1、2009 ASHRAE 887ABSTRACTThe purpose of ventilation is to dilute or remove indoor contaminants that an occupant could be exposed to. It can be provided by mechanical or natural means. ASHRAE Standards including standards 62, 119, and 136 have all considered the contribution of infiltration in various
2、ways, using methods and data from 20 years ago. The vast majority of homes in the United States and indeed the world are ventilated through natural means such as infiltration caused by air leakage. Newer homes in the western world are tight and require mechanical ventilation. As we seek to provide a
3、cceptable indoor air quality at minimum energy cost, it is important to neither over-ventilate nor under-ventilate. Thus, it becomes critically important to correctly evaluate the contribution infil-tration makes to both energy consumption and equivalent ventilation. ASHRAE Standard 62.2 specifies h
4、ow much mechanical ventilation is considered necessary to provide acceptable indoor air quality, but that standard is weak on how infiltration can contribute towards meeting the total require-ment. In the past ASHRAE Standard 136 was used to do this, but new theoretical approaches and expanded weath
5、er data have made that standard out of date. This article will describe how to properly treat infiltration as an equivalent ventilation approach and then use new data and these new approaches to demonstrate how these calculations might be done both in general and to update Standard 136. INTRODUCTION
6、Infiltration, adventitious or incidental air leakage through building envelopes, is a common phenomenon that affects both indoor air quality and building energy consumption. Infiltration can contribute significantly to the overall heating or cooling load of a building, but the magnitude of the effec
7、t depends on a host of factors, including environmental condi-tions, building design and operation, and construction quality. Typically infiltration accounts for one-third to one-half of the space conditioning load of a home.In addition to increasing the conditioning load of a build-ing, infiltratio
8、n can bring unwanted constituents into the build-ing or into the building envelope and cause building failures. For example, infiltration of hot, humid air in an air conditioned building in the summer (or exfiltration of indoor air in a heated building in the winter) can cause condensation in the bu
9、ilding envelope leading to potential structural failure and mold growth. For these reasons reducing infiltration is desirable.Infiltration, however, serves a vital purpose in most exist-ing homes: it is the dominant mechanism for providing venti-lation. The purpose of ventilation is to provide fresh
10、 (or at least outdoor) air for comfort and to ensure healthy indoor air qual-ity by diluting contaminants. Historically, people ventilate buildings to provide source control for both combustion prod-ucts and objectionable odors (Sherman 2004). Currently, a wide range of ventilation technologies is a
11、vailable to provide ventilation in dwellings including both mechanical systems and more sustainable technologies. Most of the existing hous-ing stock in the U.S. uses infiltration combined with window opening to provide ventilation. Sometimes this results in over-ventilation with subsequent energy l
12、oss or under-ventilation and poor indoor air quality. Recent residential construction methods have created tighter, more energy-saving building envelopes that create a potential for under-ventilation (Sherman and Dickerhoff 1994, Sherman and Matson 2002). McWilliams and Sherman (2005) have reviewed
13、ventilation standards and related Infiltration in ASHRAEs Residential Ventilation StandardsMax Sherman, PhDFellow ASHRAEMax Sherman is a senior scientist at the Lawrence Berkeley National Laboratory, Berkeley, CA, where he runs the Energy Performance of Buildings Group in the Indoor Environment Depa
14、rtment.LO-09-085 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2009, vol. 115, part 2. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permi
15、tted without ASHRAEs prior written permission.888 ASHRAE Transactionsfactors. Infiltration rates in new homes average a half to a quar-ter of the rates in existing stock. As a result, new homes often need mechanical ventilation systems to meet current ventila-tion standards. Unless buildings are bui
16、lt completely tight and fully mechanically ventilated, infiltration will always contribute towards ventilation. Ignoring that contribution can lead to over-ventilation and unnecessary energy expense; over-esti-mating of that contribution can lead to poor indoor air quality. This report uses simulati
17、on methods to help determine how infiltration can and should be properly valued in the context of residential ventilation.ASHRAE STANDARDSA key motivation for understanding infiltrations role in ventilation is for setting minimum standards both for energy and indoor air quality. The American Society
