Infiltration effects on residential pollutant concentrations for continuous and intermittent mechanical ventilation approaches.
Indoor air quality affects occupant health (Jacobs et al. 2007). Exposures in the residential environment are important to the total air pollutant exposure an individual experiences, as demonstrated in numerous studies (Edwards et al. 2001; Weisel et al. 2005). As outdoor air pollutant concentrations decrease and residential air exchange rates are lowered with improved air tightness (Sherman and Matson 2002), the contribution of indoor pollutant sources to overall exposure is expected to become increasingly more significant. The California Air Resources Board (CARB) recently published a report showing that pollution risks indoors often exceed risks in the outdoor environment (CARB 2005). According to the American Lung Association (ALA 2010), a number of factors within homes are increasingly recognized as threats to respiratory health. Appropriate residential ventilation can address many of these indoor air quality problems.
Many hazardous pollutants have been measured in the indoor residential environment. Several organizations, both within the United States and abroad, have established exposure standards and guidelines for exposure to these pollutants. These standards are often compared to pollutants concentrations outdoors but have only sparingly been compared to indoor concentrations to identify hazards and risks indoors. The main organizations that publish standards and guidelines are the World Health Organization (WHO), the United States Environmental Protection Agency (USEPA), and the California EPA (CalEPA) (WHO 2005; USEPA 2008; Office of Environmental Health Hazard Assessment (OEHHA) 2010). These groups publish both chronic and acute standards that are thought to represent "safe" levels. Acute standards are set at levels that are thought to prevent immediate health effects for short-term exposure. These standards are set for 1 h, 8 h, or 24 h. Chronic standards are set at levels that are thought to prevent long-term health effects from a lifetime of exposure. These standards are usually set for a time period of 1 year to life, and the total exposure is the time-averaged exposure over the relevant time frame, meaning that variability in concentration is ignored. Residential ventilation standards focus, at most, on chronic effects, even though variations in concentrations over shorter time periods due to varying ventilation scenarios may exceed acute standards.
The dominant standard for residential ventilation system design is the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 62.2-2010 (ASHRAE 2010). Most implementations of Standard 62.2 use constant mechanical ventilation and assume that infiltration is high enough to meet the standard.
There are economic and energy-efficiency rationales for reducing total residential mechanical ventilation and for designing and operating ventilation systems with variable amounts of ventilation airflow (Concannon 2002; Russell et al. 2005). Benefits of reduced and variable mechanical ventilation include reducing energy use during peak load periods, avoiding the intake of outside air during high-pollution events, and reducing the energy used to heat or cool inside air.
Standard 62.2 allows for the use of infiltration credits and intermittent operation in residential buildings to accommodate innovative ventilation systems and variable outdoor conditions. However, these ventilation strategies increase the magnitude of fluctuations in indoor pollutant concentrations because they result in periods of reduced and/or no mechanical ventilation. Weather-induced changes in infiltration and the allowable variability in time-scheduled operation of mechanical ventilation can both substantially affect indoor pollutant concentrations. Thus, when ventilation systems are designed to maximize the benefits of infiltration credits and intermittent operation, steps must be taken to ensure that indoor air quality is not compromised. Designers and decision makers need a method to determine how intermittent ventilation and varying infiltration will impact indoor concentrations for both acute-exposure-relevant and chronic-exposure-relevant time frames. ASHRAE does not directly address these issues.
Ventilation system design and the time variability of infiltration (described henceforth as "ventilation scenarios") can affect indoor pollutant concentrations in various ways. Whole-house ventilation aims to keep indoor pollutant concentrations at safe levels for long-term (i.e., chronic) exposure.
This study presents a methodology to quantify the effect of ventilation scenario on short-term concentration fluctuations for continuously emitted indoor pollutants. This article does not assess whether the rates specified in ASHRAE Standard 62.2 are sufficient to control chronic exposures to contaminants of concern. Rather, it focuses on quantifying the magnitude of short-term fluctuations relative to long-term average pollutant concentrations. The result of this analysis informs the relative level of concern for acute versus chronic health effects for continuously emitted pollutants that have exposure standards for both time frames. One particular application is the question of whether fluctuations could lead to exceedance of an acute standard when chronic standards are being met.
The approach of this study uses a physics-based model to track infiltration, mechanical ventilation, and pollutant emissions to determine the time-varying pollutant concentration profile for a representative single-family house placed through simulation in various climate zones and having varying air tightness levels (expressed as normalized leakage [NL] area). The ratio between the highest short-term concentrations is determined in each simulation to the nonvarying pollutant concentrations that result from a reference simulation using a continuous steady ventilation rate. The continuous ventilation rate is set by the air flows specified in Standard 62.2, including both mechanical ventilation and an assumed steady natural infiltration rate. These ratios are then compared to ratios of acute to chronic pollutant concentrations from various health standards.
