natural ventilation
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when not to use natural ventilation
- Sites with high levels of acoustic noise.
- Sites with poor air quality.
quantifying ventilation effectiveness
To measure
the effectiveness of your ventilation strategies, you can measure both the
volume and speed of the airflow.
The volume of the airflow is important because it dictates the rate at which stale air can be replaced by fresh air, and determines how much heat the space gains or losses as a result. The volume of airflow due to wind is:
Q_wind = K • A • V
Q_wind = airflow volumetric rate (m³/h)
K = coefficient of effectiveness (unitless, see below)
A = opening area, of smaller opening (m²)
V = outdoor uninterrupted wind speed (m/h)
The coefficient of effectiveness is a number from 0 to 1, adjusting for the angle of the wind and other fluid dynamics factors, such as the relative size of inlet and outlet openings. Wind hitting an open window at a 45° angle of incidence would have a coefficient of effectiveness of roughly 0.4, while wind hitting an open window directly at a 90° angle would have a coefficient of roughly 0.8.
When placing ventilation openings, you need to place both air inlets and air outlets; often they do not have the same area. The opening area used in this equation is the smaller of the two.
The volume of the airflow is important because it dictates the rate at which stale air can be replaced by fresh air, and determines how much heat the space gains or losses as a result. The volume of airflow due to wind is:
Q_wind = K • A • V
Q_wind = airflow volumetric rate (m³/h)
K = coefficient of effectiveness (unitless, see below)
A = opening area, of smaller opening (m²)
V = outdoor uninterrupted wind speed (m/h)
The coefficient of effectiveness is a number from 0 to 1, adjusting for the angle of the wind and other fluid dynamics factors, such as the relative size of inlet and outlet openings. Wind hitting an open window at a 45° angle of incidence would have a coefficient of effectiveness of roughly 0.4, while wind hitting an open window directly at a 90° angle would have a coefficient of roughly 0.8.
When placing ventilation openings, you need to place both air inlets and air outlets; often they do not have the same area. The opening area used in this equation is the smaller of the two.
air speed and temperature in buildings
In addition
to volume, you should design for the wind speed inside your building. Wind speed is a component of human comfort,
and the speed you want depends on the climate.
Higher velocity air causes more effective cooling, because it pulls heated air away faster, and because it helps sweating be more effective by evaporating it faster. Even a moderate wind speed can cool perceived temperatures 5°C (9°F) compared to still air. This is how fans make people feel cooler even though they do not change the temperature of the air.
Higher velocity air causes more effective cooling, because it pulls heated air away faster, and because it helps sweating be more effective by evaporating it faster. Even a moderate wind speed can cool perceived temperatures 5°C (9°F) compared to still air. This is how fans make people feel cooler even though they do not change the temperature of the air.
thermal mass
- Thermal mass can have an impact on natural ventilation.
- Thermal mass is able to be used to help maintain a consistent temperature and avoid big jumps.
wind ventilation
Wind
ventilation is a kind of passive ventilation that uses the force of the wind to
pull air through the building.
Wind ventilation is the easiest, most common, and often least expensive form of passive cooling and ventilation. Successful wind ventilation is determined by having high thermal comfort and adequate fresh air for the ventilated spaces, while having little or no energy use for active HVAC cooling and ventilation.
Strategies for wind ventilation include operable windows, ventilation louvers, and rooftop vents, as well as structures to aim or funnel breezes. Windows are the most common tool. Advanced systems can have automated windows or louvers actuated by thermostats.
If air moves through openings that are intentional as a result of wind ventilation, then the building has natural ventilation. If air moves through openings that are not intentional as a result of wind ventilation, then the building has infiltration, or unwanted ventilation (air leaking in).
Wind ventilation is the easiest, most common, and often least expensive form of passive cooling and ventilation. Successful wind ventilation is determined by having high thermal comfort and adequate fresh air for the ventilated spaces, while having little or no energy use for active HVAC cooling and ventilation.
Strategies for wind ventilation include operable windows, ventilation louvers, and rooftop vents, as well as structures to aim or funnel breezes. Windows are the most common tool. Advanced systems can have automated windows or louvers actuated by thermostats.
If air moves through openings that are intentional as a result of wind ventilation, then the building has natural ventilation. If air moves through openings that are not intentional as a result of wind ventilation, then the building has infiltration, or unwanted ventilation (air leaking in).
strategies for wind ventilation
The keys to
good wind ventilation design are the building orientation and massing, as well
as sizing and placing openings appropriately for the climate. In order to maximize wind ventilation, you’ll
want the pressure difference between the windward (inlet) and leeward (outlet)
to be maximized. In almost all cases, high pressures occur on the windward side
of a building and low pressures occur on the leeward side.
