Natural ventilation - AbsoluteAstronomy.com

Natural ventilation is the process of supplying and removing air through an indoor space without using mechanical systems. It refers to the flow of external air to an indoor space as a result of pressure or temperatures differences. There are two types of natural ventilation

Ventilation (architecture)

Ventilating is the process of "changing" or replacing air in any space to provide high indoor air quality...

occurring in buildings: wind driven ventilation and stack ventilation. While wind is the main mechanism of wind driven ventilation, stack ventilation occurs as a result of the directional buoyancy force that results from temperature variation.

Process

The static pressure

Pressure

Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

of air is the pressure in a free-flowing air stream and is depicted by isobars

Contour line

A contour line of a function of two variables is a curve along which the function has a constant value. In cartography, a contour line joins points of equal elevation above a given level, such as mean sea level...

in weather map

Weather map

A weather map displays various meteorological features across a particular area at a particular point in time. Such maps have been in use since the mid-19th century and are used for research and weather forecasting purposes. Maps using isotherms show temperature gradients, which can help locate...

s. Differences in static pressure arise from global and microclimate

Microclimate

A microclimate is a local atmospheric zone where the climate differs from the surrounding area. The term may refer to areas as small as a few square feet or as large as many square miles...

thermal phenomena and create the air flow we call wind

Wind

Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...

. Dynamic pressure is the pressure exerted when the wind comes into contact with an object such as a hill or a building and it described by the following equation :

where (using SI

International System of Units

The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...

units):

	= dynamic pressure Pressure Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :... in pascals Pascal (unit) The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre... ,
	= fluid density Density The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight... in kg/m³ (e.g. density of air Density of air The density of air, ρ , is the mass per unit volume of Earth's atmosphere, and is a useful value in aeronautics and other sciences. Air density decreases with increasing altitude, as does air pressure. It also changes with variances in temperature or humidity... ),
	= fluid velocity Velocity In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ... in m/s.

The impact of wind on a building affects the ventilation and infiltration

Infiltration (HVAC)

Infiltration is the unintentional or accidental introduction of outside air into a building, typically through cracks in the building envelope and through use of doors for passage. Infiltration is sometimes called air leakage. The leakage of room air out of a building, intentionally or not, is...

rates through it and the associated heat losses or heat gains. Wind speed increases with height and is lower towards the ground due to frictional drag.

The impact of wind on the building form creates areas of positive pressure on the windward side of a building and negative pressure on the leeward and sides of the building. Thus building shape is crucial in creating the wind pressures that will drive air flow through its apertures. In practical terms wind pressure will vary considerably creating complex air flows and turbulence by its interaction with elements of the natural environment (trees, hills) and urban context (buildings, structures). Vernacular and traditional buildings in different climatic regions rely heavily on natural ventilation for maintaining thermal comfort

Thermal comfort

Thermal comfort is a term used by the American Society of Heating, Refrigerating and Air-Conditioning Engineers, an international body. It is defined as the state of mind in humans that expresses satisfaction with the surrounding environment...

conditions in the enclosed spaces.

Design

Design guidelines are offered in building regulations

Building code

A building code, or building control, is a set of rules that specify the minimum acceptable level of safety for constructed objects such as buildings and nonbuilding structures. The main purpose of building codes are to protect public health, safety and general welfare as they relate to the...

and other related literature and include a variety of recommendations on many specific areas such as:

Building location and orientation
Building form and dimensions
Indoor partitions and layout
Window
Window
A window is a transparent or translucent opening in a wall or door that allows the passage of light and, if not closed or sealed, air and sound. Windows are usually glazed or covered in some other transparent or translucent material like float glass. Windows are held in place by frames, which...

typologies, operation, location, and shapes
Other aperture types (door
Door
A door is a movable structure used to open and close off an entrance, typically consisting of a panel that swings on hinges or that slides or rotates inside of a space....

s, chimney
Chimney
A chimney is a structure for venting hot flue gases or smoke from a boiler, stove, furnace or fireplace to the outside atmosphere. Chimneys are typically vertical, or as near as possible to vertical, to ensure that the gases flow smoothly, drawing air into the combustion in what is known as the...

s)
Construction methods and detailing (infiltration)
External elements (walls, screens)
Urban planning
Urban planning
Urban planning incorporates areas such as economics, design, ecology, sociology, geography, law, political science, and statistics to guide and ensure the orderly development of settlements and communities....

conditions

The following design guidelines are selected from the Whole Building Design Guide

Whole Building Design Guide

The Whole Building Design Guide or WBDG is the most used online resource for building information in the world, with over 500,000 distinct users per month and over 3 million document downloads according to data as of January 2011. The online portal covers a wide range of topics, from performance...

, a program of the National Institute of Building Sciences

National Institute of Building Sciences

The National Institute of Building Sciences is a non-profit, non-governmental organization that successfully brings together representatives of government, the professions, industry, labor and consumer interests, and regulatory agencies to focus on the identification and resolution of problems and...

