chapter eleven Ventilation Ventilation has historically been applied and viewed as both a desirable and effective technique in improving thermal comfort and general air quality and comfort in buildings. It has been used to dilute and exhaust unwanted con- taminants such as combustion by-products, lavatory and cooking odors, heat and moisture, etc. from residential and nonresidential buildings. Exhaust ventilation is widely used in industry to remove contaminants at their source to reduce worker exposure and health risks. General dilution ventilation induced and delivered by mechanical fans is used in office, commercial and retail, and institutional buildings to maintain acceptable air quality. Ventilation is a physical process that involves the movement of air through spaces. It has two dimensions. When air flows into a space, venti- lation is characterized by mixing, dilution, and partial replacement. When air flows (is removed) from a space (because it is under negative pressure), it is replaced by air from a nearby area. General dilution ventilation describes the first case; exhaust ventilation, the second. Ventilation, because it involves the flow of air due to pressure differ- ences, is a natural phenomenon. However, natural ventilation is often too variable or inadequate. As a consequence, it is necessary to use systems that deliver controlled, mechanically induced ventilation to provide for the needs of a variety of building spaces. Ventilation in its dilution, displacement, and replacement aspects causes changes in chemical composition and environmental factors in the air envi- ronment of ventilated spaces. These changes may result in reduced overall contaminant levels or a decrease in concentrations of some substances, with an increase in others. The outcome depends on the nature of air in ventilated spaces as well as air used for ventilation. The same is true for environmental factors such as temperature and relative humidity. The desired effect of ventilation, whether it is natural or mechanically induced, is to enhance or protect the quality of air in the space being venti- © 2001 by CRC Press LLC lated. Ventilation causes an exchange of air within building spaces and between building interiors and the outside environment. Ventilation is used to dilute and remove contaminants, enhance thermal comfort, remove excess moisture, enhance air motion, improve general comfort, and in large build- ings, maintain pressure differences between zones. I. Natural ventilation All buildings are subject to natural forces that result in air exchange with the ambient (outdoor) environment. Natural ventilation depends on the inflow of air as a result of (1) pressure differences when a building is under closure conditions and being heated or cooled, (2) pressure-driven flows when building windows and doors are open, or (3) the continual movement of air through a building as it enters through some openings and exits through others (Figure 11.1). In the last case, small buildings will experience relatively high air exchange rates. Maximum exchange rates will depend on wind speed, the position of open windows and doors relative to each other, and prevailing winds. Though limited scientific data are available, air exchange rates should be at their maximum when buildings are ventilated by opening windows and doors when outdoor conditions are favorable. Residential buildings, which include both single- and multi-family struc- tures, are increasingly being provided year-round climate control. As a con- sequence, the practice of opening windows and doors for ventilation pur- poses is decreasing. Such residences, and those in seasonally cold or warm climates, are maintained under closure conditions for extended periods of time (upwards of 9 months or so). Under closure conditions, ventilation occurs as a result of infiltration and exfiltration processes. These processes involve pressure-driven flows Figure 11.1 Ventilation air flow through a single-family house under open condi- tions. (From USEPA, Introduction to Indoor Air Quality — A Self-paced Learning Module , EPA/400/3-91/002, Washington, D.C., 1991.) © 2001 by CRC Press LLC associated with temperature differences (between indoor and outdoor envi- ronments) and the speed of the wind. Infiltration occurs as a result of the inflow of air through cracks and a variety of unintentional openings (leakage areas) in the building envelope. Infiltration only occurs through leakage areas where internal pressures are negative relative to those outdoors. In residential structures, infiltration typ- ically occurs at the base and during windy conditions on the lee (downwind) side of buildings. When infiltration occurs, it replaces and displaces air in the building interior. As a consequence, air must also flow outward through cracks and other leakage areas. In residential structures, such air outflows (exfiltration) typically occur through ceiling areas and upper wall locations where internal pressures are positive. A. Stack effect On calm days or during calm periods during the day, infiltration and exfil- tration occur as a consequence of pressure differences associated with dif- ferences between the inside and outside