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Review of residential ventilation technologies.

This paper reviews current and potential ventilation technologies for residential buildings with particular emphasis on North American climates and construction. The major technologies reviewed include a variety of mechanical systems, natural ventilation, and passive ventilation. Key parameters that are related to each system include operating costs, installation costs, ventilation rates, and heat recovery potential. The paper also examines related issues, such as infiltration, duct systems, filtration options, noise, and construction issues. This report describes a wide variety of systems currently on the market that can be used to meet ASHRAE Standard 62.2-2004, Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings. While these systems generally fall into the categories of supply, exhaust, or balanced, the specifics of each system are driven by concerns that extend beyond those in the standard and are discussed. Some of these systems go beyond the current standard by providing additional features (such as air distribution or pressurization control). The market will decide the immediate value of such features, but ASHRAE may wish to consider related modifications to the standard in the future.


The purpose of ventilation is to provide fresh (or at least outdoor) air for comfort and to ensure healthy indoor air quality by diluting contaminants. Historically, people have ventilated buildings to provide source control for both combustion products and objectionable odors (Sherman 2004a). Currently, a wide range of ventilation technologies is available to provide ventilation in dwellings, including both mechanical systems and sustainable technologies. Most of the existing housing stock in the US uses infiltration combined with window opening to provide ventilation, sometimes resulting in overventilation with subsequent energy loss, sometimes resulting in underventilation and poor indoor air quality. Based on the work of Sherman and Dickerhoff (1998), Sherman and Matson (2002) have shown that recent residential construction has created tighter, energy-saving building envelopes that create a potential for underventilation. Infiltration rates in these new homes average three to four times less than rates in the existing housing stock. As a result, new homes often need ventilation systems provided to meet current ventilation standards. (McWilliams and Sherman [2005] and McKone and Sherman [2003] have reviewed such standards and related factors.)

According to ANSI/ASHRAE Standard 62.2-2004, Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings, published by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE 2004), single, detached residential buildings are required to meet a whole-house ventilation rate based on the number of bedrooms in the house, the number of occupants, plus an infiltration credit (3 cfm per 100 [ft.sup.2] plus 7.5 cfm per additional occupant, which includes a 2 cfm per 100 [ft.sup.2] allowance for infiltration). There are a variety of ways to meet this standard, either through mechanical systems or via natural forces. But for some occupants and homeowners, there is more to ventilation than just meeting the standard. They may desire added features for the comfort and health of the indoor environment or to reduce energy costs. According to Home Energy Magazine (Rudd and Lstiburek 2001), a good ventilation system should

* provide a controlled amount of unpolluted outdoor air for both comfort and dilution,

* have at least a 15-year life,

* be acceptable to operate by occupants (low noise, low cost), and

* not detract from the safety and durability of the house.

This paper will review both mechanical and sustainable ventilation technologies and the factors that affect their effectiveness. Mechanical technologies include

* continuous exhaust systems,

* intermittent exhaust systems,

* exhaust system with makeup air inlets,

* local exhaust with outside air integrated in HVAC system,

* continuous supply system,

* intermittent supply with inlet in return side of HVAC system,

* combined exhaust and supply (balanced) system, and

* houses without central forced-air distribution systems.

Sustainable technologies, which are those whose motive forces are principally temperature difference and wind, are reviewed later in this paper and include

* infiltration with operable windows,

* passive stack ventilation,

* solar chimney, and

* hybrid systems.

The effects of incidental ventilation provided by infiltration and operable windows are discussed. Finally, a variety of factors that can affect ventilation effectiveness are discussed, including cost and energy use, air cleaning and filtration, construction quality, control systems, and duct systems.


There are a variety of mechanical whole-house ventilation systems, including exhaust, supply, and balanced systems. Any of these may be in continuous operation or operate intermittently, they may be single-port or multi-port, or the system may be integrated into an existing HVAC system. Mechanical ventilation strategies provide more uniform ventilation rates than natural ventilation (Hekmat et al. 1986). Properly designed mechanical systems provide good control over ventilation rates when compared to most other ventilation systems; however, additional energy is required to operate the system. Holton et al. (1997) compared ventilation systems in newly built homes and found infiltration rates ranging from 0.1 to 0.07 ach in the summer and 0.35 to 0.15 ach in the winter. As a result, they recommend that modern houses include a mechanical ventilation system. Researchers have studied various configurations of exhaust, supply, and balanced ventilation systems, with and without whole-house recirculation by the central heating and cooling air-handler fan, and these are reviewed in the following.

Continuous Exhaust System

A continuous whole-house exhaust system provides ventilation by using a single-point or multi-point central fan to remove air from the building (Concannon 2002). Supply air enters the building envelope through gaps or provided vents (see Figure 1). If the building envelope is tight, there is a possibility that negative pressure can be created inside the building, leading to backdrafts from combustion (open flue) appliances. Often these systems incorporate a pressure relief damper to alleviate pressure imbalances. Supply air enters the building in an uncontrolled manner and may be pulled in from relatively undesirable areas, such as garages, musty basements (or crawlspaces), or dusty attics (Barley 2002). Whole-house exhaust systems may not be appropriate in areas where levels of outside environmental contaminants are high. In the case of radon, researchers have found that exhaust systems may actually increase the indoor levels of contaminants (Bonnefous et al. 1994). In severe climates, very cold supply air may create drafts, while in moist, humid climate zones, exhaust-only systems can cause moisture damage to the building structure. Filtration cannot be sensibly added to an exhaust-only ventilation system unless one considers the building envelope as part of the filtration system.

Heat recovery can be added to exhaust systems. Passively, the building envelope itself can provide some heat recovery (Walker and Sherman 2003b) and is also partially effective at removing ozone. More actively, an exhaust air heat pump can be used to recover the energy in the exhaust airstream.

The Home Ventilating Institute (HVI 2005) lists a large variety of fans that can meet current ASHRAE standards for ventilation rates if properly installed. However, several factors (such as the tightness of the building envelope, size, quality of ductwork, and placement of ducting, among others) can have a significant effect on whether the installed fan can provide the indicated ventilation rate. These fans can potentially provide ventilation rates from 50 cfm to more than 5000 cfm. Most of the operating costs result from conditioning the supply air rather than from the energy to operate the fan. The HVI directory lists the energy use for only a small percentage of the fans, with typical power consumption of about 3.5 cfm/W. Wray et al. (2000) found that from most perspectives, exhaust-only mechanical ventilation systems are the most inexpensive mechanical systems to operate.