The local climate may have strong prevailing winds in a certain direction, or light variable breezes, or may have very different wind conditions at different times. Often a great deal of adjustability by occupants is required. Consult climate data for wind rose diagrams.
The local climate may also have very hot times of the day or year, while other times are quite cold (particularly desert regions). In summer, wind is usually used to supply as much fresh air as possible while in winter, wind ventilation is normally reduced to levels sufficient only to remove excess moisture and pollutants.
The local climate may have strong prevailing winds in a certain direction, or light variable breezes, or may have very different wind conditions at different times. Often a great deal of adjustability by occupants is required. Consult climate data for wind rose diagrams.
The local climate may also have very hot times of the day or year, while other times are quite cold (particularly desert regions). In summer, wind is usually used to supply as much fresh air as possible while in winter, wind ventilation is normally reduced to levels sufficient only to remove excess moisture and pollutants.
site, massing and orientation for wind ventilation
Massing and
orientation are important because building height and depth play a huge role in
the structure's ability to effectively pull outside air through occupied
spaces. The massing and orientation pages discuss how to optimize them for
passive ventilation. In a nutshell, upper floors and roofs are exposed to more
wind than lower floors, and buildings with thin profiles facing into the path
of prevailing winds are easiest to ventilate. Atria and open-plan spaces also
help wind ventilation be more effective.
cross ventilation
When placing
ventilation openings, you are placing inlets and outlets to optimize the path
air follows through the building.
Windows or vents placed on opposite sides of the building give natural
breezes a pathway through the structure.
This is called cross-ventilation.
Cross-ventilation is generally the most effective form of wind
ventilation.
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It is generally best not to place openings exactly across from each other in a space. While this does give effective ventilation, it can cause some parts of the room to be well-cooled and ventilated while other parts are not. Placing openings across from, but not directly opposite, each other causes the room's air to mix, better distributing the cooling and fresh air. Also, you can increase cross ventilation by having larger openings on the leeward faces of the building that the windward faces and placing inlets at higher pressure zones and outlets at lower pressure zones.
Placing inlets low in the room and outlets high in the room can cool spaces more effectively, because they leverage the natural convection of air. Cooler air sinks lower, while hot air rises; therefore, locating the opening down low helps push cooler air through the space, while locating the exhaust up high helps pull warmer air out of the space. This strategy is covered more on the stack ventilation page.
Placing inlets low in the room and outlets high in the room can cool spaces more effectively, because they leverage the natural convection of air. Cooler air sinks lower, while hot air rises; therefore, locating the opening down low helps push cooler air through the space, while locating the exhaust up high helps pull warmer air out of the space. This strategy is covered more on the stack ventilation page.
Different amounts of ventilation and air mixing with different windows open.
steering breezes
Building structures can redirect prevailing winds to cross-ventilation.
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Not all
parts of buildings can be oriented for cross-ventilation. But wind can be steered by architectural
features, such as casement windows, wing walls, fences, or even
strategically-planted vegetation.
Architectural features can scoop air into a room. Such structures facing opposite directions on opposite walls can heighten this effect. These features can range from casement windows or baffles to large-scale structures such as fences, walls, or hedgerows. |
wind walls
Wing walls
project outward next to a window, so that even a slight breeze against the wall
creates a high pressure zone on one side and low on the other. The pressure
differential draws outdoor air in through one open window and out the adjacent
one. Wing walls are especially effective
on sites with low outdoor air velocity and variable wind directions.
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stack ventilation and Bernoulli's principle
Stack
ventilation and Bernoulli's principle are two kinds of passive ventilation that
use air pressure differences due to height to pull air through the
building. Lower pressures higher in the
building help pull air upward. The difference between stack ventilation and
Bernoulli's principle is where the pressure difference comes from.
Stack ventilation uses temperature differences to move air. Hot air rises because it is lower pressure. For this reason, it is sometimes called buoyancy ventilation.
Bernoulli's principle uses wind speed differences to move air. It is a general principle of fluid dynamics, saying that the faster air moves, the lower its pressure. Architecturally speaking, outdoor air farther from the ground is less obstructed, so it moves faster than lower air, and thus has lower pressure. This lower pressure can help suck fresh air through the building. A building's surroundings can greatly affect this strategy, by causing more or less obstruction.