temperature ( ∆ T). During the heat- ing season in seasonally cool to cold climates, warm air rises and creates a positive pressure on ceilings and upper walls of small residential (and non- residential) buildings. This upward flow of warm air produces negative pressures, which are at a maximum at the base of the structure. These negative pressures cause an inflow (infiltration) of cool or cold air, with maximum inflows where negative pressures are (in absolute terms) the high- est. Infiltration causes air to be drawn in from both the outdoor environment and from the ground (soil gas). An idealized characterization of pressure conditions in a single-family dwelling on a cool day is diagramed in Figure 11.2. As can be seen, an area of neutral pressure exists between negative and positive pressure environ- ments. This is the neutral pressure level or neutral pressure plane (NPP). At Figure 11.2 Generalized pressure conditions in a small house on a cool, calm day under closure conditions. (From Lstiburek, J. and Carmody, J., Moisture Control Hand- book , Van Nostrand Reinhold [John Wiley & Sons], New York. With permission.) © 2001 by CRC Press LLC the NPP, indoor and outdoor pressures are equal. In Figure 11.2, the NPP is located at approximately mid-level, suggesting that leakage areas are uni- formly distributed over the building face. The location of the NPP depends on the distribution of leakage sites. In older single-family dwellings, the NPP is often above mid-height, and in the case of houses with flue exhaust of combustion by-products, the NPP may be above the ceiling level during exhaust operation. The construction of residential dwellings has included a number of energy-conserving measures, most notably tighter building enve- lopes. Since leakage areas have been reduced, pressure characteristics have changed, resulting in reduced infiltration and exfiltration. In such houses, the NPP would be expected to occur at mid-height (absent the active oper- ation of combustion exhaust systems). In tall buildings, the NPP may vary from 30 to 70% of the building height. The inflow and outflow of air discussed above is called the stack effect because airflows are similar to those which occur in a smokestack. The magnitude of the stack effect increases significantly with building height. The change in pressure with height in a large building has been reported to be approximately 0.001” H 2 O (0.25 pascals) per story. Stack effect flows upward are particularly noticeable in elevator and other service shafts and in open stairwells. Each story, if constructed in an airtight way, can behave more or less independently, i.e., have its own stack effect. The influence of stack effect on building air exchange rates is, for the most part, proportional to ∆ T, the difference between indoor and outdoor temperatures. This relationship can be seen in model predictions graphed in Figure 11.3 for ∆ T = 0°F and 40°F. As ∆ T increases, air exchange increases, with maximum values on cold days. Minimum air exchange occurs when Figure 11.3 Building air exchange associated with different stack effect and wind speed conditions. © 2001 by CRC Press LLC indoor and outdoor temperatures are the same or little different from each other. Such conditions exist for brief periods (hours) during diurnal changes in outdoor temperatures, and for more extended periods on mild overcast days, and mild days in the fall and autumn in temperate climatic regions. They commonly occur in coastal regions where maritime climates produce outdoor temperature conditions which in many cases are in the same range as those indoors. B. Wind Infiltration and exfiltration are also significantly influenced by wind. The effect of wind on pressures both inside and outside of buildings is a relatively complex phenomenon. As wind approaches a building it decelerates, creating significant posi- tive pressure on the windward face. As wind is deflected, its flow separates at a building’s sides as well as its top or roof. This produces negative pres- sures around the sides of the building, the roof, and the leeward side. The effect of these pressure differences is to cause an inflow of air on the wind side and an outflow on all exterior surfaces which are negative relative to indoor pressures. The effects of wind on pressure conditions on a rectangular building oriented perpendicular to the wind is illustrated in Figure 11.4. The distribution of positive and negative pressures on a building depends on wind speed, building geometry and size, and the incident angle of the wind. The magnitude and distribution of infiltration air is influenced by the type of building cladding; tightness of the building envelope; and barriers to air Figure 11.4 Effect of wind on pressure conditions in/on a rectangular building oriented perpendicular to the wind. (From Allen, C., Technical Note AIC 13 , Air Infiltration Centre, Berkshire, U.K., 1984. With permission.) © 2001 by CRC Press LLC movement such as trees, shrubbery, and other buildings. Such barriers induce turbulence that reduces wind speed and alters wind direction. The effect of wind on building infiltration is a squared function of wind velocity or speed (mph, m/s). The