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Good ventilation is the key to a comfortable and healthy home. You can cool your home and remove hot, stale air by opening and closing doors and windows. This natural ventilation is often sufficient to keep a home comfortable. At other times, the forces of air pressure and gravity are not enough to circulate air through a building, so some type of mechanical device is needed to provide adequate ventilation. Fans and ventilators are an effective way to enhance air circulation. This forced ventilation can supplement or even replace air conditioning. Through careful selection and proper sizing, fans and ventilators can increase comfort levels and reduce energy costs.
Natural Ventilation is created by natural forces. It results from differences in the distribution of air pressures around a building, since air always flows from areas of high pressure to areas of low pressure. This airflow is affected by gravity and wind pressure. Natural ventilation depends on the placement and control of doors and windows, and can be enhanced by ventilators, louvers, and other design features.
Louvers and roof ventilators help prevent heat and moisture buildup in the attic. Attic ventilation can greatly reduce home cooling costs, since temperatures in an unventilated attic can reach 145 degrees F on a hot summer day. The temperatures in a ventilated attic will not exceed 115 degrees F. A cooler attic results in cooler living areas. Louvers and roof ventilators also supply air intake and exhaust for forced ventilation. The amount of ventilation required depends on how well the attic is insulated.
Airflow caused by gravity is called the Thermal or Stack Effect. It results from the temperature differences between the air inside a building and the air outside a building, combined with the differences in elevation between the highest and lowest vent. The Stack Effect works because cold air is heavier than warm air. Cold air entering through a vent placed low on a building pushes hot air up, where it escapes through a higher vent. The greater the height difference between the vents, the better the effect.
Airflow is also caused by wind blowing over and around a building, creating regions of high and low pressure. When wind hits a building, it slows down and creates a positive pressure on the windward face of the building. A cushion of air builds up and diverts the wind to the building's sides, where the flow separates from the building at the corners. Faster winds along the sides create a negative pressure or suction. A large, slow moving eddy builds up on the leeward face, creating another, smaller suction. Cross ventilation occurs when openings are placed on windward and leeward sides. Several Factors affect the wind pressure: wind velocity, prevailing wind direction, seasonal and daily variations in wind direction and wind velocity, shape of roof, interference caused by nearby buildings, trees, hills, and other obstructions.
Louvers are typically placed in attics for moisture control and ventilation, and may be combined with ventilators to provide more airflow. Louvers are fixed openings that function either as an inlet or an outlet for airflow. They are mounted low in the attic to provide an inlet for the roof ventilator. There are three main types of louvers: gable end, rectangular, and soffit. Most gable end louvers are inexpensive and inconspicuous. Rectangular louvers are placed on the gable end of the attic. Soffit vents are mounted under the eaves and may be combined with a ridge vent (a vent placed along the ridge of the roof) to provide more effective airflow. They come in units of 8 by 16 inches, or in strips 2 to 4 inches wide by any length.
A common problem with louvers is preventing insects from entering through them. Insect screens work, but also reduce airflow through the vent. Screens with ¼-inch holes keep out insects, but many building codes require screens with 1/8-inch holes.
Roof ventilators are stationary or rotating vents located near the peak of the roof. They provide a weatherproof air outlet. Most roof ventilators use an ejector action; as outside air passes over the roof surface, air velocity increases, creating a region of low pressure which draws up air from the attic by induction. Even without wind, roof ventilators operate if the attic's temperature is higher than the outside temperature. Turbine ventilators use centrifugal force, however. Their spinning blades create a region of low pressure, which draws air out of the attic. The slightest breeze activates a turbine ventilator. Aluminum turbine ventilators spin more easily than turbine ventilators made of heavier materials.
The capacity of a roof ventilator to remove hot air depends on several factors: the placement of the ventilator on the roof. For maximum flow induction, roof ventilators should be placed on the roof where they receive the most wind with the least interference, the ventilator's resistance to airflow, the design of the ventilator.
Standards for attic ventilation are set for moisture control and good indoor air quality. The Federal Housing Administration's (FHA) minimum standard for attic ventilation is one square foot of net-free vent area for every 150 square feet of attic floor. The American Ventilation Association (AVA) recommends one square inch of net-free area for each square foot of attic floor. (This equals one square foot of net-free area per 144 square feet of attic floor.) The net-free area consists of the clear or free opening of the vent or louver through which air may pass. Insect screens and weatherproofing devices reduces the free area. Most vents and louvers are stamped with their measured net-free area.
Maximum ventilation is achieved in both high and low pressure areas by circulating a large quantity of outside air through the attic. A combination of roof ventilators and louvers achieves maximum ventilation. The three most common combinations are roof ventilators with soffit vents, gable end or rectangular louvers with soffit vents, and soffit vents with ridge vents.