Single-Point Exhaust System. A single-point exhaust system is often an upgraded bathroom fan (e.g., Figure 2). Construction and installation costs are the lowest of the mechanical systems (Concannon 2002). Only one fan and possibly some simple ducting are required to exhaust the air to the outside. In some cases, the fan can be installed in an exterior wall, eliminating the need for extensive ductwork. Single-point ventilation systems suffer from a nonuniform distribution of fresh air, especially to closed rooms (Rudd and Lstiburek 2000). In an evaluation of five mechanical ventilation systems, Reardon and Shaw (1997) found that local exhaust-only strategies (which depended on kitchen and bathroom fans to provide whole-house ventilation) provide only marginally better performance than infiltration alone. This simple system suffers from a poor distribution of supply air. Standard 62.2-2004, however, has no distribution requirement; so this is not an issue for a minimally compliant system, but it is nevertheless a consideration.

Multi-Point Exhaust System. Multi-point exhaust systems are an improvement over single-port exhaust systems in that they improve the room-to-room uniformity of the whole-house ventilation, but there is extra cost required for the installation of the ductwork (Rudd 1999). One exhaust fan is ducted to many rooms of the house and may be remotely installed to reduce noise levels. In a comparison of ventilation systems, Reardon and Shaw (1997) found that if a centralized multi-point system was installed, air was distributed evenly throughout the house, even to closed bedrooms.

Intermittent Exhaust System

An intermittent exhaust system is similar to a continuous exhaust system; generally it consists of one central fan to remove stale air from the building, but it may also incorporate several fans in areas of high sources (i.e., bathrooms and kitchens). In this case, the fan(s) runs only part of the time at a higher rate and is sized to provide the necessary ventilation. The rate of ventilation when the system is operated intermittently must be larger than if it were operating continuously (Sherman 2004b). There are several advantages for using an intermittent ventilation system. The occupant can reduce the amount of outdoor air entering the building during periods of the day when the outdoor air quality is poor. Peak load concerns may make it advantageous to reduce ventilation for certain periods of the day. When the ventilation system is integrated with the heating and cooling system, cyclic operation may also make more sense.


The occupant can control the fan when needed. The disadvantage here is that the occupant controls the ventilation and must be relied on to know when ventilation is needed. The occupant may choose not to operate the system (for example, if the fan is noisy), which could result in underventilation. (If the system is Standard 62.2-2004-compliant, ventilation fans should meet sound requirements and noise should not be a substantial issue.) Many systems use a timer to automatically run the fan for a certain amount of time each day so that the occupant is not relied on to sense when ventilation is needed. However, the occupant often has control over a switch to turn the fan on high when extra ventilation is needed. More sophisticated (and costly) control systems are available, including C[O.sub.2] sensors, occupant sensors, and humidity sensors. C[O.sub.2] sensors and occupant-controlled systems do not meet the current Standard 62.2-2004 requirements unless those features are used to raise the ventilation over and above the minimum rates required by the standard.

Our own experience has shown that installation and operating costs are similar to the continuous exhaust systems but may exceed them if sophisticated control systems are installed. As with continuous exhaust systems, most of the energy requirements are for conditioning the supply air rather than fan operation. The potential exists to reduce energy consumption when compared to the continuous exhaust system if the intermittent system is used in conjunction with natural driving forces to provide adequate ventilation while reducing the energy required to condition outside air. For example, running the fan at night could reduce cooling costs. Also, the fan could be programmed to run during times when outside pollutant levels are low or, alternatively, to shut down the system when outside particulate or ozone levels are high. If time-of-use utility rates are locally in use, it may be possible to reduce operating costs by ventilating more during low-cost periods to allow reduced or even zero ventilation during high-cost periods.

Exhaust System with Makeup Air Inlets

Another mechanical ventilation system uses an exhaust fan but controls the entry of supply air into the dwelling by providing openings specifically for air supply (see Figure 3). Trickle vents, air inlets, or louvers can be located in rooms that need extra ventilation, such as the bathroom. Again, filtration of the supply air is not possible with this system; however, the entry point of the supply air can be controlled to provide cleaner air by installing trickle vents away from polluted areas such as garages, musty basements, or dusty attics. Trickle vents are not necessary to meet Standard 62.2-2004 per se but may be needed in exceptionally tight construction to reduce depressurization and related issues. They are commonly used as part of European systems both because of the tight construction and to ensure that habitable rooms have individual air supplies.

Local Exhaust with Outside Air Integrated in HVAC System

This method builds on the exhaust systems described above but adds an outside air inlet in the return duct system of the air-handling unit of the HVAC system. Depending on the path(s) of least airflow resistance, this may use the existing duct system to extract air from individual areas of the building. Because existing ductwork would be used, marginal installation costs can be kept very low. This system can provide uniform ventilation throughout the house and may be operated intermittently or continuously. There are added operating expenses for running the central fan when heating or cooling is not needed, which depends on climate and system sizing.

Often exhaust systems are designed to comply with Standard 62.2-2004, unlike the centrally integrated makeup air system that can provide not only ventilation but also air distribution and can counteract depressurization--both of these later attributes go beyond the minimum requirements of Standard 62.2-2004 and are often desirable. In principle, the makeup air system could be designed to meet Standard 62.2-2004, and the exhaust system could be used as a source control enhancement.

Continuous Supply System

Continuous supply systems allow the occupant to control the location of the supply air to maximize air quality and give the occupant the option of filtering and/or conditioning the supply air (Building Science Corporation). Air is supplied by a central fan ducted to some or all of the rooms of the dwelling, forcing stale air out through leaks in the building envelope. This system creates a positive pressure inside the building, which has both advantages and disadvantages. The amount of the pressure depends on the supply flow and the tightness of the envelope. A positive pressure prevents outside contaminants from entering the building, but it also can force moisture-laden air through the building fabric. In cold climates, the moist air may condense in the walls of the building, creating an environment for mold growth. Various studies have considered the use of whole-house fans to provide night ventilation for cooling purposes (Santamouris 2006). In these systems, air-conditioning loads may be reduced up to 56% depending on the thermal preferences of the occupants.

