed that could result in improved IAQ characteristic
8、s for homes. Because healthy houses typically require the introduction of costly systems or features, most of the work to date has focused on upscale” and “high-end” housing. However, significant numbers of low-cost and low-income housing residents (and especially children) are believed to have expe
9、rienced health problems that are associated with or aggravated by IAQ defi- ciencies. How can a healthy house also be achieved for low- income homeowners? Is it possible to design, construct, and maintain a low-cost house that is free of mold, mildew, dust mites, and other chemical pollutants that c
10、ommonly occur in much of todays housing? The sources of pollutants that impact IAQ can be classi- fied into three main categories: materials, inhabitants activi- ties and lifestyles, and outdoor pollutants. Materials include those used in construction as well as furnishings. Inhabitants activities a
11、nd lifestyles include metabolic activity that results in reduction of oxygen and the increase of carbon dioxide (CO,) levels. Outdoor pollutants include pollen, dust in the atmosphere, and airborne industrial chemicals. Outdoor pollutants are introduced into the home through windows, doors, envelope
12、 infiltration, or the ventilation system. Over the last two decades, design and construction of energy-efficient housing has been on the rise. An important design parameter of an energy-efficient envelope is its air- tightness with minimal infiltratiodexfiltration to reduce energy losses. A tight en
13、velope can aggravate potential IAQ problems. The Occupational Safety use wall-mounted lighting fixtures -A single plumbing wall to minimize floor penetrations and save costs; all floor penetrations completely sealed -Well sealed windows and door to lower infiltration -Exterior walls designed to dry
14、to inside; no vapor barrier at drywall -Sealed attic and crawlspace from conditioned space -A single plumbing wall to minimize floor penetrations and save costs -1 15 cfm capacity turbulent flow precipitation (TFP) air cleaner (smallest commercially available unit) provided to filter particles large
15、r than 0.3 pm (micrometers) from the fresh air intake -Centrally located heat pump air-handling unit to mini- mize ductwork -Ducts located within the conditioned space -All appliances are electric; no combustion equipment used Construction methods Equipment INSTRUMENTATION AND DATA ACQUISITION Measu
16、rements of key IAQ parameters have been made using sensors and loggers that were configured for automated data acquisition. The attic, crawlspace, and living room were monitored for temperature and relative humidity. The carbon dioxide (CO,) levels in the living room were measured at 3 ft above floo
17、r level. O, VOC, combustible gases (measured as percentage of lower explosive level, LEL), and carbon monox- ide (CO) were also measured in the living room at 3 ft above floor level. The airborne particulate monitor was also installed in the living room. A data acquisition unit was used to collect d
18、ata from the monitors and download it to a laptop for process- ing. (Additional details about the instrumentation are located in Appendix A.) EXPERIMENTAL RESULTS AND DISCUSSION Heating Season IAQ Characteristics and Energy Consumption In order to establish a baseline, initial tests of the house wer
19、e conducted with the house unoccupied and unfurnished. During the first week of heating season monitoring (Feb. 8- 15, 2002), measurements were taken without running the filtered fresh air ventilation fan. CO, levels ranged from 380 to 460 ppm (typical outdoor is 300 to 500 ppm), and VOCs were measu
20、red at the optimal level. The electrical meter indicated that 84 kWh had been expended during the week. For these first two weeks of monitoring, all electricity consuming devices except the heat pump and the data logger were turned off. (The particulate meter had not yet been installed.) During the
21、second week of monitoring (Feb. 15-22, 2002), the fresh air ventilation fan with HEPA filter was turned on and kept on for the entire one-week period. The ventilation fan is rated at 125-130 cfm. (There were a limited number of sizes of ventilation fans with HEPA filters available. The one selected-
22、the smallest commercially available at the time- was intended for a 2000-ft2 house. ASHRAE recommends a minimum of 15 cfm 7.5 L/s per person, or 60 cfm for a family of four, which this structure was designed to house.) Measurements were again taken and readouts were similar to the previous week. CO,
23、 levels were the same as the previous week-380 to 460 ppm. VOCs were measured at optimal levels again. The electrical meter showed that 107 kWh had been expended during the previous week. During the first week, the outdoor temperature was an average of 5F lower than during the second week, and yet e
24、nergy consumption was 27% higher during the second week, as would be expected. Outside RH was comparable for the two weeks, but during the first week, inside RH ranged from 33% to 47%. The EPA and the Consumer Product Safety Commis- sion (CPSC) recommend maintaining indoor RH levels between 30% and
25、50%. If humidity is too low, viral and bacte- rial populations tend to flourish, thus contributing to respira- tory infections. If humidity is too high, fungal growth and dust mites can thrive, which can also contribute to health problems. During the second week when the ventilation fan was running,
26、 indoor relative humidity was reflective of outdoor humidity. Over half of the time during the second week, indoor relative humidity was measured either above or below the 30%-50% recommended optimum range. The first two weeks of monitoring have provided prelim- inary evidence that running a ventila
27、tion fan 100% of the time does not improve indoor air quality in an unoccupied house. In fact, it contributed to indoor RH levels outside the recom- mended optimum range. Furthermore, running the ventilation fan led to significantly higher energy consumption due to the ASHRAE Transactions: Research
28、79 1 O0 90 80 70 r 60 CL 50 ? 40 30 20 10 n 1 Ilil! windows Closed I Filter On 1 1 I. ,-. I ll/l/,l II o “ o 24 48 72 93lP144168 1333-k m 13Jxts w w O 24 48 72 96 120 144 168 13OOHB hours 1300Hrs ApQ7 Maw Figure 2a Relative humidity versus time with the windows Figure 26 Relative humidity versus tim
29、e with the windows closed and air cleaner on (swing season). open and the air cleaner off (swing season). ventilation fan and the added heating from the heat pump. In the future, introducing occupants into the house who cook, breathe, perspire, and take showers will tend to raise RH levels. When the
30、 ventilation fan is running and outdoor humid- ity is high, occupant activities will probably raise FU3 even further beyond the recommended range than it was during the second week of monitoring. On the other hand, running the fan did not lead to better CO2 or VOC levels. Furniture is a major potent
31、ial source of VOCs. The house had not yet been furnished, but it will be in the future. Without occupants, the house would be expected to have low CO, levels, too. Baseline measurements without occupants have been taken, and in the future CO, measure- ments will be taken with occupants in the house.