Maximize wind-induced ventilation by siting the ridge of a building perpendicular to the summer winds
Widths of naturally ventilated zone should be narrow (max 13.7 m [45 feet])
Each room should have two separate supply and exhaust openings. Locate exhaust high above inlet to maximize stack effect. Orient windows across the room and offset from each other to maximize mixing within the room while minimizing the obstructions to airflow within the room.
Window openings should be operable by the occupants
Consider the use of clerestories or vented skylights.

Wind driven ventilation

Wind driven ventilation depends on wind behavior, on the interactions with the building envelope and on openings or other air exchange devices such as inlets or chimneys. For a simple volume with two openings, the cross wind flow rate can be calculated using the following equation :

Q=U_wind√((Cp₁-Cp₂)/(1/A₁²C₁²)+(1/A₂²C₂²) (1)

The knowledge of the urban climatology i.e. the wind around the buildings is crucial when evaluating the air quality and thermal comfort inside buildings as air and heat exchange depends on the wind pressure on facades. As we can see in the equation (1), the air exchange depends linearly on the wind speed in the urban place where the architectural project will be built. CFD (Computational Fluid Dynamics

Computational fluid dynamics

Computational fluid dynamics, usually abbreviated as CFD, is a branch of fluid mechanics that uses numerical methods and algorithms to solve and analyze problems that involve fluid flows. Computers are used to perform the calculations required to simulate the interaction of liquids and gases with...

) tools and zonal modelings are usually used to design naturally ventilated buildings. Windcatchers are able to aid wind driven ventilation by directing air in and out of buildings.

Some of the important limitations of wind driven ventilation:

Unpredictability and difficulties in harnessing due to speed and direction variations
The quality of air
Indoor air quality
Indoor air quality is a term referring to the air quality within and around buildings and structures, especially as it relates to the health and comfort of building occupants....

it introduces in buildings may be polluted for example due to proximity to an urban or industrial area
May create a strong draught, discomfort.

Stack driven ventilation

(For more details, see Stack effect
Stack effect
Stack effect is the movement of air into and out of buildings, chimneys, flue gas stacks, or other containers, and is driven by buoyancy. Buoyancy occurs due to a difference in indoor-to-outdoor air density resulting from temperature and moisture differences. The result is either a positive or...

)

Stack effect is temperature induced. When there is a temperature difference between two adjoining volumes of air the warmer air will have lower density and be more buoyant thus will rise above the cold air creating an upward air stream. Forced stack effect in a building takes place in a traditional fire place. Passive stack ventilators are common in most bathrooms and other type of spaces without direct access to the outdoors.

In order for a building to be ventilated adequately via stack effect the inside and outside temperatures must be different so that warmer indoor air rises and escapes the building at higher apertures, while colder, denser air from the exterior enters the building through lower level openings. Stack effect increases with greater temperature difference and increased height between the higher and lower apertures. The neutral plane in a building occurs at the location between the high and low openings at which the internal pressure will be the same as the external pressure (in the absence of wind). Above the neutral plane, the air pressure will be positive and air will rise. Below the neutral plane the air pressure will be negative and external air will be drawn into the space. Stack driven ventilation has several significant benefits:

Does not rely on wind: can take place on still, hot summer days when it is most needed.
Stable air flow (compared to wind)
Greater control in choosing areas of air intake
Sustainable method

Limitations of stack driven ventilation:

Lower magnitude compared to wind ventilation
Relies on temperature differences (inside/outside)
Design restrictions (height, location of apertures) and may incur extra costs (ventilator stacks, taller spaces)
The quality of air it introduces in buildings may be polluted for example due to proximity to an urban or industrial area

Natural ventilation in buildings relies mostly in wind pressure differences but stack effect can augment this type of ventilation and partly restore air flow rates during hot, still days. Stack ventilation can be implemented in ways that air inflow in the building does not rely solely on wind direction. In this respect it may provide improved air quality in some types of polluted environments such as cities. For example air can be drawn through the backside or courtyards of buildings avoiding the direct pollution and noise of the street facade. Wind can augment the stack effect but also reduce its effect depending on its speed, direction and the design of air inlets and outlets. Therefore prevailing winds must be taken into account when designing for stack effect ventilation.

Estimating stack effect ventilation

The natural ventilation flow rate can be estimated with this equation:

English units:

where:
Q_S	= Stack vent airflow rate, ft³/s
A	= cross-sectional area of opening, ft² (assumes equal area for inlet and outlet)
C_d	= Discharge coefficient for opening (typical value is 0,62)
g	= gravitational acceleration Gravitational acceleration In physics, gravitational acceleration is the acceleration on an object caused by gravity. Neglecting friction such as air resistance, all small bodies accelerate in a gravitational field at the same rate relative to the center of mass.... , around 32.2 ft/s² on Earth
H_d	= Height from midpoint of lower opening to neutral pressure level (NPL), ft
NPL	= location/s in the building envelope with no pressure difference between inside and outside (ASHRAE American Society of Heating, Refrigerating and Air-Conditioning Engineers The American Society of Heating, Refrigerating and Air Conditioning Engineers is an international technical society for all individuals and organizations interested in heating, ventilation, air-conditioning, and refrigeration... 2001, p.26.11)
T_I	= Average indoor temperature between the inlet and outlet, °R
T_O	= Outdoor temperature, °R