For most of the existing U.S. housing stock, ventilation is provided by uncontrolled infiltration combined with occupant use of windows, doors, and bath and kitchen exhaust fans. Sometimes this results in over-ventilation and corresponding excess energy consumption or under-ventilation and poor indoor air quality (Orme 2001). Residential construction methods designed to produce energy-efficient buildings have created tight building envelopes that can result in under-ventilation (Sherman and Dickerhoff 1994; Sherman and Matson 2002). Thus, new homes often need mechanical ventilation systems to meet ventilation standards.
ASHRAE Standard 62.2 has requirements for source control ventilation in kitchens and baths and for whole-house ventilation. The required whole-house mechanical ventilation rate is based on the assumption that infiltration contributes 2 cubic feet per minute (cfm) per 100 square feet ([ft.sup.2])or0.1 L [s.sup.-1] [m.sup.-2]. In addition to this infiltration, the standard prescribes the whole-house mechanical ventilation rate given by Equation 1 :
Q(cfm) = 0.01 [A.sub.floo]([ft.sup.2]) + 7.5(N + 1), (1a)
Q (L / S) = 0.05 [A.sub.floor]([m.sup.2]) + 3.5(N + 1), (1b)
where g is the required ventilation rate, [A.sub.floor] is the house floor area, and N is the number of bedrooms. The size of the house is a surrogate for pollutants from materials intrinsic in the building, and the number of bedrooms is a surrogate for home-occupancy-dependent activities and associated emissions. Standard 62.2 also allows for specific infiltration credits and for intermittent operation of mechanical ventilation systems with some restrictions.
Standard 62.2 requires source control, including local kitchen and bath exhausts, but a large part of the standard focuses on the requirements for continual mechanical whole-house ventilation. Standard 62.2's whole-house ventilation requirement implicitly assumes a constant, additive infiltration rate. However, infiltration is highly variable and dependent on the air-tightness of the building envelope as well as on fluctuating driving conditions, such as wind and indoor-outdoor temperature differences. The metric for air-tightness is NL, defined by ASHRAE Standard 119 (1988) as:
NL = 1000 ELA / Floor Area [([N.sub.story]).sup.2], (2)
where ELA is the effective leakage area, N is the number of stories in the home, and Floor Area is the floor area of the home. NL is a dimensionless term that represents the fraction of the home that is open to airflow normalized for the effects of house size and height. NL typically ranges from 0.1 to 1.0; the former is exceptionally tight with very little infiltration, and the latter is very loose with excessive infiltration. Chan et al. (2005) found that the median NL value in U.S. homes was 0.52, with the central 50% of homes having values that range from 0.33 to 0.84.
McWilliams and Sherman (2005) reviewed ventilation standards and studied the impacts of intermittent mechanical ventilation and ventilation credits for infiltration. The study's major finding was that ventilation is increasingly becoming an important component of a healthy dwelling and that occupants generally think ventilation is important, even if their understanding of ventilation systems is low.
Method and approach
This study used numerical modeling to compare indoor concentrations across a variety of ventilation scenarios for a typical single-family dwelling: a 2,000-[ft.sup.2] (185-[m.sup.2]) three-bedroom single-story detached house with 8-ft (2.5-m) ceilings. Time-dependent outdoor air exchange rate for the house was calculated for various mechanical ventilation approaches, envelope leakages, and climates. The effect of weather-driven temporal variations on infiltration was examined by simulating placement of the house in six U.S. cities located in distinct climates: Long Beach, CA; Phoenix, AZ; Miami, FL; Chicago, IL; Boston, MA; and Bethel, AK. These climates range from mild (Long Beach, CA) to extreme (Chicago, IL) and are the same cities studied by Sherman (2008).
Hourly pollutant concentrations were calculated by mass balance assuming a constant whole-house emission rate with dilution and pollutant removal varying with total outdoor air exchange (ventilation). Pollutant concentrations were examined over averaging times relevant to chronic (annual) and acute (1-h, 8-h, and 24-h) exposures and health-based standards. All results were compared to the pollutant concentrations calculated for a reference case meeting all constant airflow assumptions of ASHRAE Standard 62.2.
The reference case to which alternate ventilation scenarios are compared is the typical house described above, ventilated in compliance with ASHRAE Standard 62.2; the whole-house mechanical ventilation is set to the value specified in the standard, and infiltration is set to the level assumed in the standard. The mechanical ventilation is assumed to be constantly running, and infiltration is assumed to be constant, non-varying, and adding linearly to the mechanical ventilation to give the total air change rate. The Standard 62.2 reference case of adding a constant infiltration contribution is an idealized case, because in reality, infiltration varies hour by hour and over the year. That issue is further explored below when looking at the default scenario.
Standard 62.2 specifies that local mechanical exhaust shall be installed to be operated as needed by the occupant in each bathroom and kitchen. The standard specifies kitchen range hood exhaust fan airflow of at least 100 cfm (47 L [s.sup.-1]) or a whole kitchen exhaust rate of 5 ach or more. Bathroom fans are specified at 50 cfm (24 L [s.sup.-1]) or more. This ventilation is user operated and is presumed to run to exhaust the intermittent and local pollutants generated by the activities in those rooms. Because the focus here is on pollutants with constant emissions, it is assumed that local ventilation is not used in either the reference case or in the different scenarios. For the reference case in this study, the total ventilation (i.e., the combined air exchange rate from mechanical ventilation and infiltration) is a constant 0.3375 air changes per hour.