Because outdoor air is often not in the thermal comfort zone, the temperature of the supply air is a design concern. Supply systems need to address this concern by conditioning or tempering the air in some way during the periods when it would be perceived as unacceptable. One method, for example, is to mix the supply air with indoor air before it reaches the occupants. Standard 62.2-2004 has no requirements for tempering. As with continuous exhaust systems, there are two main designs: single-point and multi-point systems.


Single-Point Supply System. In this strategy a supply fan provides fresh air via a small amount of ducting to a main room of the house. The air is distributed about the house by natural process. Often there is a return duct in a separate room. This system has low equipment costs; only the fan and a small amount of ducting are needed. However, the system suffers from a poor distribution of supply air, especially to closed rooms in the house (Rudd and Lstiburek 2000) even compared to single-point exhaust. Tempering or conditioning of this air is almost always needed if one wishes to avoid comfort complaints.

Multi-Point Supply System. The multi-point system has the advantage of improving ventilation uniformity throughout the house but with the extra installation cost of the ductwork. Because each supply is of a lower flow, the needs for tempering or conditioning may be reduced. From the perspective of Standard 62.2-2004, however, there are no differences between single- and multi-point supply systems.

Intermittent Supply with Inlet in Return Side of HVAC System

Integrating the supply air into the existing HVAC system provides a low-cost option to supply and distribute fresh air through the existing duct system and is the ventilation system most acceptable to large production home builders (Rudd and Lstiburek 2001). In this system, the existing central forced-air system is used to supply fresh air in a distributed manner through the building's ducting. An outside air inlet is placed in the return of the HVAC system to allow fresh air to enter when the air handler fan operates (see Figure 4).

All mechanical ventilation systems benefited from intermittent operation of the central fan. This resulted in more uniformity of ventilation air among the various rooms of the house (Rudd and Lstiburek 2000). By operating the ventilation system intermittently as opposed to continuously, Rudd (1999) estimated a 28% annual savings in total energy use. Computer modeling studies showed the cost-effectiveness of this system when compared to a separate supply ventilation system as well as the marginal costs of operation compared to no mechanical ventilation ($3 to $27 per year) (Rudd and Lstiburek 1998). Plus, they estimated it would take ten years to recover the initial costs of a separately ducted supply ventilation system. According to the computer modeling, the continuous and intermittent simulated systems had average outside air exchange rates of between 40 and 50 cfm, including the combined effects of ventilation and infiltration. These rates met Standard 62-1989 but would not meet the current Standard 62.2-2004.


A supply system can create positive pressure in the house, so a pressure relief vent is often installed. We often see pressure relief achieved through backdraft dampers of bathroom and kitchen exhaust fan ducting, as well as incidental leakage sites around windows, doors, or other building penetrations.

It is possible to add filtration to the supply air to remove contaminants. Installation costs are minimal for the return inlet itself; only a small amount of extra ducting and possibly a damper are required. Depending on the design, extra costs may be incurred for control devices and/or dampers.

Heat recovery potential for intermittent supply is low since heat exchange only occurs as the exhaust air exits via exfiltration through the building fabric. Currently air-handler fans are available to meet airflow rate standards in an energy-efficient manner. Simple control systems are available to operate the system when the HVAC system is heating or cooling or to operate the system on a timer so that fresh air is supplied when heating or cooling are not required (Walker and Sherman 2003a, 2003b). Energy efficiency is maximized when the entire air distribution system is airtight and located in a conditioned space (Rudd and Lstiburek 1998).

Combined Exhaust and Supply (Balanced) Systems

A balanced ventilation system uses two fans with separate ducting systems, one to supply fresh air and one to remove stale air from the building (see Figure 5). The system should not affect the pressure balance of the interior space unless the return path between the supply and exhaust is blocked. This ventilation strategy can be used effectively in any climate. It is possible to include a heat exchanger (or heat pump) to recover heat from the exhaust air and use it to precondition the supply air. Extensive ducting is used to supply fresh air to living and sleeping rooms, while a separate exhaust system removes stale, often moist, air from the kitchen and bathrooms. Advantages include prefiltration of the supply air and energy savings from the heat recovery of the exhaust air. Some disadvantages include installation costs, maintenance costs (because there are multiple fans), and possibly fan noise (for fans not meeting Standard 62.2-2004 noise requirements). Noise generated from the fan(s) and ducting system can be transmitted to each room of the house and reach 30 to 40 dB. Veld and Passlack-Zwaans (1998) describe various strategies for soundproofing, including insulating ducts and preventing fan vibrations. Reducing noise from ventilation systems has a positive impact on indoor air quality by reducing the likelihood that occupants will block vents or turn off the system.


Supply Integrated into HVAC System with Continuous Exhaust. If the house has an existing central forced-air system, it is possible to save on installation costs by integrating the supply inlet into the return of the HVAC system. A separate exhaust fan would run continuously to remove the stale air. This system can sometimes be problematic in humid climates, where moist air is injected into cool supply air ducts, resulting in condensation, and independent humidity control may be required.

Supply Integrated into HVAC System with Intermittent Exhaust. In this strategy (similar to the above) the exhaust fan would operate intermittently. Advanced control strategies can in principle be used to operate the exhaust fan only as needed to supplement the supply air to the return.

Houses without Central Forced-Air Distribution Systems

Most new homes in the US are built with central forced-air systems, but not all. Houses with radiant, hydronic, and/or baseboard systems may not have any central air distribution system and cannot use any of the HVAC-integrated systems discussed above. Any of the other systems, however, can be used to meet Standard 62.2-2004.

If air distribution is a concern, however, some systems may perform better than others for houses without central forced-air systems. If the building envelope is tight, an exhaust system with trickle vents or air inlets can increase the likelihood that each room will get some outdoor air. Supply or balanced approaches require a dedicated distribution system (i.e., multi-point supply) in order to achieve good air distribution.


All of the systems described above focus on a mechanical ventilation solution. Standard 62.2-2004 does not mention any other way to provide ventilation to new construction, but it does allow (in Section 4.1.2) alternative approaches if approved by a licensed design professional. There is a variety of potential ventilation options that do not require fans. Here we examine such sustainable technologies with the understanding that they do not meet Standard 62.2-2004, but they do allow advanced solutions in the future.