Placement of vents is also important for achieving maximum ventilation. Ideally, inlet vents face directly into prevailing winds, and outlets, such as roof ventilators, are placed opposite inlets in low pressure areas. The amount of air forced through a vent is affected by wind velocity and wind direction, as well as by the size of the net-free area.
Another means of ventilating a home is by using windows and wingwalls. The amount of airflow through a building increases as the window area increases. The larger the open area, the greater the airflow. Certain types of windows, such as casement and awning units, open fully to provide maximum free area. Double-hung and slider windows only open 50 percent. Windows that are wider than they are tall are best for trapping winds.
Cross ventilation is created when open windows face the windward and leeward sides of a building. Open windows of equal size on the windward side and on the side wall instead of the leeward side creates even better ventilation because of stronger suction along the side walls. In areas where the direction of the wind varies, better ventilation occurs in rooms with windows on three adjacent walls instead of two opposite walls.
By opening and closing windows, different parts of a building can be ventilated. The size and location of the inlet and outlet areas determine the rate of airflow. The greatest volume of airflow occurs when the inlet and outlet areas are equal. The velocity of local airflow is greater when there is an imbalance between inlet and outlet areas. Larger outlets create a faster airflow near the smaller inlets, and larger inlets create a faster airflow near the smaller outlets. Therefore, the airflow in certain areas of a building can be changed by simply opening and closing windows. For example, opening all windows on the leeward side of a building, and closing all the windows on the windward side except in one room maximizes airflow in that room.
Wingwalls help ventilate rooms that have only one exterior wall and therefore have poor cross ventilation. In such a room, windows should be placed as far apart as possible. Wingwalls are built onto the building's exterior along the inner edges of all the windows, and should extend from the ground to the eaves. Wingwalls cause fluctuations in the natural wind direction, thereby creating moderate pressure differences across the window. This is most effective when the wind is perpendicular to the windows. Properly placed casement windows can help achieve the same effect. Unfortunately, wingwalls are only effective on the windward side of the building. They may also be architecturally unappealing.
When designing a building for natural ventilation, there are several guidelines to follow:
If possible, design the building to respond to winds from any direction.
Provide inlets and outlets for airflow in each room.
Locate inlets and outlets so air will flow through parts of the room most likely to be occupied, such as the sitting area in a den, and avoid creating stagnant areas.
Use windows and doors that open fully.
Ensure that vents and windows are accessible and easy to use.
Avoid blocking windows with exterior objects such as shrubs and fences, but do not eliminate shading. Tall trees allow air to enter while providing shade.
Concentrate vent openings in spaces most likely to require cooling.
Use overhangs, porches, and eaves to protect windows and vents from rain. This extends the amount of time that natural ventilation can be used.
Be sure vent openings can be tightly sealed in winter or when using an air conditioner.
Savings from natural ventilation depends on how frequently it is used in place of air conditioning or forced ventilation. For example, if natural ventilation was able to eliminate the need for air conditioning on a certain day, the savings would be the cost of running the air conditioner that day. However, calculated savings are not as straightforward on days when both natural and mechanical cooling are used.
In extremely humid weather, natural ventilation may bring excessive moisture into the building. As temperatures rise, an air conditioner has to run longer to extract this moisture, which reduces potential savings of natural ventilation. The longer a building can stay cool with natural ventilation, however, the more the energy costs decrease. By supplementing natural ventilation with forced ventilation, savings can be extended even more.
Sometimes natural ventilation cannot circulate enough air through a building. Forced ventilation creates airflow in rooms or building that would otherwise be stagnant. Electric fans are the most common devices of forced ventilation. They use electricity to create a comfortable environment at a wider range of conditions. Simply stated, the breeze from a fan blows across the occupants' skin, making them feel cooler at a higher temperature. Fans used with an air conditioner increase circulation, which speeds up the evaporative cooling process. This means that the thermostat can be set up to 4 degrees F higher while the comfort level remains the same, resulting in energy savings of 28% to 40%. The three main types of electric fans are whole house fans, ceiling fans, and portable fans.
Whole-house fans can be used as either the sole source of cooling, or in conjunction with an air conditioner. A whole-house fan exhausts hot, stale air through attic vents and creates a negative pressure in the house. Fresh air is drawn in through the windows, creating a breeze which makes temperatures feel cooler than they actually are. They function best when the indoor temperature is greater than the outdoor temperature. However, they still can be effective when outside temperatures are as high as 85 degrees F, as long as the humidity level is below 75%.
The fan is installed in a centrally located hallway ceiling that leads to the attic. It draws outside air in through open screened windows and doors, pulls it through the house, and exhausts it through attic vents.
