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.Sun, Y., Zhang, Y., Bao, L., Fan, Z. and Sundell, J., 2011. Ventilation and dampness in dorms and their associations with allergy among college students in China: a case-control study. Indoor Air, 21(4), pp. 277–283. There are three types of natural ventilation occurring in buildings: wind-driven ventilation, pressure-driven flows, and stack ventilation.How Natural Ventilation Works by Steven J. Hoff and Jay D. Harmon. Ames, IA: Department of Agricultural and Biosystems Engineering, Iowa State University, November 1994. 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), exhaustWhole-House Ventilation | Department of Energy 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.de Gids W.F., Jicha M., 2010. "Ventilation Information Paper 32: Hybrid Ventilation ", Air Infiltration and Ventilation Centre (AIVC), 2010 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).

Standards for residential buildings For residential buildings, which mostly rely on infiltration for meeting their ventilation needs, a common ventilation rate measure is the air change rate (or air changes per hour): the hourly ventilation rate divided by the volume of the space (I or ACH; units of 1/h). During the winter, ACH may range from 0.50 to 0.41 in a tightly air-sealed house to 1.11 to 1.47 in a loosely air-sealed house.Kavanaugh, Steve. Infiltration and Ventilation In Residential Structures. February 2004

ASHRAE now recommends ventilation rates dependent upon floor area, as a revision to the 62-2001 standard, in which the minimum ACH was 0.35, but no less than 15 CFM/person (7.1 L/s/person). As of 2003, the standard has been changed to 3 CFM/100 sq. ft. (15 L/s/100 sq. m.) plus 7.5 CFM/person (3.5 L/s/person).

Standards for commercial buildings Ventilation rate procedure Ventilation Rate Procedure is rate based on standard and prescribes the rate at which ventilation air must be delivered to space and various means to the condition that air.ASHRAE Standard 62 Air quality is assessed (through CO2 measurement) and ventilation rates are mathematically derived using constants. The Indoor Air Quality Procedure uses one or more guidelines for the specification of acceptable concentrations of certain contaminants in indoor air but does not prescribe ventilation rates or air treatment methods. This addresses both quantitative and subjective evaluations and is based on the Ventilation Rate Procedure. It also accounts for potential contaminants that may have no measured limits, or for which no limits are not set (such as formaldehyde off-gassing from carpet and furniture).

Airborne diseases Natural ventilation is a key factor in reducing the spread of airborne illnesses such as tuberculosis, the common cold, influenza, meningitis or COVID-19. Opening doors and windows are good ways to maximize natural ventilation, which would make the risk of airborne contagion much lower than with costly and maintenance-requiring mechanical systems. Old-fashioned clinical areas with high ceilings and large windows provide the greatest protection. Natural ventilation costs little and is maintenance-free, and is particularly suited to limited-resource settings and tropical climates, where the burden of TB and institutional TB transmission is highest. In settings where respiratory isolation is difficult and climate permits, windows and doors should be opened to reduce the risk of airborne contagion. Natural ventilation requires little maintenance and is inexpensive.

Natural ventilation is not practical in much of the infrastructure because of climate. This means that the facilities need to have effective mechanical ventilation systems and or use Ceiling Level UV or FAR UV ventilation systems.

Ventilation is measured in terms of air changes per hour (ACH). , the CDC recommends that all spaces have a minimum of 5 ACH.Centers For Disease Control and Prevention (CDC) For hospital rooms with airborne contagions the CDC recommends a minimum of 12 ACH.Centers For Disease Control and Prevention (CDC) However, there is a need to revise these recommendations to account for the combined effects of circulation patterns and ACH. Challenges in facility ventilation are public unawareness,Dr. Edward A. Nardell Professor of Global Health and Social Medicine, Harvard Medical School , Morawska, L, Allen, J, Bahnfleth, W et al. (36 more authors) (2021) A paradigm shift to combat indoor respiratory infection. Science, 372 (6543). pp. 689–691. ISSN 0036-8075 ineffective government oversight, poor building codes that are based on comfort levels, poor system operations, poor maintenance, and lack of transparency.Video

Pressure, both political and economic, to improve energy conservation has led to decreased ventilation rates. Heating, ventilation, and air conditioning rates have dropped since the energy crisis in the 1970s and the banning of cigarette smoke in the 1980s and 1990s.

Mechanical ventilation

Mechanical ventilation of buildings and structures can be achieved by the use of the following techniques:

Whole-house ventilation Mixing ventilation Displacement ventilation Dedicated subaerial air supply

Demand-controlled ventilation (DCV)