modeled effect of wind speed on house air exchange rates can be seen in Figure 11.3 for a ∆ T of 0°F (0°C) and 40°F (22°C). Though the effect of wind speed on air exchange rates is exponential, significant increases in infiltration-induced ventilation are only seen at high wind speeds (>8 mph, 3.56 m/s). As can been seen in Figure 11.3, the combined effect of stack effect and wind speed on building air exchange rates (ventilation) can be significant. Figure 11.3 describes the effect of indoor/outdoor temperature differ- ences and wind speed on a single house. Because of differences in building tightness, distribution of leakage areas, and orientation to the wind, air exchange rates in other houses under similar stack effect and wind speed conditions are likely to be different (though the general form of the rela- tionship will be much the same). These curves are based on the following linear model: I = A + B( ∆ T) + C(v 2 ) (11.1) where I = infiltration rate (ACH) A = intercept coefficient, ACH ( ∆ T = 0, v = 0) B = temperature coefficient C = wind velocity coefficient ∆ T = indoor/outdoor temperature difference, (°F) v = wind speed (mph, m/sec) Both the temperature and wind speed coefficients are empirically derived and differ for each building. Coefficient differences among buildings are relatively small. C. Infiltration and exfiltration air exchange rates As indicated above, building air exchange rates associated with infiltra- tion/exfiltration-induced airflows vary with indoor/outdoor temperature differences (which vary considerably themselves), wind speed, and tightness of the building envelope. They may also be influenced by pressure changes associated with the operation of vented combustion appliances, bath- room/lavatory fans, and leaky supply and return air ducts. Each of these can increase infiltration and air exchange rates above those associated with combined stack effect and wind infiltration and exfiltration values. Leaky supply/return air ducts may be responsible for upwards of 30+% of infil- tration/exfiltration-related air exchange in residential buildings. In response to energy concerns in the late 1970s and early 1980s, the U.S. Department of Energy supported several studies to evaluate infiltration and exfiltration rates in U.S. housing stock. The average air exchange rate for © 2001 by CRC Press LLC more than 300 U.S. houses was measured on a one-time basis. As can be seen in Figure 11.5, approximately 80 to 85% of houses tested had a daily average air exchange rate of <1 air change per hour (ACH). Infiltration/exfil- tration rates in low-income housing were on average significantly higher; approximately 40% had infiltration values >1 ACH. On a population basis, these one-time measurements of infiltration/exfil- tration-induced air exchange were likely to have demonstrated a reasonable estimate of ventilation conditions in housing stock existing at the time (early 1980s). Since then, construction practices have changed (tighter building envelopes are now the norm), and significant weatherization measures have been implemented to reduce energy losses in low-income housing. Weath- erization measures using retrofit tightening of building envelopes in low- income housing have, however, only been moderately effective (on average, ≤ 25% reduction in building leakage and infiltration-associated air exchange). It is highly probable that construction practices in the past several decades have significantly increased the stock of housing units in North America, northern Europe, and other developed regions and countries which have lower air exchange (and thus ventilation) rates than older houses. Decreasing natural ventilation rates have been a cause for concern among policy makers in various governmental agencies, utilities which have supported weather- ization measures, public health groups, and research scientists. It was and is widely believed among environmental and public health professionals that decreasing natural ventilation rates associated with infiltration/exfiltration- reducing measures are likely to cause an increase in indoor contaminant levels and health risks associated with increased exposures. D. Leakage characteristics Air exchange rates in buildings associated with thermal and wind-induced pressure differences are affected to a significant degree by building leakage characteristics. Typical leakage areas are indicated in Figure 11.6 for a single- Figure 11.5 Infiltration rates measured in 312 North American houses in the early 1980s. (From Grimsrud, D.T. et al., LBL-9416, Lawrence Berkeley Laboratory, Berke- ley, CA, 1983.) © 2001 by CRC Press LLC story house on a basement. Major structure-related leakage areas include the sole plate where the building frame is fastened to the substructure, and cracks around windows, doors, exterior electrical boxes, light fixtures, plumbing vents, and various joints. Leakage also occurs through exhaust fans, supply/return ducts, and combustion appliance flues. Leakage is par- ticularly pronounced when combustion appliances such as furnaces, hot water heaters, and fireplaces are in operation. Leakage can also occur through duct systems which provide heating and cooling when