32、 Swing Season IAQ Characteristics and Energy Consumption The first evaluations of the IAQ with all monitoring equipment in place were conducted during the springlsummer swing season of 2002. During this period, the outdoor temper- atures did not necessitate the use of the HVAC system (neither coolin
33、g nor heating was needed). The IAQ was evaluated for one week under a natural ventilation (windows open) period and for one week with forced ventilation through the TFP air cleaner. All windows were kept closed during the forced ventilation period. The TFP air cleaner was used at its full capacity (
34、rated speed of 125- 130 cfm) supply of fresh air to the house. Assuming an occupancy of four people, this amount of fresh air is roughly two times higher than the minimum recommended by ASHRAE. Measurements such as temperature, O, COz, VOC, RH, and dust concentration were taken continuously and reco
35、rded every hour. The RH and dust particle concentration were espe- cially important because high pollen counts during the swing season increase the particulates indoors, and the high RH can contribute to mildew and mold growth. The two “swing season“ weeks had a similar average maximum outdoor tempe
36、rature. The average maximum outdoor temperature for the weeks when the TFP air cleaner was on and the windows r) QI E I U EOB O Figure 3 Comparison of particulate concentrations for week of natural ventilation versus week of fresh air fan operation. were closed was 82OF, and the range was between 75
37、F and 87F. The average maximum outdoor temperature when the windows were open and the air cleaner was off was 8 1 OF, and the range was between 73F and 85F. The range for the outdoor relative humidiy for these two weeks was also very close-between 40% and 97% when the TFP air cleaner was on (Figure
38、2a) and between 38 and 95% during the week when the windows were open (Figure 2b). Both averages and ranges are very close and, thus, a direct comparison is reasonable. Figure 2a illustrates the relationship between both outdoor and indoor RH throughout the week when windows were closed and the TFP
39、air cleaner was on. Figure 2b shows outdoor and indoor RH during the week when the TFP air cleaner was off and the windows were open. The range in the relative humidity in each case was between 40% and 60%. Forced ventilation through the TFP air cleaner did not result in significantly different indo
40、or relative humidity than leaving the windows open. The particulate concentrations in the living room for the periods of natural ventilation and forced ventilation are shown in Figure 3. This figure does indicate that the concentration of dust particles was lower when the air cleaner was used than 8
41、0 ASHRAE Transactions: Research when the windows were left open. The average dust concen- tration inside the house during the week when the windows were open was 0.04 mg/m3 (0.022 ppm (v). When the TFP air cleaner was on, dust concentration was 0.015 mg/m3 (0.0083 ppm (v), which is only 37.5% of the
42、 dust concentra- tion with the windows open. The TFP air cleaner filters out particles that are greater than 0.3 pm in diameter. Most pollen spores have diameters larger than 20 pm. (While the air cleaner can filter out most particles of pollen that would enter the house through the fresh air fan, i
43、t does nothing to prevent entry ofpollen through opened windows or doors. The filter will not remove these particles once they have entered because it is connected only to the fresh air supply.) Running the air cleaner during the swing season consumed an average of 3 kWh per day. (At $0.07 per kWh,
44、this translates to $6.30 per month, a fairly nominal recurring expense.) Using the TFP air cleaner in the house did improve indoor air quality relative to partic- ulates during the swing season, and it cost little to operate. However, the fresh air fan did not maintain RH within the desired range of
45、 30 to 50%. Impacts of High-Occupancy Conditions The IAQ and efficacy of the ventilation system during high-occupancy conditions were evaluated next. Stable pre- test conditions of indoor temperature and RH were established by air-conditioningcooling thc indoor air. The TFP air cleaner was kept runn
46、ing with the AC unit for 24 hours prior to starting the test. The high-occupancy condition was established by having 19 people inside the living room involved in sedentary activity (i.e., watching an educational video). The high occu- pancy was abruptly terminated after 80 minutes. During the high-o
47、ccupancy period, the data acquisition systems recorded the temperature, RH, O, CO, particulate concentrations, and VOCs every five minutes. The measurements were continued for another 20 hours after the termination of the high-occu- pancy test. Figure 4a shows RH during a 22-hour period that encom-
48、passes the high-occupancy event. As would be expected, a spike in RH is evident soon after the start of the high-occu- pancy period. Figure 4b shows the RH rising at the beginning of the crush load and dropping gradually during the occupancy period. The 1.5-ton heat pump brought RH back down to arou
49、nd 50%. The periodic spikes in the indoor RH (Figure 4a) are the kick-in signature of the AC unit. Figure 4a also reveals that the indoor RH in the living space (except during the high- occupancy period) reflects the outdoor RH when the TFP air cleaner was on-except at a much lower magnitude. The noticeable spike in the outdoor RH in Figure 4a was due to light rain after the test was completed. The AC coil maintained the indoor RH below 60% even when the outdoor RH approached 97%. At about 70% outdoor RH, the indoor RH was kept at about 50%. In the current experime
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