The advantage of Bernoulli’s principle over the stack effect is that it multiplies the effectiveness of wind ventilation. The advantage of stack ventilation over Bernoulli's principle is that it does not need wind: it works just as well on still, breezeless days when it may be most needed. In many cases, designing for one effectively designs for both, but some strategies can be employed to emphasize one or the other. For instance, a simple chimney optimizes for the stack effect, while wind scoops optimize for Bernoulli’s principle.
For example, the specially-designed wind cowls in the Bed ZED development use the faster winds above rooftops for passive ventilation. They have both intake and outlet, so that fast rooftop winds get scooped into the buildings, and the larger outlets create lower pressures to naturally suck air out. The stack effect also helps pull air out through the same exhaust vent.
After wind ventilation, stack ventilation is the most commonly used form of passive ventilation. It and Bernoulli's principle can be extremely effective and inexpensive to implement. Typically, at night, wind speeds are slower, so ventilation strategies driven by wind is less effective. Therefore, stack ventilation is an important strategy.
Successful passive ventilation using these strategies is measured by having high thermal comfort and adequate fresh air for the ventilated spaces, while having little or no energy use for active HVAC cooling and ventilation.
Stack ventilation uses temperature differences to move air. Hot air rises because it is lower pressure. For this reason, it is sometimes called buoyancy ventilation.
Bernoulli's principle uses wind speed differences to move air. It is a general principle of fluid dynamics, saying that the faster air moves, the lower its pressure. Architecturally speaking, outdoor air farther from the ground is less obstructed, so it moves faster than lower air, and thus has lower pressure. This lower pressure can help suck fresh air through the building. A building's surroundings can greatly affect this strategy, by causing more or less obstruction.
The advantage of Bernoulli’s principle over the stack effect is that it multiplies the effectiveness of wind ventilation. The advantage of stack ventilation over Bernoulli's principle is that it does not need wind: it works just as well on still, breezeless days when it may be most needed. In many cases, designing for one effectively designs for both, but some strategies can be employed to emphasize one or the other. For instance, a simple chimney optimizes for the stack effect, while wind scoops optimize for Bernoulli’s principle.
For example, the specially-designed wind cowls in the Bed ZED development use the faster winds above rooftops for passive ventilation. They have both intake and outlet, so that fast rooftop winds get scooped into the buildings, and the larger outlets create lower pressures to naturally suck air out. The stack effect also helps pull air out through the same exhaust vent.
After wind ventilation, stack ventilation is the most commonly used form of passive ventilation. It and Bernoulli's principle can be extremely effective and inexpensive to implement. Typically, at night, wind speeds are slower, so ventilation strategies driven by wind is less effective. Therefore, stack ventilation is an important strategy.
Successful passive ventilation using these strategies is measured by having high thermal comfort and adequate fresh air for the ventilated spaces, while having little or no energy use for active HVAC cooling and ventilation.
strategies for stack ventilation and Bernoulli's principle
Designing
for stack ventilation and Bernoulli's principle are similar, and a structure
built for one will generally have both phenomena at work. In both strategies, cool air is sucked in
through low inlet openings and hotter exhaust air escapes through high outlet
openings. The ventilation rate is
proportional to the area of the openings. Placing openings at the bottom and
top of an open space will encourage natural ventilation through stack
effect. The warm air will exhaust
through the top openings, resulting in cooler air being pulled into the
building from the outside through the openings at the bottom. Openings at the top and bottom should be
roughly the same size to encourage even air flow through the vertical space.
To design for these effects, the most important consideration is to have a large difference in height between air inlets and outlets. The bigger the difference, the better.
Towers and chimneys can be useful to carry air up and out, or skylights or clerestories in more modest buildings. For these strategies to work, air must be able to flow between levels. Multi-story buildings should have vertical atria or shafts connecting the airflows of different floors.
Solar radiation can be used to enhance stack ventilation in tall open spaces. By allowing solar radiation into the space (by using equator facing glazing for example), you can heat up the interior surfaces and increase the temperature that will accelerate stack ventilation between the top and bottom openings.
Installing weatherproof vents to passively ventilate attic spaces in hot climates is an important design strategy that is often overlooked. In addition to simply preventing overheating1, ventilated attics can use these principles to actually help cool a building. There are several styles of passive roof vents: Open stack, turbine, gable, and ridge vents, to name a few.
To allow adjustability in the amount of cooling and fresh air provided by stack ventilation and Bernoulli systems, the inlet openings should be adjustable with operable windows or ventilation louvers. Such systems can be mechanized and controlled by thermostats to optimize performance.