During early evening hours, or when temperatures are lower than 85 degrees F, the windows should be opened and the fan turned on to draw in fresh air and provide a steady breeze that will keep occupants comfortable. Later in the evening, unoccupied rooms should be closed off, and bedroom windows should be opened to ventilate sleeping areas. On days when temperatures are less than 85 degrees F, the whole-house fan can cool your home more cheaply than an air conditioner.
During early morning hours, while it's still cool outside, the whole-house fan flushes out heat and draws in cool air. After brief operation, the fan should be turned off, the windows should be closed with curtains drawn, and all entrances to the house should be closed. In a properly insulated home, the cool air remains indoors well into the day, delaying or eliminating the need for air conditioning.
It can also be used to flush hot air out of a closed house prior to running an air conditioner. This removes excess heat, reducing the burden on the air conditioner. Running the air conditioner less saves energy and can extend the life of the air conditioner.
Whole-house fans have several drawbacks. They cannot control temperatures or humidity in a building. They may draw dirt, dust, and pollen in from outside. Screens reduce this infiltration but cannot prevent it. The noise the fan makes disturbs some people, but others like the "white" noise because it drowns outdoor sounds.
Whole-house fans should be sized to provide 60 air changes per hour (ACH), or one air change per minute. Fans are rated by the cubic feet of air that they can move per minute (CFM). To determine the volume of air that a fan will be required to move, multiply the floor area of the space to be ventilated by the ceiling height (leaving out storage areas and the garage). For example, a 1,500 square foot house with 8 foot ceilings has a volume of 12,000 cubic feet, and requires a fan with a rating of at least 12,000 CFM. A minimum of one square foot of net-free vent area must be provided in the attic for each 750 CFM of air delivered by the fan. This allows air pulled up by the fan to exhaust out the vents. For example, a fan that provides 12,000 CFM requires 16 square feet (12,000 divided by 750=16) of net-free vent area to exhaust air from the attic. Without sufficient exhaust area, the whole-house fan delivers less air and consumes more power than intended.
When selecting a whole-house fan, several factors ought to be considered:
Size - Select a fan that provides 60 ACH
Speeds - Variable speed settings allow better control of airflow.
Noise - Larger fans are generally quieter than smaller fans because they can operate at lower speeds. Check if the fan has been tested for noise output.
Installation - Check requirements and cost of installation. Some may require extensive carpentry.
Safety - Some fans offer fire safety features.
Warranty - Fans should always come with a warranty.
Whole-house fans are installed horizontally in the ceiling below the attic. Louvers are often installed below the fan to conceal it and to close off the opening when the fan is not in use. Inlets to the fan should open directly into the living space, and the outlets to the fan should have at least 3 feet of clearance in the attic to allow air to exhaust freely. Whole-house fans should not be used in homes with blown-in insulation unless the insulation is covered. The ceiling must be reinforced to compensate for any framing that is cut during installation. Some fans have mounting systems that eliminate the need for cutting into the framing. The switch to operate the fan should be located in the living area for easy access. An automatic timer can be set to turn the fan on during early evenings when temperatures are cooler, and to shut off automatically when temperatures rise. A firestat ( a fusible link in the fan power circuit) should be used to interrupt power to the fan if a fire starts. Finally, the rate of airflow through the inlets and outlets should be kept below 750 CFM. Higher flow rates create a strong breeze that may be uncomfortable for occupants.
Whole-house fans generally have diameters of 20 to 42 inches. Prices range from $180 and up for a 24-inch, 1/3 horsepower model, to $320 and up for a 36-inch, 3/4 horsepower model. Prices will vary by 25%, depending on the make and quality of the model. Installation costs range from around $100 to $300, depending on the size of the fan, but it may cost more if your home requires extensive carpentry. Prices also vary due to the amount per hour the installer charges, and whether an initial fee is charged for the visit.
Whole-house fans consume about 300 to 550 watts, while a central air conditioner consumes about 3,000 to 4,000 watts. Using a whole-house fan to supplement the air conditioner can reduce energy consumption for cooling by 30% to 45%, even considering the electricity used to run the fan. Using a whole-house fan instead of the air conditioner can result in savings of 30% to 60%. In the South, savings can reach $100 to $200 a year. In the North, a whole-house fan could feasibly replace the air conditioner entirely. However, if a whole-house fan replaces an air conditioner that works, the net savings will be less, taking into account the purchase price of the fan. Savings are directly affected by how consistently the fan is used, the placement of the inlets and outlets, and how well the building is insulated.
Ceiling fans are another means to circulate air. They supplement the air conditioning in a home, and can possibly replace it in cool climates. Some ceiling fans are reversible; blades turn clockwise in summer to create a downdraft, and counterclockwise in the winter to circulate the heated air collecting at the ceiling down towards the floor. Others come with light fixtures. Ceiling fans can also add a decorative touch to a home.