Modeling indoor concentrations
The combined hourly infiltration and mechanical ventilation were simulated using the method described by Sherman (2008), which is based on the ASHRAE (2009) Handbook of Fundamentals, a mass balance model approach. The home was treated as a perfectly well mixed space with a constant pollutant emission rate. The derivative form of a mass balance of the home is:
[V.sub.house] [dC / dt] = S - A x C x [V.sub.house], (3)
where C is the concentration in the home (mass/volume), S is the emission rate of the pollutant (mass per unit time), A is the air exchange rate (1/time), and [V.sub.house] is the volume of the house. Negligible outdoor concentrations were assumed of the pollutant of interest.
A script was written to determine the hourly pollutant concentration for each of the climates and each of the ventilation scenarios. A simple time-step modeling approach was used to calculate indoor concentrations as a function of mechanical and natural ventilation. Since the derivative form of the mass balance equation is unstable for a 1-h time step for many of the simulations explored, the integrated form of the equation was used. In order to integrate Equation 3, it was assumed that A was steady over the course of a given time step and equal to the average air exhange rate over that time period. The integral form of Equation 3, assuming A is constant, is
C(t) = [C.sub.o] exp(-At) + [[S(1 - exp(-At))] / A[V.sub.house]]. (4)
This form of the equation can only be applied from on time step to another in these scenarios and cannot be used continuously. The discretized form of Equation 4 is
[C.sub.i] = [C.sub.i-1] exp(-[A.sub.i] [DELTA]t) + [[S(1 - exp(-[A.sub.i] [DELTA]t))] / [A.sub.i][V.sub.house]] (5)
Solutions were compared using the integrated and derivative versions of the mass balance equation and no difference was found in the calculated solution. A time step [DELTA]T of 1 h was found to be small enough to keep the Equation 5 calculations stable. For each run, initially [c.sub.i = 0] was set to zero, and the irst month of results was discarded as spin-up time after which a full year of simulations was completed. Spin-up time is the time it takes the model to reach dynamic equilibrium and to eliminate the effect of the initial conditions on the solution.
Both infiltration and mechanical ventilation contribute to the air exchange rate [A.sub.i]. ASHRAE Standard 136 (1993) gives a reference method of combining mechanical ventilation and natural infiltration:
[A.sub.i] = [A.sub.bal,i] + [square root of [A.sup.2.sub.unbal,i] + [A.sup.2.sub.inf,i]], (6)
where [A.sub.bal,i] is the air exchange rate at time step i due to balanced mechanical ventilation alone, [A.sub.unbal,i] is the air exchang rate at time step i due to unbalanced mechanical ventilation alone, and [A.sub.inf,i] is the air exchange rate at time step i due to natural infiltration alone. Balanced mechanical ventilation uses mechanical equipment to provide both supply and exhaust air flow at equal rates. When mechanical equipment is used to provide only supply or exhaust airflow, airflow in the other direction through the building envelope is induced through the resulting pressure differential, and the system is described as unbalanced. Infiltration is natural ventilation that is driven by the indoor-outdoor temperature difference and outdoor wind speed through envelope leaks.
For the reference case, mechanical ventilation was assumed to be balanced. For the remaining cases, the mechanical ventilation was assumed to be unbalanced
Infiltration is dependent on the air tightness of the building envelope. Tightness is quantiied using NL, which is a nondimensional value that represents the area of the leaks and cracks in a home that allow for airflow, normalized by home size. There are several methods of modeling air infiltration rates ranging from multi-zone indoor air quality models (Emmerich 2001) to simple numerical models based on building and meteorological properties. For this work, the relationship between NL and [A.sub.inf] used was described in detail by Sherman (2008) as
[A.sub.inf,i] = [k.sub.i] N L [S.sub.i], (7)
[s.sub.i] = [square root of [f.sup.2.sub.s] x [DELTA][T.sub.i] + [f.sup.2.sub.w] x [v.sup.2.sub.i]]. (8)
Here, [k.sub.i] (1.44 s [hr.sup.-1][m.sup.-1]), [f.sub.s] (0.12 m [s.sup.-1] [K.sup.-1/2]), and [f.sub.w] (0.132) are all constants, [f.sub.w] is dimensionless, [DELTA]T is the difference between the outdoor and indoor temperatures, and v is wind speed. The values for [k.sub.i], [f.sub.s], and [f.sub.w] were taken from ASHRAE Standard119 (1988). The combined infiltration and mechanical ventilation rate will vary diurnally and seasonally. The outdoor temperature and wind velocity data were taken from the Typical Meteorological Year Version 3 (TMY3) Database (National Renewable Energy Laboratory (NREL) 2008). The TMY3 database provides hourly wind and outdoor temperature data for a representative year for each climate explored. The database is intended for use in computer simulations of building systems. An indoor temperature of 22[degrees]C (71.6[degrees]F) was used for all of the simulations.