duct runs are in crawlspaces, attics, and garages. The size of leakage areas and their distribution in a building determine the magnitude of infiltration and exfiltration air exchange when measured under similar environmental conditions. They also determine the nature of air flow patterns into and out of buildings. Building leakage potentials are commonly assessed using fan-pressur- ization or blower-door techniques. By pressurizing buildings with a fan installed into a test door, the overall leakage potential of residential buildings can be determined to identify leakage areas that need to be caulked or sealed. Such leakage characterization is commonly conducted in weatherization programs which target low-income housing. II. Measuring building air exchange rates Building air exchange rates (ACH) associated with wind and thermally induced pressure flows, as well as those associated with mechanical venti- Figure 11.6 Air leakage sites on a single-family house with basement substructure. © 2001 by CRC Press LLC lation, can be measured using tracer gas techniques. Tracer gases used in such measurements are characteristically unreactive, nontoxic, and easily measured at low concentrations. Sulfur hexafluoride and perfluorocarbons are commonly used to measure air exchange rates because they can be detected and quantitatively determined in the parts per billion range (ppbv). On occasion, nitrous oxide (N 2 O) or carbon dioxide (CO 2 ) are used. Carbon dioxide has the advantage of being measured in real time on relatively inexpensive continuous monitors. It has the disadvantage of being produced by humans. Thus it cannot be used when building spaces are occupied. Silicon hexafluoride, perfluorocarbons, and N 2 O are collected using one-time sampling techniques and require analysis on sophisticated, expensive instru- ments. Perfluorocarbon measurements are usually made using permeation tubes as sources and passive samplers as collectors. As such, air exchange measurements based on perfluorocarbons typically provide 7-day averages. The concentration decay method (previously described in Chapter 8) is the most widely used air exchange measuring technique. It involves initial injection of a tracer gas into a space or building with the assumption that the tracer gas is well mixed in building/space air. The decrease, or decay as it is often described, of the tracer gas concentration is measured over time. From these measurements the air exchange rate I in ACH can be determined from the following exponential equations: C t = C 0 e –(Q/V) t (11.2) where C t = tracer gas concentration at the end of the time interval ( µ g/m 3 , ppmv, ppbv) C 0 = tracer gas concentration at time t = 0 ( µ g/m 3 , ppmv, ppbv) V = volume of space (m 3 ) Q = ventilation rate (m 3 /hour) e = natural log base t = time (hours) The ratio Q/V, considered in the context of hours, yields the air exchange rate I in ACH. C t = C 0 e –It (11.3) Equation 11.4, used to determine I, the air exchange rate, can be derived from Equation 11.2. I = (ln C 0 /C t )/t (11.4) Let us assume that the initial tracer gas concentration was 100 ppbv and at the end of 2 hours it was 25 ppbv. We could then calculate I as follows: © 2001 by CRC Press LLC I = (ln 100/25)/2 (11.5) I = 0.69 ACH In perfluorocarbon determinations of air exchange, perfluorocarbons are injected at a constant rate. The air exchange I is calculated from the ventila- tion rate (Q), which is the ratio of the rate of injection (F) to the measured concentration C. Q = F/C (11.6) Since Q is expressed in m 3 /sec or cubic feet per minute (CFM), it must be multiplied by appropriate time units and divided by the volume of the space to obtain ACH values. III. Mechanical ventilation Most large commercial, office, and institutional buildings constructed in developed countries over the past three decades are mechanically ventilated. Use of mechanical ventilation is often required in building codes and repre- sents what can be described as good practice for building system designers and architects. Increasingly, buildings are being designed to provide year- round climate control. To ensure optimum operation of heating, ventilating, and air-conditioning (HVAC) systems, windows are sealed so they cannot be opened by occupants to provide ventilation. The availability of outdoor air for space ventilation depends on the design and operation of HVAC systems as well as air that enters by infiltration and exfiltration processes. Mechanical ventilation is used in buildings to achieve and maintain a comfortable and healthy indoor environment. Two ventilation principles are used to accomplish this goal; general dilution and exhaust ventilation. Both principles are used in most buildings. General dilution ventilation is the dominant ventilation principle used to ventilate buildings. Local exhaust ventilation is used for special applications: removing lavatory and kitchen odors, combustion by-products, combustion appliance flue gases, etc. A. General dilution ventilation Ventilating buildings to provide a relatively comfortable, healthy, and odor- free environment is based on the premise that a continual supply of outdoor air can be introduced into building spaces. As ventilation air mixes with contaminated air, contaminant levels are reduced by dilution. In general dilution theory, a doubling of the air volume available for dilution is expected, under episodic or constant conditions of contaminant generation, to reduce contaminant concentration by 50%. If the volume of ventilation air were to be doubled again, the original concentration would be reduced to 25% of its original value. By decreasing ventilation air required © 2001 by CRC Press LLC [...]