Tradition: Infiltration with Operable Windows

Many existing homes rely on infiltration through a porous building envelope for background ventilation with operable windows to provide increased ventilation when needed. Natural climatic forces create differences in air pressure between the outside and inside of the building, which can ventilate a building. Pressure differences depend on changes in temperature and wind speed. Wind causes a positive pressure on the windward side of the building and a negative pressure on the leeward side of the building (see Figure 6). The resulting amount of ventilation is dependent on the placement and number of openings in the building envelope and on wind direction and speed. This makes the ventilation rate unpredictable and uncontrollable since the driving mechanism is variable over the year and the flow paths are diffused over the building envelope (Allard and Ghiaus 2006). The average ventilation rate may be predictable, but the average ventilation rate itself is not the key factor.


Sherman and Matson (1997) have shown that typical existing homes have an annual average air change rate of over one air change an hour due to infiltration; this high rate can satisfy existing ventilation standards so that many existing homes do not need an extra ventilation system. Dwellings in cold, harsher climates and new residential construction are three to four times tighter, creating a tight building envelope and the potential for underventilation (Sherman and Matson 2002).

This basic system of operable windows has no extra construction costs or explicit operating costs. The energy implications are almost exclusively from the need to condition the outdoor air. The system relies on the occupants to open and close windows to provide adequate ventilation, particularly when the building envelope is tight; however, there is poor control over ventilation rates when the envelope is leaky. The lack of control can result in energy loss due to high air change rates, especially in winter when temperature differences and wind speeds are high. Alternatively, the system may underventilate during the hot summer months. When climatic conditions are favorable, natural ventilation can be used for cooling and can replace air-conditioning systems for part of the year.

But in urban settings there are considerable limitations to such an open ventilation system including noise, security, and pollution (Santamouris 2006). Additional limitations arise from the unique climatic conditions of cities. Both higher temperatures (the heat island effect) and decreased wind speeds in urban canyons can decrease the potential of natural ventilation systems. Geros et al. (2001) studied the reduction of airflow in naturally ventilated buildings in ten urban canyons in Athens, Greece, and found that because of the reduced wind speed, the airflow through the buildings decreased up to 90%. A few strategies exist for reducing noise in buildings using operable windows, and they are capable of reducing traffic noise by 7.5 to 8.5 dB without compromising the airflow path resistance (Oldham et al. 2004).

Since climate plays an important factor in the effectiveness of natural ventilation, many groups have analyzed the suitability of various climatic conditions. The potential of natural ventilation depends not only on the outdoor climate but also the building site and the design of the building site. Yang et al. (2005) have created a model to evaluate the potential of a particular site to provide the natural forces necessary to meet ventilation standards with only natural ventilation. It is clear that many climates are too harsh for infiltration to be used as a primary source of ventilation. Conversely, there are climates where the driving forces are too weak for infiltration to be a practical source of primary ventilation. All of which leads Wilson and Walker (1992) to conclude, "There is no hole for all seasons."

Infiltration does provide ventilation automatically without using any transport energy, but it almost always requires more space-conditioning energy to supply the equivalent ventilation as a constant mechanical system. Infiltration can provide some heat recovery and filtration through the building envelope, but unless it is well designed (e.g., the "dynamic insulation" used in Scandinavia) it is not likely to provide very much. Infiltration depends on the weather, so there is no "right" amount of air leakage. Infiltration will always provide more ventilation than is needed during extreme periods in order to meet average demands. For more information on operable windows and infiltration, see the "Incidental Ventilation" section below.

Passive Stack Ventilation

Passive stack ventilation is designed to provide more control over natural ventilation rates by incorporating one or more stacks or towers into the building structure to extract stale air while fresh air enters through provided openings such as trickle vents or louvers. Passive stack airflows are created from a combination of two climatic forces: differences between the inside and outside temperature and wind speed. The negative pressure at the stack top is often the critical factor. As shown in Figure 7, wind speed creates negative pressure on the leeward side of the building in many situations. The combination of cooler, entering air with warmer, less dense indoor air and negative wind pressure at the top of the stack result, in air being exhausted from the stacks.

Although rare in the United States, passive ventilation systems are widely used in the European Union. Axley (2001) found that in England and the Netherlands most single-family dwellings use passive ventilation (90% and 65%, respectively). Emmerich and Dols (2003) have used some of Axley's approach to create a passive ventilation design and analysis tool for use in a multizone environment.

Stack height and position are important in maintaining a negative pressure at the stack terminus and preventing backflows into the building. A taller stack is less sensitive to wind speed and wind direction. Installation guidelines and building codes reflect the importance of stack position relative to the roof. A variety of terminal caps are available that are designed and located to provide consistently negative pressures (independent of wind direction) at the stack exit (Axley 2001). Stacks need to have a larger diameter than mechanical ducting systems to reduce flow resistance for low pressure drop conditions. At present there is insufficient information to recommend specific minimum or maximum values for performance parameters, but there are references worth considering, including those in the AIVC database and in Stephen et al. (1994). A draft European Standard for testing cowls and roof outlets is in preparation (prEN 131415).


Ventilation flow rates can vary significantly from room to room. Upper, leeward rooms in particular may be underventilated and can easily have no outdoor air. Careful design measures can be taken to control and distribute flow rates. Typically, systems are designed with trickle vents or louvers that can be manually adjusted to control the flow rate, but these work best when uncontrolled infiltration rates are low (and building envelopes are tight). Each room must have a transfer grille or vent to allow free distribution of the air. While these same criteria are relevant for mechanical ventilation, the issue is often more critical for passive ventilation because of the lower driving forces. Many anecdotal cases indicate that passive ventilation systems have shown the capability of providing adequate long-term ventilation but fall short when required to provide short-term high ventilation during peak episodes of contaminant production (i.e., bathing or cooking).

Because they are designed similarly to mechanical systems but without mechanical components, passive stack ventilation systems can reduce construction and operating costs of residential buildings. Careful design of internal spaces should be considered during construction to allow air to flow between the rooms of the building and from the supply openings through the exhaust spaces. Relatively larger ducts than those used in mechanical systems are required since flow resistance is an issue. Operating (air transport) costs are nonexistent; however, there are usually some days of the year when weather conditions (low wind speed and/or small indoor/outdoor temperature differences) create insufficient airflow.

