Ventilation (architecture)

Ventilation is the intentional introduction of outdoor air into a space, primarily to control indoor air quality by diluting and displacing indoor pollutants. It can also influence indoor temperature, humidity, and air movement, affecting thermal comfort and building energy use. Ventilation is important where combustion occurs indoors, as fireplaces, heaters, and other combustion sources consume oxygen and produce gases, smoke, and water vapor that may need to be removed. Ventilation is commonly classified as natural ventilation, mechanical ventilation, or mixed-mode ventilation. Natural ventilation relies on passive forces such as wind pressure and the stack effect, while mechanical ventilation uses fans to move outdoor air into or out of a building. Mixed-mode systems combine natural and mechanical processes. Ventilation design considers both the amount and distribution of airflow. Ventilation rates may be expressed as volumetric flow, as airflow per person or unit of floor area, or as air changes per hour (ACH). Airflow distribution affects ventilation effectiveness, while required ventilation rates can depend on occupancy and pollutant sources. Natural ventilation can reduce the risk of airborne disease transmission. If outdoor air is polluted, introducing it without adequate treatment can worsen indoor air quality. Demand-controlled and smart ventilation systems can adjust ventilation rates according to occupancy or environmental conditions to balance indoor air quality and energy use. Ventilation rates are addressed by building codes and standards, including ASHRAE Standards 62.1 and 62.2. Passive ventilation techniques have been used since antiquity, while mechanically driven systems have been developed from the 18th century; modern ventilation-rate standards began to emerge in the 19th century.

Types

Natural

Natural ventilation is the intentional passive flow of outdoor air into a building through planned openings (such as louvers, doors, and windows). Natural ventilation does not require mechanical systems to move outdoor air. Instead, it relies entirely on passive physical phenomena, such as wind pressure, or the stack effect. Almost all historic buildings were ventilated naturally. The technique was generally abandoned in larger US buildings during the late 20th century as the use of air conditioning became more widespread. However, with the advent of advanced Building Performance Simulation (BPS) software, improved Building Automation Systems (BAS), Leadership in Energy and Environmental Design (LEED) design requirements, and improved window manufacturing techniques; natural ventilation has made a resurgence in commercial buildings both globally and throughout the US. Natural ventilation openings may be fixed, or adjustable. Adjustable openings may be controlled automatically (automated), owned by occupants (operable), or a combination of both. Cross ventilation is a phenomenon of natural ventilation. Poor ventilation in rooms is identified to significantly increase the localized moldy smell in specific places of the room including room corners. There are three types of natural ventilation occurring in buildings: wind-driven ventilation, pressure-driven flows, and stack ventilation. The pressures generated by 'the stack effect' rely upon the buoyancy of heated or rising air. Wind-driven ventilation relies upon the force of the prevailing wind to pull and push air through the enclosed space as well as through breaches in the building's envelope. The benefits of natural ventilation include:

Improved indoor air quality (IAQ) Energy savings Reduction of greenhouse gas emissions Occupant control Reduction in occupant illness associated with sick building syndrome Increased worker productivity Techniques and architectural features used to ventilate buildings and structures naturally include, but are not limited to:

Operable windows Clerestory windows and vented skylights Night purge ventilation Building orientation Wind capture façades

Mechanical Mechanical ventilation is the intentional fan-driven flow of outdoor air into and/or out from a building. Mechanical ventilation systems may include supply fans (which push outdoor air into a building), exhaust fans (which draw air out of a building and thereby cause equal ventilation flow into a building), or a combination of both (called balanced ventilation if it neither pressurizes nor depressurizes the inside air, or only slightly depressurizes it). Mechanical ventilation is often provided by equipment that is also used to heat and cool a space.

Mixed-mode Mixed-mode ventilation systems use both mechanical and natural processes. The mechanical and natural components may be used at the same time, at different times of day, or in different seasons of the year. Since natural ventilation flow depends on environmental conditions, it may not always provide an appropriate amount of ventilation. In this case, mechanical systems may be used to supplement or regulate the naturally driven flow.

Design of air flow in rooms The air in a room can be supplied and removed in several ways, for example via ceiling ventilation, cross ventilation, floor ventilation or displacement ventilation.

Furthermore, the air can be circulated in the room using vortexes which can be initiated in various ways:

Ventilation rates for indoor air quality

The ventilation rate, for commercial, industrial, and institutional (CII) buildings, is normally expressed by the volumetric flow rate of outdoor air, introduced to the building. The typical units used are cubic feet per minute (CFM) in the imperial system, or liters per second (L/s) in the metric system (even though cubic meter per second is the preferred unit for volumetric flow rate in the SI system of units). The ventilation rate can also be expressed on a per person or per unit floor area basis, such as CFM/p or CFM/ft², or as air changes per hour (ACH).

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