The analysis assumes a constant whole-house pollutant emission rate that is unaffected by air exchange rate or home conditions. The analysis focuses on the relative concentration, which is independent of S and allows the assessment of the effect of ventilation scenario on indoor concentrations for any constant indoor source strength.
Description of ventilation scenarios
For all of the ventilation schemes modeled, indoor concentrations were calculated based on combined mechanical ventilation and natural infiltration. The amount and delivery method of mechanical ventilation varied among scenarios and was specified in each run of the model. Unlike in the reference case, the amount of infiltration was not specified in each scenario. Rather, the total air change rate was calculated based on the mechanical ventilation rate NL and weather parameters for each climate using Equations 6 and 7. Hourly concentrations were then calculated, and any appropriate averages taken. For each scenario, the time-varying indoor pollutant concentration for the reference house was calculated using weather data for each city and for NL values ranging from 0.1 to 1. The ventilation scenarios are listed in the following.
The constant default scenario is what is currently implemented in most new homes. Mechanical ventilation is set to the value specified by ASHRAE Standard 62.2-2010 (Equation 1 or Table 1). For the simulations here, the total ventilation was calculated based on weather conditions and leakage as described above.
Constant with infiltration credit
Standard 62.2-2010 allows the required mechanical ventilation rate to be reduced under certain circumstances when the air leakage has been measured. The constant with infiltration credit scenario sets an assumed constant infiltration airflow rate based on NL and a w value provided in ASHRAE Standard 136. The w value is a representation of localized weather conditions. Based on this credit, the airflow rate of the designed mechanical ventilation system can be reduced by half of the calculated increase in infiltration above the assumed 62.2 infiltration of 2 cfm/100 [ft.sup.2] or 0.1 L [s.sup.-1] [m.sup.-2] (from section 4.1.3 of Standard 62.2). For each climate and each NL value, the mechanical ventilation flow rate was reduced based on this credit.
The constant optimized ventilation scenario looks at optimizing the amount of constant mechanical ventilation to reduce the total air exchange rate to the lowest level possible while still achieving an annual average concentration that is no higher than the reference case. This can be viewed as an alternative infiltration credit approach that achieves the most efficient constant ventilation rate. Although ASHRAE Standard 62.2 does not state a maximum relative chronic exposure, it sets the minimum ventilation for acceptable air quality. This scenario implicitly assumes that the rate and assumptions specified in Standard 62.2 would provide "acceptable indoor air quality" as intended.
A set of simulations was executed for each climate and NL value in which the mechanical ventilation system was adjusted across a wide range of values from zero to a value high enough that concentrations were reduced to nearly zero. For each level of mechanical ventilation, hourly indoor concentrations were calculated as described above and the annual average concentration was determined. For each NL and climate, the lowest mechanical ventilation flow rate that resulted in an annual average concentration that was the same or lower than the reference case was identified. This value was used to calculate hourly concentrations for the representative year.
The intermittent optimized scenario looks at the effect of intermittent mechanical ventilation on indoor concentrations but is otherwise similar to the optimized ventilation scenario. The ventilation scenarios above assumed that mechanical ventilation operated continuously. Standard 62.2, however, supports the use of intermittent ventilation, i.e., shutting off the ventilation system during a certain portion of a day and increasing the ventilation during the rest of the day to provide the same average indoor concentration as if the system ran continuously. The intermittent ventilation equations in ASHRAE Standard 62.2 are based on Sherman (2004), using the concept of relative dose. The general idea of relative dose is that if the time-averaged concentration over a given period is the same as it would be for a constant concentration over that same time period, then the chronic health effects for those two ventilation scenarios may be considered equivalent.
Intermittent ventilation increases the range of concentrations occurring over shorter averaging times because overall ventilation rates are reduced during periods without mechanical ventilation. This, in turn, leads to higher short-term concentrations relative to constant ventilation. The highest concentrations occur during the longest periods without mechanical ventilation. The case in which the mechanical ventilation is off for 12 continuous hours and on for 12 continuous hours, as might occur with night ventilation or an economizer, was selected. As with the previous scenario, the mechanical ventilation is set to the minimum value to keep the annual average concentration at the reference level or below. This value was determined in the same way as in the previous case, by conducting simulations for a range of mechanical ventilation values and determining, for each climate and NL, the minimum mechanical ventilation rate that resulted in a annual average concentration as low as the reference case.
Effect of infiltration on air exchange rate
Figure 1 presents the calculated daily average ventilation rate over one year for the modeled house using climate data from Bethel, AK, and Long Beach, CA. Results in the left panel were calculated for a tight envelope (NL = 0.125), whereas the results in the right panel are for a moderately leaky condition (NL = 0.5). Also shown in each panel is the air exchange rate for the reference case used in this analysis, namely the home where infiltration airflow is fixed and additive to mechanical airflow, as assumed in Standard 62.2.