There is inherently some uncertainty in any system performance that is dependent on natural driving forces. Underventilation or overventilation can be expected at certain times of the year (Yoshino et al. 2003). Wilson and Walker (1992) showed that even with several large passive ventilation openings, single-family residences could not be adequately ventilated (relative to Standard 62-1989) during periods of light winds (less than 10 km/h) or small temperature differences ([DELTA]10[degrees]C). These conditions are common in the spring and fall. At these times proper ventilation may only be attained if the occupant opens a window or otherwise supplements the system. The usual natural forces are highest on cold days, creating overventilation, cold drafts, and energy loss. Self-regulating vents are available that can reduce or control overventilation. Pressure-sensitive ventilators are available that can provide constant ventilation rates over a wide range of pressures, but these passive control units are relatively scarce (Axley 2001).


Passive systems fall short when compared to mechanical systems in the areas of filtration and heat recovery. Filtration of the supply air is not feasible, and heat recovery is also relatively uncommon. Shao et al. (1998) have shown that heat pipes can be used with 50% heat recovery efficiency. Another strategy for heat recovery is to install inlet vents into the subfloor (see Figure 8). This strategy will temper cold supply air and help avoid cold drafts and will also reduce the sensitivity of the ventilation rate to wind direction (Hayashi and Yamada 1996).

Solar Chimney

A solar chimney is a passive stack system fitted with a solar collection panel (or often glazed walls on the south side of the building) that is used to heat the air in the stack, resulting in an increased buoyancy of the air in the stack. By increasing the temperature differential between the inside and outside of the stack, ventilation rates are substantially improved on warm, sunny days with low wind speeds (Bansal et al. 1994). This can improve the year-round effectiveness of the passive stack ventilation system. Airflow rates can be increased 20% over passive stacks without solar chimneys (Jaros and Charvat 2004). Khedari et al. (2003) reported that a solar chimney could reduce the load on the air-conditioning system (by using ventilative cooling), resulting in an average electrical savings of 10% to 20%. The advantages of this system are the increased reliability of the passive stack system plus the system being silent and transparent to the occupant. The disadvantages are the extra design, installation, and cost of the solar glazed panels. This system would be most appropriate for a sunny, warm climate.

Hybrid Systems

Hybrid systems are passive systems with a low-power fan to boost the flow of air through the stacks or vents, thus combining the advantages of a passive system with the reliability of a mechanical system. The combination of the two systems improves the indoor air quality while reducing energy demand through an intelligent controller (Heiselberg 2006; Li and Heiselberg 2003). There are a number of ways the two systems may be combined. The building may have two independent systems linked by a controller to switch from one to the other (a mechanical exhaust fan for the summer and winter and natural ventilation for the moderate seasons, for example). Another combination is fan-assisted natural ventilation, where the main ventilation is provided by natural forces but a low-power fan can be switched on to assist ventilation during periods of weak natural forces. A third, similar strategy is to include a small fan in a passive stack system to assist in creating optimal pressure differences in the stack.

Yoshino et al. (2003) have shown that a hybrid system can provide adequate ventilation rates even when weather conditions create poor ventilation in the passive system. By using a fan to boost stack ventilation during times of low wind speed, underventilation was prevented. And by using damper control at the vents, overventilation was prevented when temperature differences were large. Often, these systems incorporate the use of sophisticated control systems such as CO2 sensors, room temperature, airflow sensors, motorized windows, and even a weather station (Dorer et al. 2004a). Filtration of supply air is not common. The main disadvantage of hybrid systems is the complex control system. This adds an extra cost to the installation in terms of expensive parts and trained personnel to install them. Most occupants feel comfortable with (or prefer) a simpler user interface.


Incidental (or adventitious) ventilation refers to features or effects that were not designed to provide whole-house ventilation but, in fact, may. When they are truly incidental, one does not "count" them in the ventilation design, but one may need to take account of them in order to determine the actual energy and indoor climate impacts of a specific design.

For example, an air-to-air heat exchanger can only recover the energy of the air that goes through it. If the building is leaky and a significant fraction of the actual ventilation air bypasses the exchange, energy performance will be severely compromised. By contrast, the performance of an exhaust air heat pump is less dependent on envelope airtightness, although not completely independent.


Air leakage through the building envelope can have a detrimental effect on ventilation effectiveness regardless of the ventilation system. Infiltration rates are not constant since they are dependent on the weather. Both mechanically and passively ventilated leaking homes will lose energy when infiltration rates are high during the heating season. Very little heat recovery occurs in the building envelope (Walker and Sherman 2003b), which generally results in a loss of energy used to condition the infiltrating air. Balanced ventilation systems will also suffer a reduction in performance when air bypasses the heat recovery unit. However, buildings that are too tight may also suffer from a reduction in indoor air quality. Mechanical exhaust systems can create a negative pressure inside the dwelling when infiltration is low. This can lead to back-drafts from combustion appliances, poor indoor air quality, and high fan power requirements.

There are several methods available to measure leakage of the building envelope (Sherman 1990; Sherman and Chan 2006; Ask 2003; Dorer et al. 2004b). Ideally a building would leak no more than the air required for healthy indoor air. The amount of infiltration will depend on the airtightness of the building, the difference in indoor and outdoor temperatures, and the wind pressure. A tight building envelope will provide very little ventilation from infiltration and will require a ventilation system. Infiltration rates need to be taken into consideration when designing an HVAC system.

Sherman (1995) created a map of infiltration zones required to meet Standard 62-1989 ventilation standards based on the climatic data of each zone. In mild climates (such as the coast of California) infiltration alone is not enough to provide adequate ventilation in newer well-insulated homes, while in harsher climates, infiltration rates may be so high as to cause overventilation, energy loss, and comfort issues due to drafts. This zone would require the tightest home construction.

Operable Windows

Most homes are required to have operable windows in each room of the house. Occupants are more likely to feel comfortable when they have control over the ventilation system, and windows provide a familiar system of ventilation. If used on a daily basis, windows can provide the ventilation necessary to meet current codes. Liddament (2001) reviewed several studies on occupant behavior and ventilation and found that windows were most likely to be opened under the following conditions: sunny days, higher occupant density, higher outdoor temperature, low wind speed, during cleaning or cooking activities, and when smoking. However, there are many circumstances when opening a window is not practical, such as noise, rain or high winds, outdoor pollutants, cold drafts, privacy, security and safety issues, and energy loss, or the window may be difficult to operate. These observations suggest that window opening or closing is not always in response to ventilation needs.