... Environ., 16, 123, 1981 Spengler, J.D., Samet, J.M., and McCarthy, J.F., Eds., Indoor Air Quality Handbook, McGraw-Hill Publishers, 2000, chapters 7, 8, 13 USEPA, Building Air Quality Manual, EPA 402-F-9 1-1 02, USEPA, Washington, D.C., 1991 Woods, J.E., Measurement of HVAC system performance, Ann ACGIH: Evaluating Office Environmental Problems, 10, 77, 1984 Questions 1 What is general dilution ventilation?... resistant to corrosion Readings ASHRAE, Ventilation for Acceptable Indoor Air Quality, Standard 6 2-1 989, American Society of Heating, Refrigerating and Air-conditioning Engineers, Atlanta, 1989 Bearge, D.W., Indoor Air Quality and HV AC Systems, CRC Press/Lewis Publishers, Boca Raton, F., 1993 Fanger, P.O., The comfort equation for indoor air quality, ASHRAE J., October, 33, 1989 Godish, T., Sick Buildings:... building Cross-contamination occurs when pressure differences exist between building zones served by different AHUs Cross-contamination is one of the most frequently encountered problems in problem-building investigations It is commonly perceived by occupants as chemical odors or as an odor-out-of-place phenomenon Cross-contamination complaints arise when contaminants migrate from special-use areas where... J.J., Ventilation for acceptable indoor air quality: ASHRAE Standard 6 2-1 981, Ann ACGIH: Evaluating Office Environmental Problems, 10, 59, 1984 Maroni, M., Siefert, B., and Lindvall, T., Indoor Air Quality A Comprehensive Reference Book, Elsevier, Amsterdam, 1995, chap 29 McNall, P.E and Persily, A.K., Ventilation concepts for office buildings, Ann ACGIH: Evaluating Office Environmental Problems, 10, 49,... standard Figure 11. 13 appears to confirm the wisdom of ASHRAE’s recommendations on ventilation requirements for mechanically ventilated office buildings c Flush-out ventilation Conventional ventilation systems can be operated to speed up the decay rate of initially high material-, finishes-, and furnishings-based VOC concentrations in new or remodeled buildings and thereby improve indoor air quality Such... building wake or break through it as a result of upward momentum Figure 11. 17 Building roof-top wake and re-entry phenomena © 2001 by CRC Press LLC v Entrainment In re-entry, exhaust gases, vapors, and particles may become entrained in outdoor air intake flows, resulting in varying degrees of indoor air contamination Entrainment and subsequent indoor contamination may also occur from contaminants generated... exhaust is commonly used in oil-burning and all high-efficiency natural gas and propane-fueled furnaces As indicated in Chapter 3, combustion gas exhaust systems are subject to flue gas spillage associated with equipment malfunction and weather conditions b Soil-gas ventilation Soil gas contaminated with radon is commonly removed in radon mitigation efforts by the use of sub-slab ventilation, a special... occupancy can be seen in Figures 11. 12a and 11. 12b The effect of different ventilation rates on © 2001 by CRC Press LLC Figure 11. 12 Effect of occupancy and ventilation on CO2 levels in a San Francisco office building (From Turiel, I., et al., Atmos Environ., 17, 51, 1983 With permission.) CO2 levels can also be seen when Figures 11. 12a and 11. 12b are compared In Figure 11. 12a, the HVAC system is being... variations These include all-air, air–water, and all-water systems i All-air systems In all-air systems, air is conditioned as it passes over heating and cooling coils It is then delivered to occupied spaces through a single duct or through individual hot and cold ducts to a mixing box (and then to occupied spaces) Air flow through these systems may be at a constant rate (constant-air-volume [CAV] systems),... Variable-air-volume systems were developed in response to energy management concerns and, when properly installed and operated, can reduce building energy consumption © 2001 by CRC Press LLC Figure 11. 8 Simplified constant-volume HVAC system design (From McNall, P.E and Persily, A.K., in Ann ACGIH: Evaluating Office Environmental Problems, 10, 77, ACGIH, Cincinnati, 1984 With permission.) Figure 11. 9 Simplified . involve pressure-driven flows Figure 11. 1 Ventilation air flow through a single-family house under open condi- tions. (From USEPA, Introduction to Indoor Air Quality — A Self-paced Learning. both single- and multi-family struc- tures, are increasingly being provided year-round climate control. As a con- sequence, the practice of opening windows and doors for ventilation pur- poses is. (constant-air-volume [CAV] systems), or air flow may be varied to individual spaces (variable-air-volume [VAV] systems). Simplified CAV and VAV system designs are illustrated in Figures 11. 8 and 11. 9. In