SI units:

where:
Q_S	= Stack vent airflow rate, m³/s
A	= cross-sectional area of opening, m² (assumes equal area for inlet and outlet)
C_d	= Discharge coefficient for opening (typical value is 0,62)
g	= gravitational acceleration Gravitational acceleration In physics, gravitational acceleration is the acceleration on an object caused by gravity. Neglecting friction such as air resistance, all small bodies accelerate in a gravitational field at the same rate relative to the center of mass.... , around 9.81 m/s² on Earth
H_d	= Height from midpoint of lower opening to neutral pressure level (NPL), m
NPL	= location/s in the building envelope with no pressure difference between inside and outside (ASHRAE 2001, p.26.11)
T_I	= Average indoor temperature between the inlet and outlet, K Kelvin The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
T_O	= Outdoor temperature, K

Assessing performance

One way to measure the performance of a naturally ventilated space is to measure the air changes per hour

Air changes per hour

Air changes per hour is a measure of how many times the air within a defined space is replaced.Air changes in a confined space are important for a variety of reasons, mainly though, we need fresh air to live...

in an interior space. In order for ventilation to be effective, there must be exchange between outdoor air and room air. A common method for measuring ventilation effectiveness is to use a tracer gas. The first step is to close all windows, doors, and openings in the space. Then, a tracer gas is added to the air. The reference, American Society for Testing and Materials (ASTM) Standard E741: Standard Test Method for Determining Air Change in a Single Zone by Means of a Tracer Gas Dilution, describes which tracer gases can be used for this kind of testing and provides information about the chemical properties, health impacts, and ease of detection. Once the tracer gas has been added, mixing fans can be used to distribute the tracer gas as uniformly as possible throughout the space. To do a decay test, the concentration of the tracer gas is first measured when the concentration of the tracer gas is constant. Windows and doors are then opened and the concentration of the tracer gas in the space is measured at regular time intervals to determine the decay rate of the tracer gas. The airflow can be deduced by looking at the change in concentration of the tracer gas over time. For further details on this test method, refer to ASTM Standard E741.

Standards

For standards relating to ventilation rates, in the United Stated refer to ASHRAE Standard 62.1-2010: Ventilation for Acceptable Indoor Air Quality

Indoor air quality

Indoor air quality is a term referring to the air quality within and around buildings and structures, especially as it relates to the health and comfort of building occupants....

. These requirements are for "all spaces intended for human occupancy except those within single-family houses, multifamily structures of three stories or fewer above grade, vehicles, and aircraft." In the revision to the standard in 2010, Section 6.4 was modified to specify that most buildings designed to have systems to naturally condition spaces must also "include a mechanical ventilation system designed to meet the Ventilation Rate or IAQ procedures [in ASHRAE 62.1-2010]. The mechanical system is to be used when windows are closed due to extreme outdoor temperatures noise and security concerns". The standard states that two exceptions in which naturally conditioned buildings do not require mechanical systems are when:

Natural ventilation openings that comply with the requirements of Section 6.4 are permanently open or have controls that prevent the openings from being closed during period of expected occupancy, or
The zone is not served by heating or cooling equipment.

Also, an authority having jurisdiction may allow for the design of conditioning system that does not have a mechanical system but relies only on natural systems. In reference for how controls of conditioning systems should be designed, the standard states that they must take into consideration measures to "properly coordinate operation of the natural and mechanical ventilation systems."

Another reference is ASHRAE Standard 62.2-2010: Ventilation and Acceptable Indoor Air Quality in low-rise

Low-rise

A low-rise is a building that is only a few stories tall.There is no universally accepted height requirement for a building to be considered a low-rise...

Residential Buildings. These requirements are for "single-family houses and multifamily structures of three stories or fewer above grade, including manufactured and modular houses," but is not applicable "to transient housing such as hotels, motels, nursing homes, dormitories, or jails."

For standards relating to ventilation rates, in the United Stated refer to ASHRAE Standard 55-2010: Thermal Environmental Conditions for Human Occupancy. Throughout its revisions, its scope has been consistent with its currently articulated purpose, “to specify the combinations of indoor thermal environmental factors and personal factors that will produce thermal environmental conditions acceptable to a majority of the occupants within the space.” The standard was revised in 2004 after field study results from the ASHRAE research project, RP-884: developing an adaptive model of thermal comfort and preference, indicated that there are differences between naturally and mechanically conditioned spaces with regards to occupant thermal response, change in clothing, availability of control, and shifts in occupant expectations. The addition to the standard, 5.3: Optional Method For Determining Acceptable Thermal Conditions in Naturally Ventilated Spaces, uses an adaptive thermal comfort