Figure 1 shows the effect of weather (as a driving force) and NL on infiltration and the resulting overall air exchange rate. When NL = 0.125, both cities have daily averaged air exchange rates lower than the reference case for most days. A higher NL leads to higher air exchange rates in both locations; with NL = 0.5, the daily values are generally on par with or higher than the reference case. Daily fluctuations are driven by local weather conditions; however, the overall annual average concentration depends strongly on NL. Bethel has a much lower-temperate climate than Long Beach, resulting in more fluctuations in concentration over the year. Infiltration is higher in Bethel than in Long Beach because of greater indoor-outdoor temperature differences. As NL increases, the magnitude of the day-to-day fluctuations increases for both cities. For a constantly emitted pollutant, variations in ventilation cause variations in indoor concentration.
[FIGURE 1 OMITTED]
Effect of varying infiltration on relative indoor concentration
Figures 2 through 5 present results for the simulated ventilation scenarios in relation to the reference case. The scenarios all include time-varying infiltration resulting from time-varying weather conditions, and each ventilation scenario was simulated for a range of leakages in each climate. All of the figures are formatted in the same manner to allow for easy comparison. In each subfigure, results are presented for each city, with air leakage (NL) values ranging from 0.1 to 1.0. In all of these figures, subfigure (a) shows the normalized annual average concentration, subfigure (b) shows the normalized maximum 24-h averaged concentration, subfigure (c) shows the normalized maximum 8-h averaged concentration, and subfigure (d) shows the normalized maximum 1-h concentration. The maximum concentration is defined as the highest concentration for the stated averaging time over the one-year simulation. In all of the subfigures, the concentrations are normalized by the same value as for the comparable concentrations in the reference case. Because the ventilation is constant in the reference case, the indoor concentration was constant, and the normalizing values are all the same and equal to the annual average concentration in the reference case.
Constant mechanical ventilation
Figure 2 shows the concentration ratios as a function of air tightness for the constant default mechanical ventilation scenario. With more leakage, infiltration increases, and concentrations decrease for the analyzed conditions relative to the Standard 62.2 reference case (balanced constant mechanical and constant infiltration). The peak ratios for each of the graphs and for each of the cities occur at the minimum NL value considered. For tight houses, infiltration airflow is less, and concentrations are higher than in the Standard 62.2 reference case. For each climate, the annual average concentration is reduced below the reference value when infiltration airflow exceeds the value assumed in Standard 62.2. For each case, the critical value of NL at which this occurs can be identified. For the locations examined in this analysis, the critical NLs for annual average ventilation were in the range of 0.23 to 0.4. However, the maximum hourly concentrations for Miami, Phoenix, and Chicago are higher than the reference case for NL values up to 1.
[FIGURE 2 OMITTED]
Figure 2 shows, not surprisingly, that for constant mechanical ventilation, increasing air leakage leads to lower indoor concentrations. It also shows that the damping effect of increased air leakage is reduced for shorter averaging times because infiltration varies over the year; air leakage is less effective than mechanical ventilation at reducing concentration peaks.
Figure 3 displays simulation results for the constant with infiltration credit scenario. For low NL values, there is no difference between the results in Figure 2 and the results in Figure 3, because the air leakage is too low to allow for much of a reduction in mechanical ventilation. Again, the annual average concentration reaches a maximum value when there is no leakage. As NL increases, the maximum concentrations for 1-h, 8-h, and 24-h averaging times increase until a peak is reached at NL values of 0.2 to 0.5. As NL increases further, the mechanical ventilation is reduced in favor of natural infiltration until a critical NL value where mechanical ventilation reduces to zero. Before the critical value, air-leakage-induced ventilation is replacing constant mechanical ventilation, and the short-term averages increase because temporal variations in infiltration lead to temporal variations in overall ventilation rates. At leakage areas above the critical NL, mechanical ventilation is no longer required by the standard and, thus, is not included in the simulation. Beyond this point, higher NLs lead to higher air exchange rates that, even with variability, produce lower peak short-term concentrations.
[FIGURE 3 OMITTED]
Figure 4 displays the results for the constant optimized scenario, in which mechanical ventilation is set at the minimum level needed to achieve annual average concentrations that are the same as the reference case. Subfigure (a) indicates that, consistent with the proposed mechanical ventilation scenario, the annual average concentration is kept at the same level as in the reference case until the NL value is large enough to turn off the mechanical ventilation altogether. The value of this critical NL varies from climate to climate. As mechanical ventilation is reduced with increasing NL, short-term maximum average concentrations increase. At some point in each climate, the envelope becomes sufficiently leaky that no mechanical ventilation is required. At this point, the maximum 1-h, 8-h, and 24-h concentrations reach a peak value. As the house gets even leakier, the maximum short-term ratios decrease, as does the annual average concentration ratio. The maximum short-term concentrations ranged from 1.9 to 2.7 compared to 1.8 to 2.4 for the Standard 62.2 infiltration credit.