With ceiling fans, the thermostat setting can be raised from 75 degrees F to a maximum of 85 degrees F (depending on the occupants' preference) because the fan creates a breeze which makes people feel comfortable at higher temperatures. Separate fans should be placed in all frequently used rooms. They should be located over areas that are likely to be occupied, such as over the seating area in a den or over the bed in a bedroom. Ceiling fans work best when the blades are 7 to 9 feet above the floor and 10 to 12 inches below the ceiling. Placing fans so the blades are closer than 8 inches to the ceiling can decrease the efficiency by 40%. Fans also require at least 18 inches of clearance between the blade tips and walls. They should never be hung where excessive moisture could damage the wiring or warp wooden blades. If you purchase a reversible fan to circulate warm air in the winter, be sure the fan has a setting low enough to circulate the air without creating too much of a breeze. These fans are best for rooms that tend to build up heat, such as sunrooms or rooms with a woodstove.
A larger fan provides a greater range of airflow settings and ventilates a larger area at lower velocities, with less noise, and only slightly more power than similar smaller units. A 36- or 42-inch fan works best for rooms 12 feet by 12 feet or smaller. A 48- or 52-inch fan works best for rooms 12 feet by 18 feet or smaller. Two medium-sized fans work best in a room longer than 18 feet. Larger fans work best in central halls. Small- and medium-sized fans move air in a 4- to 6-foot radius and should be placed close to the center of the room. Large fans are effective up to 10 feet from the center of the room because they have a greater airflow radius.
There are several factors to consider when selecting a ceiling fan:
Warranty - Fans with longer warranties (over 2 years) may cost more but tend to be more durable and last longer.
Metal or plastic motor housing - Metal housing requires oiling once a year. Plastic requires no maintenance, but may not last as long as metal.
Multiple speeds - Fans with two or more speeds are more durable.
Blade material - Different materials perform equally well, so select the type that looks best in your home.
Blade size - The larger the blade span, the greater the airflow. A 52-inch fan creates a greater breeze than a 36-inch fan.
Noise - Listen to a fan before purchasing one. Loud fans can become a nuisance. A more expensive but stable, smooth operating fan is better than a cheaper, wobbly, noisy fan. Check for noise ratings.
Installation - Cost may be high if complex wiring and mounting is necessary.
Multiple fans - You may want to install fans in more than one room to maintain the comfort level throughout the house.
Ceiling fans cost between $75 and $200. The typical cost of a professionally installed fan is about $250, but many fans can be installed by the homeowner. Fans with features such as light fixtures, reverse or multiple speed settings, and extended warranties may cost more. Supplementing the air conditioner with ceiling fans can result in energy savings for cooling of 15% to 35% (when the thermostat is turned up). Replacing the air conditioner with ceiling fans saves up to 60%.
Portable fans are the least expensive means of forced ventilation. They can be used to circulate indoor air, or to exhaust indoor air and pull cooler outdoor air inside. The three main types of portable fans are window, box, and oscillating. Window fans are mounted in windows to create an airflow through the room, and draw in or exhaust air. During the day, window fans can exhaust hot air that accumulates indoors, and can be reversed at night to pull in cooler outdoor air. Box fans are generally square and made of metal or plastic. Oscillating fans circulate air by rotating side to side. They can be placed wherever they are needed. All of these fans are relatively inexpensive. Some models operate at different speeds.
Ventilation reduces cooling costs and creates a comfortable environment by efficiently circulating or exhausting indoor air and pulling in fresh outdoor air. Depending on weather conditions and a building's annual cooling load, forced and natural ventilation can supplement or on some days even replace air conditioning.
Natural ventilation relies on natural forces such as wind flow and gravity to circulate air within a building. Building designs can incorporate roof ventilators, vents, louvers, windows, and wingwalls to increase the effects of natural ventilation. By positioning and operating inlets and outlets correctly, wind flow can be controlled to ventilate an entire building or specific rooms.
Attic ventilation is important for reducing energy costs. Hot air collecting in an unventilated attic can overheat the whole house. Attic fans, ventilators, vents, and louvers allow fresh outdoor air to circulate through the attic, exhausting hot attic air. Adequate ventilation can reduce attic temperatures by over 30 degrees F, reducing the cooling load of the entire building.
Forced ventilation using mechanical devices circulate air in areas that cannot be ventilated sufficiently by natural means. Whole-house fans, ceiling fans, and portable fans are means of circulating air throughout an entire building, or in specific areas of a building. Forced ventilation can be used to supplement air conditioning, and in cooler weather can often be used instead of air conditioning.
Ventilation can effectively reduce a building's cooling load, and hence reduce energy costs. Remember, however, that ventilators must be installed and operated properly to be effective. Consider your options carefully before deciding how to ventilate your home.