18、 of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE) is the key organization and the only one to have American National Standards on residential ventilation and infiltration. The three key standards are Standards 62.2, 119 and 136.Standard 62.2 (2007) sets requirements for residential
19、ventilation and acceptable indoor air quality. There are source control requirements and there are minimum ventilation requirements. The standard is mostly about mechanical venti-lation systems but has a default infiltration credit and allows mechanical ventilation rates to be reduced based on an in
20、fil-tration credit measured using ASHRAE Standard 136.Standard 136 (1993) uses pre-calculated weather factors and the air tightness measured using normalized leakage (of Standard 119) to estimate the impact that infiltration would have on indoor air quality and thus determine its equivalent ventilat
21、ion. This concept will be described in more detail in later sections.Standard 119 (1988) defines normalized leakage and also specifies tightness levels based on energy conservation concerns. Herein, we are concerned with the metric (Normal-ized Leakage) that is used in the ASHRAE Standards and the s
22、tandardized infiltration model based on it. We will look at the impacts of infiltration towards provid-ing acceptable indoor air quality and examine the need to change these standards, particularly standard 136, accordingly.BACKGROUNDTo understand the contribution of infiltration, we review the role
23、 of air tightness and weather in driving infiltration and the role ventilation has in providing acceptable indoor air quality.AIR TIGHTNESS“Air Tightness” is the property of building envelopes that is most important to understanding infiltration. It is quantified in a variety of ways, all of which t
24、ypically are called “air leak-age”. Air tightness is also important from a variety of perspec-tives, such as insulation performance or envelope durability, but those issues will not be discussed herein. There are a vari-ety of definitions of infiltration, but, fundamentally, infiltra-tion is the mov
25、ement of air through leaks, cracks, or other adventitious openings in the building envelope. The modeling of infiltration (and thus ventilation) requires a measure of air tightness as a starting point. More extensive information on air tightness can be found in Sherman and Chan (2003), who review th
26、e state of the art. This information is also part of a broader state of the art review on ventilation compiled by Santamouris and Wouters (2005).Sherman and Chan (2003) also discuss the topic of metrics, reference pressures, and one versus two parameter descriptions in some detail. There are various
27、 metrics one could use other than Normalized Leakage. For example, airflow at a fixed pressure is the easiest one to measure but suffers from accuracy issues and does not account for house size in any way. Similarly, leakage per unit exposed surface area is a good estimate of the porosity of the env
28、elope, but does not scale correctly for either energy purposes or indoor air quality purposes.We have chosen to use the metric of Normalized Leakage (NL) as defined by ASHRAE Standard 119 (1988) as our primary metric to describe air tightness of houses because it removes the influence of house size
29、and height, because it scales better with house size, and because it is used in the other ASHRAE standards. The metric is as follows:(1)where NL is the normalized leakage as defined by ASHRAE (See Standard 119, 1988), ELA is the Effective Leakage Area measured by methods such as ASTM E779 (2003) in
30、the same units as the floor area (FloorArea) and Nstoryis the number of stories of the house.The interaction of air tightness with external driving forces (most notably winds speed and temperature difference) creates infiltration. Infiltration combines then with mechani-cal ventilation to provide th
31、e total ventilation. To link NL (or any other air leakage metric) to the total ventilation, we must use an infiltration model.INFILTRATION MODELTo estimate the infiltration we use the infiltration model contained in ASHRAE Standard 119 (1988); broader discus-sions of infiltration modeling can be fou
32、nd in the ASHRAE Handbook of Fundamentals. (While we could have chosen to use a more fundamental air flow model, the arbitrariness of the additional input parameters would not result in increased accuracy compared to this simplified physical model.) This infiltration model is a specific instance of
33、the LBL infiltration model, shown below, which estimates volumetric flow through the building shell, Q, based on the leakage area of the shell (ELA), wind and stack factors (fw, fs), and temperature difference ( ) and wind speed (v) at the house site:NL 1000ELAFloorArea- Nstory()0.3=TASHRAE Transact