Intermittent mechanical ventilation
Figure 5 shows results for the intermittent optimized scenario in which mechanical ventilation is provided on a 12 h on, 12 h off cycle. The mechanical ventilation was set at the minimum level to achieve an annual average concentration that was at least as low as in the reference case. This scenario is compliant with Standard 62.2 (i.e., mechanical ventilation rates are higher than or equal to the value determined by equation 4.2 in section 4.4 of Standard 62.2:
[Q.sub.f] = [Q.sub.r] / [[epsilon].sub.f] (9)
[Q.sub.f] is the fan flow rate, and [Q.sub.r] is the ventilation rate required by Equations 1a and 1b; [epsilon] is the ventilation effectiveness; and f is the fractional on time of the system at low NL values, 0.125-0.2, based on climate.
[FIGURE 4 OMITTED]
Since this is an optimized simulation, as NL increases beyond those levels, the mechanical ventilation rate drops below the value specified by the standard.
There are two competing effects going on in this case. Subfigures (a) and (b) are very similar to their counterparts in Figure 4 because the averaging times for those subfigures are long enough to encompass an entire 24-h ventilation cycle. Variability in these graphs is due to variability in infiltration alone. In subfigures (c) and (d), the maximum short-term concentrations are at the lowest NL value, indicating the importance of the ventilation cycle for determining the maximum time-averaged concentration over smaller integrating periods. As the NL increases, the effect of intermittent ventilation becomes a less important contributor to indoor concentration fluctuations. Beyond the critical NL, at which mechanical ventilation is no longer needed to achieve the same annual average concentration as in the reference case, there is a second maximum because of the influence of infiltration on concentrations. At low NL, the mechanical ventilation cycle is the dominant contributor to concentration fluctuations. As NL increases, the importance of the mechanical ventilation cycle decreases, and fluctuations in infiltration are the dominant contributor to concentration fluctuations. As NL increases beyond that second maximum, the graphs overlay exactly with Figure 4 because the total air change rate becomes completely dependent on infiltration.
[FIGURE 5 OMITTED]
Effect of ventilation scenarios on concentrations
Figure 2 shows the variability in pollutant concentrations that would result from climate- and house-specific leakage variations under the assumptions in the current prescriptive path of ASHRAE Standard 62.2. While the assumptions of constant infiltration and balanced mechanical ventilation are unrealistic, the calculated pollutant concentrations associated with these assumptions are the appropriate reference against which to compare pollutant concentrations resulting from other ventilation scenarios. The analysis of the idealized Standard 62.2 scenarios shows that, in general, tighter homes have lower infiltration and, thus, higher pollutant concentrations over both shorter and longer averaging times. As leakage increases, infiltration airflow varies more over time, and indoor pollutant concentrations will, therefore, vary more over shorter averaging times, resulting in larger differences between the highest short-term concentrations and annual average concentrations. The more infiltration controls time-varying air exchange rates, the larger the spread between the annual average and short-term average concentrations.
Annual average indoor concentrations will be higher than the reference case for homes with low NL. The NLs required to achieve annual average concentrations comparable to those of the reference case ranged from 0.2 for Bethel, AK, to 0.4 for Long Beach, CA. Sherman and Dickerhoff (1994) and Chan et al. (2005) found that the median NL of the existing housing stock for states where data were available was higher than this range. However, as noted previously, Chan et al. (2005) found that more than 25% of the homes they studied had NL values less than 0.4 and newer homes have NL signiicantly lower than older housing stock. Thus, while most older homes may well meet the intent of Standard 62.2 with prescriptive ventilation rates, newer, tighter homes may not.
This study's analysis of mechanical ventilation systems assumed unbalanced, exhaust-only ventilation. With an unbalanced system, additional leakage-driven airflow increases total airflow and air change rate by quadrature, not by addition; therefore, it has a greater effect at low natural infiltration and decreasing effect at infiltration increases. This means that increased infiltration will have a greater effect on whole-home ventilation than when the ventilation system is balanced. This difference is insignificant for very tight and very leaky houses, but at some point, the NL becomes large enough in both balanced and unbalanced systems that the annual average concentration will decrease below the reference case value. To be more conservative in the analysis, only the unbalanced case has been presented. The only impact of considering the balanced case would be to lower the NL at which the relative dose became equal to unity.
As Figure 2 shows, homes with weather-driven infiltration and constant mechanical ventilation, as specified by Equation 1 with NL below that critical value, will have higher pollutant concentrations than would be predicted using the assumed (constant) infiltration rate. Applying the infiltration credit (Figure 3) effectively lowers the mechanical ventilation requirement for any NL above that critical value. There is some ambiguity around this critical NL; 62.2 calculates it using ASHRAE Standard 136, while this study simulates it more exactly. Above that critical value, the Figure 3 values are somewhat higher than Figure 2 values.
