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Description
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.
INTRODUCTION
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.
MECHANICAL WHOLE-HOUSE VENTILATION
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.
[FIGURE 1 OMITTED]
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.
[FIGURE 2 OMITTED]
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... |
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