34、ions 889(2)where the specific infiltration, s, at any moment of time is given by:(3)Specific infiltration, s, has units of velocity (e.g., m/s, ft/min). The details of the model can be further explored in, for exam-ple, Sherman and Matson (1997), but we will use the numer-ical values from ASHRAE Sta
35、ndard 119 (1988), which were based on combinations of presumed leakage distribution, wind pressure coefficients, and terrain factors. Specific values were chosen in that standard to represent typical residential construction:(4)Infiltration and ventilation are often referred to in air changes per ho
36、ur. To convert between the air flow and air changes per hour it is necessary to use the volume of the space. The air change rate in air changes per hour (ACH) for a single story home due to infiltration can be found from the specific infiltration and normalized leakage as follows:(5)where for unit c
37、onversion: kI=1.44 s/hr-m (0.0073 min/hr-ft)assuming a typical specific infiltration, s, for a single-story height of approximately 2.5 m (8 ft). (This constant would otherwise decrease inversely proportional to the story height.)VENTILATION EFFECTIVENESS From the perspective of acceptable indoor ai
38、r quality, the purpose of ventilation is to dilute the concentration of contam-inants. We generally seek to control the average concentration of contaminants received over some period of interest. With constant emission strength and constant total ventilation, this is a simple calculation. When the
39、ventilation varies over time, however, the pollutant concentration is non-linear with respect to ventilation rate and a simple average of the ventilation rate cannot be used. Instead, the term effective ventilation rate is defined as the constant ventilation rate that would yield the same average po
40、llutant concentration as the actual time vary-ing ventilation; it is dependent on the time period chosen for the evaluation. ASHRAE Standard 136 (1993) was a first attempt to determine the contribution of infiltration towards providing ventilation. The algorithm used was based on a dimensional analy
41、sis by Yuill (1991) based on his work (Yuill 1986) and the work of Sherman and Wilson (1986) and used a variety of weather sources available at the time. The result was a table of “W” values that link the effective ventilation due to infiltration to the normalized leakage:(6)The Sherman and Wilson (
42、1986) approach of effective ventilation included dynamic effects which are not considered currently in standard 136. We will use the Sherman-Wilson definition herein to quantify the contributions of time varying ventilation. It is important to note that the contaminant source strength is assumed to
43、be constant over the period of interest. This holds for many building contaminants where the source emission varies slowly with time or operates in a stepwise fashion and is unaffected by ventilation rate. Some important exceptions are certain instances of radon or formaldehyde, where the emission r
44、ate can be affected by the ventilation of the building in some circumstances. If such special cases are relevant, more detailed techniques may be required. The derivation is contained in the reference, but we shall include a quick summary. The basic concept is to find the equivalent steady-state ven
45、tilation that produces the same average concentration of a continuously emitted contaminant as the actual pattern. The ventilation rate is called the effective ventilation. As Sherman and Wilson (1986) describe effective ventilation is calculated by first calculating the inverse, the characteristic
46、time (e) for the pollutant concentration to reach steady state, which is given below. (7)The mean ventilation efficiency is a non-dimensional quantity which is defined as the ratio of the mean effective ventilation to the mean instantaneous ventilation. It is shown in terms of the characteristic tim
47、e. The closer the actual venti-lation rate is to steady state over the period of interest the higher the ventilation efficiency will be.(8)where the over-bars indicate an average over time. The effective ventilation for that period will be the aver-age ventilation for that period multiplied by the m
48、ean (tempo-ral) ventilation efficiency (sometimes called efficacy) for that period.EXPOSURE PERIODThe ventilation effectiveness derivation above requires that we take averages over some period of time of the quanti-ties involved. Since we are doing this analysis using annual weather data, the nomina
49、l time period to average over would be a year. This would be appropriate if the relationship between the impact of the contaminants only depended on the average concentration of the contaminants, but that may not always be the case. Therefore we need to determine the rele-vant exposure period.Contaminants in the indoor environment may interact with the body in different ways, which means their relevant exposure metric can be quite different. For example, one end of the spectrum is when the risk of disease is related to the total QELAs=sfs2Tfw2v2+=fs0.12