The constant optimized scenario achieves long-term indoor concentrations that are at or below the reference case level. At low NL values, when the minimum required mechanical ventilation specified by Standard 62.2 is provided, the total ventilation rate will be insufficient to dilute the contaminants whether the section 4.1.3 infiltration credit is applied or not (compare Figures 2 and 3). Adding the credit at low NL values will increase indoor pollutant concentrations even more. If the NL is higher than the critical value where there is enough infiltration to reduce annual average concentrations to the reference case level, then adding the infiltration credit will reduce home thermal-conditioning costs without compromising indoor air quality. Knowing the NL value for a home could give an indication of whether applying section 4.1.3 would lead to under-ventilation or over-ventilation.
Potential health concerns from indoor concentrations
Table 1 presents the maximum indoor pollutant concentration ratios for various averaging times; these values are the highest concentrations calculated over the stated averaging time divided by the nonvarying concentration calculated for the reference case. These ratios represent the degree to which short-term concentrations in the various ventilation scenarios deviate from the long-term (annual average) concentrations for the reference case. The ratios in Table 1 can be used to assess the degree to which acute health standards are of concern in relation to chronic health standards.
While this analysis was done for the idealized case of a constant indoor emission rate and negligible outdoor concentration, the results are more generally relevant to any pollutant with continuous indoor source(s) and outdoor concentrations that are much lower than indoor levels. The analysis does not necessarily apply to pollutants with intermittent or episodic emissions or pollutants that predominately enter the indoor space via infiltration.
There is substantial uncertainty regarding whether the whole-house ventilation assumptions in Standard 62.2 lead to concentrations that are below relevant chronic health standards for all indoor pollutants. In particular, it has been shown that formaldehyde concentrations can exceed chronic standards in new homes meeting ASHRAE 62.2 (Offermann 2009).
It is important to note that the focus here is on demonstrating a methodology for understanding the effect of ventilation approach on the relationship between short-term peaks and annual average concentrations. This approach is used to investigate whether ventilation approaches that achieve chronic health standards are also adequate to meet acute health standards. Specifically, the results of this analysis are used to explore two important questions. (1) If the ASHRAE reference case meets the chronic exposure standard for some identified contaminant of concern, do the ventilation scenarios explored here meet both chronic and acute health standards for that same contaminant? (2) If a given ventilation scenario meets a chronic health standard for some contaminant, can we safely assume that an acute standard for the same contaminant will be met as well?
To answer the questions indicated above, the maximum acceptable ratios of short-term to long-term indoor concentrations when chronic standards are met must first be established. For simplicity, these ratios will be referred to in the rest of the text as the acute-to-chronic standards ratios (ACSR). This is done by dividing the most conservative acute standards for three different time frames (1 h, 8 h, and 24 h) by the most conservative chronic standard for lifetime exposure for pollutants identified as hazards in the indoor residential environment. The ACSRs are then compared to the maximum concentration ratios calculated for each of the ventilation scenarios (Table 1).
Logue et al. (2010) identified 23 pollutants Logue et al. (2010) further reduced this list to 9 key contaminants that are presented more broadly, but this study uses the larger list of 23. that are chronic hazards in U.S. homes. Table 2 lists the most health-protective chronic and acute standard and guidelines put forth by each institution (WHO 2005; USEPA 2008; OEHHA 2010). For detailed descriptions of how these levels were developed and chosen, see Logue et al. (2010). While some of these pollutants have substantial or even primarily intermittent or outdoor sources, they are all included here to provide a larger pool of acute to chronic ratios against which to evaluate the demonstrated approach.
Considering the idealized case in which Standard 62.2 most efficiently achieves the objective of providing acceptable indoor air quality, the reference case will have an annual average concentration equal to the controlling chronic health standard(s). For each ventilation scenario, an annual average concentration ratio of 1.0 in the calculations indicates acceptable indoor air quality with regard to chronic health standards.
The first two ventilation scenarios (constant default and constant with infiltration credit) result in long-term concentration ratios greater than unity for low NL values, indicating potential chronic exposure concerns for tighter homes.
For all of the ventilation scenarios, maximum ratios of short-term to annual average concentrations for 1-h and 8-h time frames are well below the ACSR values, independent of whether annual average concentrations exceed unity. The maximum 24-h concentrations are closer to this limit than is the case for the other acute time frames but are still lower than the ASCRs. This indicates that weather-induced variability in infiltration does not lead to excessive peak concentrations for constantly emitting pollutants with negligible outdoor concentrations in these ventilation scenarios, as long as the ventilation scenario meets the chronic health standards. This holds true for intermittent ventilation. However, if chronic standards are not met, intermittent ventilation could lead to concentrations above acute standards.
For ventilation scenarios that result in long-term concentrations at or below the chronic standards, it can be assumed that acute exposures to continuously emitting contaminants with negligible outdoor sources will not have to be addressed separately from the chronic exposures, as long as the scenario keeps the maximum concentrations below the following levels: the 1-h indoor concentration at less than 4.7 times the reference case, the maximum 8-h average at less than 5.4 times the reference case, and the maximum 24-h concentration average at less than below 2.5 times the reference case. This is the case for the scenarios presented here.