Local Exhaust Fans

Local exhaust fans are often used in rooms with high moisture to provide source control when needed, most commonly kitchens and bathrooms, but laundries, utility rooms, and lavatories may also have local exhaust fans. Local exhausts fans are not intended to dilute contaminants but rather to remove them while they are still concentrated. As such, they are a source control measure rather than ventilation in the normal sense.

While doing their source-removal job, they may also increase the overall ventilation of the building and, in that sense, are incidental ventilation. For example, a high-capacity kitchen exhaust of 400 cfm ensures that the overall ventilation rate will be (temporarily) at least 400 cfm, which is well above minimum Standard 62.2-2004 rates. Because the duty cycle of these local exhaust fans is determined by the occupants and presumably related to a source-generating activity, one cannot count on them toward meeting minimum ventilation requirements.

A notable exception to that last statement is the "double duty" bath fan. In this design, a continuously operating local exhaust fan simultaneously meets the need for local exhaust and also whole-house ventilation. Provided the fan meets the appropriate requirements (e.g., sizing, noise), Standard 62.2-2004 allows this approach.


Standard 62.2-2004--or any other ventilation standard or code--is a set of minimum requirements that, if followed, will provide a certain minimum level of indoor air quality. In deciding how to apply such requirements, however, a variety of real-world factors need to be considered, such as construction, installation and energy costs, control and distribution systems, and indoor air quality. Often these decisions are determined by the needs of the client (or builder) more so than the requirements of the standard (Rudd and Lstiburek 2001).

Construction and Installation Issues

A potential problem exists when ventilation technologies are not properly installed or designed (Dorer and Breer 1998). Any ventilation system will not reach its performance potential if components are poorly manufactured or installed improperly. In 2001 a group of recently constructed homes in Minnesota were examined for various performance measures. Sheltersource, Inc. (2002) found that the average measured bathroom fan exhaust capacity was only 71% to 75% of the total rated capacity. Several factors contributed to poor performance, including long duct lengths. Compression in flexible ducts can also increase pressure drops up to a factor of nine. This resulted in a loss of ventilation rate and a significant increase in power and energy consumption by the HVAC system (Abushakra et al. 2003). Building airtightness is another area where the quality of the construction and the design of the building are as important as the materials in determining the desirable airtightness of the building envelope (Sherman and Chan 2006).

Energy and Costs

Ventilation requires energy not only to move the air but also to condition the supply air. Also, costs are involved for purchasing, designing, and installing the equipment. Energy use for ventilation and infiltration is significant and can account for one-third to one-half of the total space-conditioning energy (Sherman and Matson 1993). Building energy uses account for approximately 40% of total primary energy use in developed countries. Of this, the residential sector uses 60% to 70% for space conditioning (Orme 1998). Practical measures can be taken to conserve energy while still providing healthy ventilation rates. These include avoiding unnecessary air changes (due to leaky buildings), using good control strategies (not opening windows during periods of heating and cooling), and optimizing fan and equipment efficiencies. Orme (2001) has indicated that energy losses from air change are as important as conduction and equipment losses.

Mechanical ventilation systems reduce energy costs related to conditioning supply air if the building envelope is tight and infiltration is limited. Sherman and Matson (1993) estimated that 2.1 EJ per year could be saved by tightening the existing US housing stock. Most of the US housing stock uses infiltration as the ventilation system. The average ventilation rate has been estimated at more than 1 ach, with an estimated energy load of 4 EJ annually. If the existing housing stock were tightened and a continuous mechanical ventilation system were installed to provide an national average air change rate of 0.52 ach, the researchers estimated the energy load to be 1.8 EJ with a cost savings of $2.4 billion (Sherman and Matson 1997). Energy consumption can be reduced 9% to 21% by installing a mechanical ventilation system with heat recovery (Hekmat et al. 1986).

Besides energy costs, ventilation systems involve the extra cost of purchasing and installing the equipment. Table 1 compares the costs for various supply and exhaust mechanical systems. In Table 1, all the ventilation systems are run continuously, but a cost estimate is made for running the central fan for mixing purposes (this may be an option for some houses). The results show that a single-point exhaust system is the least expensive to purchase and install, with an estimated total cost of $72. This is supported by Wray et. al. (2000), who also found a mechanical exhaust system to be the least expensive to operate. While a four-point energy recovery ventilation system would be the most expensive to purchase and install ($1772), the benefits of improved air distribution, filtration opportunities, and energy savings may outweigh the initial costs. As expected, retrofitting an existing house is more expensive than new construction, and multi-point distribution systems were more expensive than a single-point system. If the house has an existing central fan system, then it need not be cost-prohibitive to integrate a supply ventilation system with a single-point exhaust.

Even though cost estimates are available, they are not necessarily sufficient to enable optimal selection of the ventilation system. Individual users may place high values on criteria that were not considered or heavily weighted. To evaluate such a multi-objective system requires exotic optimization approaches. For example, Roberson et al. (1998) developed a unique optimization for overall cost-effectiveness of a ventilation system (which included considerations for installation costs, operating costs, distribution effectiveness, and the potential for depressurization and for condensation). A multi-point supply system was found to be the best system overall. In cold climates, the group recommended a balanced system (multi-point supply with single-point exhaust) to prevent moisture problems in the building walls. In most of these cases, however, a simple continuous exhaust system would have proven to be more cost-effective if the only objective was to meet Standard 62.2-2004.

Climate can have a large impact on energy use. In hot, humid climates, dehumidification is necessary in houses with controlled ventilation systems. According to Rudd et al. (2003), mechanical ventilation with a separate dehumidification system provided the best overall value, including humidity control, installation costs, and operating costs. Some key factors contributing to the energy savings were locating the ducts inside the conditioned space, using insulation, and installing high-performance windows.