Stack ventilation and the Bernoulli Effect can be combined with cross-ventilation as well. This matrix shows how multiple different horizontal and vertical air pathways can be combined.
Solar Chimneys A solar chimney uses the sun's heat to provide cooling, using the stack effect. Solar heat gain warms a column of air, which then rises, pulling new outside air through the building. They are also called thermal chimneys, thermosiphons, or thermosiphons.
The simplest solar chimney is merely a chimney painted black. Many outhouses in parks use such chimneys to provide passive ventilation. Solar chimneys need their exhaust higher than roof level, and need generous sun exposure. They are generally most effective for climates with a lot of sun and little wind; climates with more wind on hot days can usually get more ventilation using the wind itself.
Advanced solar chimneys can involve Trombe walls or other means of absorbing and storing heat in the chimney to maximize the sun's effect, and keep it working after sunset. Unlike a Trombe wall, solar chimneys are generally best when insulated from occupied spaces, so they do not transfer the sun's heat to those spaces but only provide cooling.
Herma chimneys can also be combined with means of cooling the incoming air, such as evaporative cooling or geothermal cooling.
Solar chimneys can also be used for heating, much like a Trombe wall is. If the top exterior vents are closed, the heated air is not exhausted out the top; at the same time, if high interior vents are opened to let the heated air into occupied spaces, it will provide convective air heating.
This works even on cold and relatively cloudy days. It can be useful for locations with hot summers and cold winters, switching between cooling and heating by adjusting which vents are open and closed.
To design for these effects, the most important consideration is to have a large difference in height between air inlets and outlets. The bigger the difference, the better.
Towers and chimneys can be useful to carry air up and out, or skylights or clerestories in more modest buildings. For these strategies to work, air must be able to flow between levels. Multi-story buildings should have vertical atria or shafts connecting the airflows of different floors.
Solar radiation can be used to enhance stack ventilation in tall open spaces. By allowing solar radiation into the space (by using equator facing glazing for example), you can heat up the interior surfaces and increase the temperature that will accelerate stack ventilation between the top and bottom openings.
Installing weatherproof vents to passively ventilate attic spaces in hot climates is an important design strategy that is often overlooked. In addition to simply preventing overheating1, ventilated attics can use these principles to actually help cool a building. There are several styles of passive roof vents: Open stack, turbine, gable, and ridge vents, to name a few.
To allow adjustability in the amount of cooling and fresh air provided by stack ventilation and Bernoulli systems, the inlet openings should be adjustable with operable windows or ventilation louvers. Such systems can be mechanized and controlled by thermostats to optimize performance.
Stack ventilation and the Bernoulli Effect can be combined with cross-ventilation as well. This matrix shows how multiple different horizontal and vertical air pathways can be combined.
Solar Chimneys A solar chimney uses the sun's heat to provide cooling, using the stack effect. Solar heat gain warms a column of air, which then rises, pulling new outside air through the building. They are also called thermal chimneys, thermosiphons, or thermosiphons.
The simplest solar chimney is merely a chimney painted black. Many outhouses in parks use such chimneys to provide passive ventilation. Solar chimneys need their exhaust higher than roof level, and need generous sun exposure. They are generally most effective for climates with a lot of sun and little wind; climates with more wind on hot days can usually get more ventilation using the wind itself.
Advanced solar chimneys can involve Trombe walls or other means of absorbing and storing heat in the chimney to maximize the sun's effect, and keep it working after sunset. Unlike a Trombe wall, solar chimneys are generally best when insulated from occupied spaces, so they do not transfer the sun's heat to those spaces but only provide cooling.
Herma chimneys can also be combined with means of cooling the incoming air, such as evaporative cooling or geothermal cooling.
Solar chimneys can also be used for heating, much like a Trombe wall is. If the top exterior vents are closed, the heated air is not exhausted out the top; at the same time, if high interior vents are opened to let the heated air into occupied spaces, it will provide convective air heating.
This works even on cold and relatively cloudy days. It can be useful for locations with hot summers and cold winters, switching between cooling and heating by adjusting which vents are open and closed.
night-purge ventilation
- Night-Purge ventilation keeps windows and other passive ventilation openings closed during the day but open at night to flush warm air out of the building and cooling thermal mass for the next day.
- Successful night-purge ventilation is determined by how much heat energy is removed from a building by bringing in night-time air, without using active HVAC cooling and ventilation.
Information Source: http://sustainabilityworkshop.autodesk.com/buildings/wind-ventilation