Summary, conclusions, and recommendation
This article has developed and demonstrated an approach to assess the effects of air leakage and intermittent ventilation on the ratio of peak short-term to time-averaged pollutant concentrations for various ventilation strategies. This approach was applied to assess ratios of the highest short-term indoor concentrations (averaged over 1-h, 8-h, and 24-h periods) to annual average exposures for four ventilation scenarios: default (constant ventilation as specified in ASHRAE Standard 62.2), infiltration credit, constant optimized, and intermittent optimized. The reference scenario was the (physically unrealistic) condition of constant infiltration airflow, as assumed in ASHRAE Standard 62.2. The other scenarios examine potential alternative applications of Standard 62.2.
Infiltration and intermittent ventilation can increase maximum, short-term indoor pollutant concentrations relative to constant ventilation; however, it was found that if the ventilation scenario meets long-term chronic standards, the maximum short-term concentrations are below acute standards.
Results indicate that chronic exposure is the most relevant metric for assessing indoor air quality of different whole-house ventilation scenarios for constantly emitting pollutants with negligible outdoor concentrations. Two of the scenarios presented here had annual average concentrations higher than the reference case; however, none had concentrations that exceed ACSRs.
Ventilation scenarios that incorporate air leakage as part of the design can provide equivalent indoor air quality. That is, it is possible to provide adequate indoor air quality without any mechanical ventilation at high NL values. The energy efficiency and cost effectiveness of such a strategy may not make it desirable to do so, but that issue was not investigated.
Providing sufficient ventilation to keep chronic concentrations of continuously emitted pollutants at safe levels should be the goal of whole-house ventilation. Although research is needed to determine whether application of Standard 62.2 does, in fact, keep indoor concentrations below chronic exposure standards, the results show that when the chronic levels are met, the concept of relative dose (i.e., average annual relative exposure) can be a guiding principle for ensuring that residential ventilation standards meet acute exposure standards.
Funding was provided by the U.S. Department of Energy Building Technologies Program, Office of Energy Efficiency and Renewable Energy under DOE contract no. DE-AC02-05CH11231, by the U.S. Department of Housing and Urban Development Office of Healthy Homes and Lead Hazard Control through Interagency Agreement I-PHI-01070, and by the California Energy Commission through contract 500-08-06.
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Max H. Sherman, Jennifer M. Logue, * and Brett C. Singer
Lawrence Berkeley National Lab, Energy and Performance of Buildings Group, EETD, Berkeley, CA, USA
* Corresponding author e-mail: email@example.com
Received August 5, 2010; accepted October 4, 2010
Dr. Max H. Sherman, PhD, Fellow ASHRAE, is senior scientist and group leader. Dr. Jennifer M. Logue, PhD, is postdoctoral fellow. Brett C. Singer, PhD, Member ASHRAE, is staff scientist.
Table 1. Maximum concentration ratio of indoor concentrations for different averaging times compared to the reference case for each ventilation scenario. Maximum concentration ratio Ventilation scenario Annual 24 h 8 h 1 h Constant-default 1.7 1.8 1.8 1.8 Constant with 1.7 1.8 2.2 2.3 infiltration credit Constant optimized 1.0 2.1 2.6 2.8 Intermittent optimized 1.0 2.1 2.6 3.4 Table 2. Concentration standards and guidelines for pollutants that present hazards in the indoor residential environment ([micro],g/ [m.sup.3]) (a). Compound Chronic 24 h 8 h 1 h Acetaldehyde * 3.7 300 470 Acrolein * 0.02 0.7 2.5 Acrylonitrile 0.03 Benzene * 0.34 1, 300 Benzyl chloride 0.2 5, 174 240 Butadiene, 1,3- * 0.06 Cadmium 0.0024 Carbon tetrachloride 0.24 1, 900 Chloroform 1. 98 150 Chromium 6.7E-5 Dichlorobenzene, 1,4- * 0.91 45, 000 Dichloropropane,1,2- 4 350, 000 Ethanol 1, 900, 000 Ethylbenzene 4 Formaldehyde * 1.67 9 55 Hexachlorobutadiene 0.45 Methylene chloride 10 14, 000 Naphthalene * 0.29 50, 000 NO2 * 40 189 PM2.5* 6.0pt1,150.0pt 10 25 Tetrachloroethane, 0.17 35, 000 1,1,2,2- Tetrachlorothene 1.69 20, 000 Vinyl chloride 0.13 1, 800, 000 Lowest acute to 2.5 5.4 4.7 chronic ratio (-) (a) Micrograms per square meter. * Compounds identified by Logue et al. (2010) as key contaminants.
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|Author:||Sherman, Max H.; Logue, Jennifer M.; Singer, Brett C.|
|Publication:||HVAC & R Research|
|Date:||Mar 1, 2011|
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