A variety of control systems from the simple to the complex are available to adjust the ventilation rate to achieve comfort and energy savings. Available systems include timers, occupant sensors, C[O.sub.2] sensors, and outside temperature or humidity sensors. The least reliable system is relying on the occupant to open and/or close windows. The occupant will respond to odors, drafts, noise, or a need for privacy rather than the need for a certain ventilation rate (Liddament 2001). The area of residential ventilation controls will continue to grow as users wish to take advantage of intermittent ventilation options, to have pollutant or weather-sensitive mechanical systems, etc.

Distribution Systems

A distribution system provides uniform ventilation and is an important component of all ventilation systems. In general, central exhaust systems and natural and passive ventilation systems do not distribute the fresh air as well as a multi-point supply system or a mechanical system that uses the ductwork of an existing HVAC system (Rudd and Lstiburek 2000). These systems allow the supply air to enter the building envelope in a rather uncontrolled manner and inevitably some rooms do not receive enough air, while others are overventilated.

The distribution system is an integral part of many mechanical ventilation systems and can have a significant effect on the ventilation rate and efficiency of a building. Leaky ducts are a source of energy loss, ventilation rate loss, and, in the case of return ducts, a source of indoor pollution (Delmotte 2003). In particular, the location of the ductwork is important. Modera (1993) has shown an energy loss of 30% to 40% when ductwork is installed in unconditioned spaces. He also showed through field testing and modeling that leakage through the average duct system was 37% higher than infiltration through the building envelope. Houses with leaky ductwork and air handlers located outside the conditioned space are at risk for increased infiltration rates, especially in hot, humid climates. This has large impacts on the actual ventilation rate found in the average house. The ventilation rate in many houses may not meet ASHRAE standards even though the equipment was designed to provide adequate ventilation, since leaky ductwork can prevent effective distribution of the supply air.

One strategy to save conditioning energy is to close the registers or grilles in rooms that are not being used. This strategy can increase the pressure in the entire duct system and increases the leakage rate in the ducts. A recent study found that the energy saved due to conditioning the air was only partially offset by increased duct system losses (Walker 2003).

Indoor Air Quality

Exposure to indoor pollutants can pose a serious health risk, especially for sensitive populations such as the young, asthmatic, or elderly (Sherman and Hodgson 2004; Seppanen and Fisk 2004). Indoor pollution originates from both indoor and outdoor sources and may be in the form of suspended particulates, volatile organic chemicals (VOCs), human bio-effluents, and microbiological contaminants (Seppanen 2006). Occupant activities such as cooking, bathing, smoking, vacuuming, using cleaning products, painting, as well as chemical emissions from building materials, electrical equipment, and appliances are all examples of indoor sources. Outdoor sources primarily result from vehicle exhaust but also agricultural activities, construction, manufacturing activities, ground sources (radon), and allergens (Levin 2004). The most effective method for controlling pollutants is by reducing or eliminating the source of the emission, but this is not always possible for some pollutants (Sherman and Matson 2003; Levin 2004). A number of strategies exist for improving indoor air quality, including increasing ventilation rates to dilute the pollutant, filtration to remove particulates, or air cleaning to capture VOCs, or a combination of all three strategies. Proper maintenance and operation of the ventilation system, appropriate building design to limit sources of pollution, avoiding excessive depressurization, providing local ventilation at sources that produce pollution (combustion appliances), and moisture control are all important strategies in controlling indoor air quality (Hadlich and Grimsrud 1999).

Dilution Ventilation. Appropriate whole-house ventilation can dilute the level of indoor pollutants with fresh outdoor air (assuming the outdoor air in not more contaminated than the indoor air). Diluting pollutants with more fresh air has historically been the function of ventilation; however, it is not a pollutant-specific strategy and not all pollutants can be treated the same way. Almost all of the ventilation technologies described previously can provide the necessary ventilation rates for effective dilution. For natural ventilation and/or passive systems there is some inherent lack of control of ventilation rates that may result in times when indoor pollution is high. Although these systems may provide an annual average acceptable ventilation rate, they cannot effectively deal with peak periods of pollution (Sherman and Wilson 1986). On the other hand, all mechanical systems offer high levels of ventilation rate control so that indoor pollutants can always be diluted. Plus, many mechanical systems also include local fans in areas where production of pollutants is high, such as bathrooms and kitchens, in order to minimize the spread of pollutants into other parts of the house. Also, mechanical systems can provide the higher ventilation rates that are required to dilute VOCs, such as formaldehyde, than those needed to control human bio-effluents, such as C[O.sub.2] (Sherman and Hodgson 2004; Grimsrud and Hadlich 1999).

Filtration. Sherman and Matson (2003) have shown that dilution ventilation is not always effective at reducing particle concentrations. Effective filtration can reduce the concentration of particulates that cannot be reduced at the source; this can also reduce the need for ventilation dilution. There are several methods that can be used to reduce particulate levels, including filtration, electrostatic precipitators, and simply by the deposition that occurs in the HVAC system.

Thatcher and Layton (1995) showed that the shell of the building offers little if any filtration of total particles and that indoor particle concentrations are significantly impacted by the activity level of the residents in the house. Even light activity, such as walking, can significantly increase the suspended particulate concentration for supermicron particles. Wallace et al. (2004) showed that the use of a central fan in a forced-air system alone could reduce the whole-house particle concentration (P[M.sub.2.5]) by 14%, and that installation of an in-duct mechanical filter could reduce the levels of particles by 23%. An electrostatic precipitator can reduce particles, especially fine particles, by 51%, but these are more expensive than mechanical filters and require routine maintenance to remain effective.

Filtration is most commonly used in mechanically ventilated buildings with supply systems and can be used to filter recirculated air or to filter the incoming supply air. Particle filters are rated by the ASHRAE Minimum Efficiency Reporting Value (MERV) scale (ASHRAE 1999). Typical furnace filters are rated at MERV 4 or lower and are not effective at removing respirable particles, but they can remove large pollens and visible dust particles. MERV filters rated 6 to 8 can remove smaller particles in the range of 10 [micro]m (P[M.sub.10]) and filters with a MERV rating of 9h[micro] to 13 can remove fine respirable 2.5 [micro]m (P[M.sub.2.5]) particles. Currently Standard 62.2-2004 recommends using a MERV 6 filter to protect the HVAC system from accumulating particles and becoming itself a source of indoor pollutants.

Since residential HVAC systems operate cyclically, filters used as part of the HVAC system perform better when the fraction run-time is high. Fugler and Bowser (2002) showed that high-efficiency furnace filters have a minimal effect on indoor particulate (P[M.sub.10]) levels when the occupants are active, but during low activity times (sleeping), P[M.sub.10] could be reduced 70%.

Filtration performance is selective; it often has poorer efficiencies for the finest of particle sizes and will fail unless care is taken in the installation and maintenance of the system (Liddament 2001). If the building envelope is tight and the filtration system is maintained, there is a potential to reduce both the indoor particulate levels and the ingress of outdoor particulates into the indoor environment. According to Sherman and Matson (2003), a MERV 11 filter installed in a supply ventilation system can reduce cat and dust mite allergens 30% to 40%. They recommend installing a MERV 9 to 12 filter and reducing duct leaks, preventing filter bypass, reducing uncontrolled infiltration, and running the fan continuously in order to maximize the filtration efficiency. This comes at a high energy cost.

Outdoor pollution presents a serious limitation for naturally (or passively) ventilated buildings, especially in urban areas. Researchers have shown that outdoor particles penetrate fully (almost 100%) into the indoor environment of houses with very leaky building envelopes and/or open windows that do not provide much opportunity for interaction between the airstream and the envelope (Thatcher and Layton 1995; Thatcher et al. 2001; Partti-Pellinen et al. 2000). In a comparison of mechanical ventilation systems with and without filtration, however, the Canadian Mortgage and Housing Corporation (CMHC 2003) found that unfiltered exhaust systems provided some protection from outdoor particles when compared to unfiltered supply or balanced ventilation systems. These provided no protection from the ingress of outdoor particles. Emmerich and Nabinger (2001) found penetration factors of 60% to 80% in test houses.

These studies suggest that the building envelope offers some protection from pollen, allergens, and diesel particles. Ventilation systems that move the supply air through the building envelope (such as natural infiltration, passive systems, and mechanical exhaust systems) can provide some minimal filtration from these types of outdoor particles. The CMHC found that the best protection from outdoor particles was provided by a ventilation system that positively pressurized the house and used a high-efficiency particle filter (HEPA), which can be expensive. In this case, a HEPA filter supply ventilation system was able to remove 99% of the outdoor particles.

Radon. However, in the case of radon, mechanical exhaust systems cannot always reduce the indoor radon concentration and may even increase it (Bonnefous et al. 1994). This result may apply to other soil gas contaminants as well. The researchers recommend a balanced ventilation system with heat recovery for low radon concentrations and an expensive subslab ventilation system to reduce radon flow into the building. Sherman (1992b) has shown that supply ventilation is generally superior for radon control but that other ventilation types can work quite well depending on the climate and construction type.


In this report we have reviewed the literature and used our expertise to evaluate technologies for meeting residential ventilation requirements. Our principle focus was on meeting Standard 62.2-2004 requirements, but in doing so we found that there are a lot of other issues that influence the actual decisions about what gets installed in houses.

There are a wide variety of systems currently on the market that can be used to meet Standard 62.2-2004. While these systems generally fall into the categories of supply, exhaust, or balanced ventilation systems, the specifics of each system are driven by concerns that extend beyond those in the standard. Some of these systems go beyond the current standard by providing additional features (such as air distribution or pressurization control). The market will decide the immediate value of such features, but ASHRAE may wish to consider relevant modifications to the standard in the future.

ASHRAE may also wish to consider expanding the standard to allow sustainable technologies--that is, passive or hybrid technologies that principally rely on natural driving forces rather than fans to transport the air. Such systems have been used for millennia and are currently used in Europe to satisfy ventilation requirements. Research and development are necessary to develop such systems for the US market since they have great potential for green buildings.


This report was jointly produced by the Building Science Corporation and Lawrence Berkeley National Laboratory. This project was funded through ARTI-21CR Work Statement: Project SI/IEQ-30090, "Evaluation of Whole-House Mechanical Ventilation System Options--Phase I Simulation Study." This work was also supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Building Technologies Program, of the US Department of Energy under contract No. DE-AC02-05CH11231.


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Marion Russell

Max Sherman, PhD


Armin Rudd


Received April 24, 2006; accepted August 23, 2006

Marion Russell is principal research associate and Mex Sherman is senior scientist and group leader, Indoor Environment Department, Lawrence Berkeley National Laboratory, Berkeley, CA. Armin Rudd is principal research engineer, Building Science Corporation, Westford, MA.
Table 1. Comparison of 2006 Equipment and Installation Costs for New and
Retrofit Mechanical Ventilation Systems Relative to an Installed
Standard Builder Exhaust Fan

Ventilation System Central Fan Equipment Installation Costs
Description Use (a) Costs ($US) Costs ($US) ($US)

Single-point exhaust, No 72 0 72
 new construction, Yes 128 21 149
 one upgraded bath
Single-point exhaust, No 103 206 309
 retrofit Yes 160 247 407
Multi-point exhaust, No 144 0 144
 new construction, Yes 201 21 222
 two upgraded bath
Multi-point exhaust, No 464 3 points, 412 876
 new construction, 4 points, 515 979
 remote fan
Multi-point exhaust, No 464 3 points, 824 1288
 retrofit, remote 4 points, 1494
 fan 1030
Single-point supply, No 361 361 722
 new construction, Yes 417 381 798
 remote fan
Multi-point supply, No 361 3 points, 566 927
 new construction, 4 points, 669 1030
 remote fan
Single-point HRV, (b) No 824 567 1391
 new construction Yes 824 587 1411
Multi-point HRV, new No 824 3 points, 773 1597
 construction 4 points, 793 1617
Single-point ERV, (c) No 824 567 1391
 new construction Yes 824 587 1411
Multi-point ERV, new No 979 3 points, 772 1751
 construction 4 points, 793 1772
Central-fan No 129 103 232
 integrated supply Yes 129 103 232
 with continuous Yes, with 185 124 309
 single-point damper
Central-fan- No 165 103 268
 integrated supply Yes 165 103 268
 with intermittent

a. Central fan is used to mix and to distribute the air.
b. Heat recovery ventilator.
c. Energy recovery ventilator.
COPYRIGHT 2007 American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2007 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Russell, Marion; Sherman, Max; Rudd, Armin
Publication:HVAC & R Research
Geographic Code:4EUUK
Date:Mar 1, 2007
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