Rationale and Background to Ventilation
Rationale and Background to Ventilation
The information below is extracted from ‘A Guide to Energy Efficient Ventilation’
What is Ventilation?
Why is Ventilation Needed?
How Does Ventilation Work?
How Much Ventilation is Needed?
When Is Ventilation Not Appropriate?
How Polluted Can a Building Become?
What is the Energy Impact of Ventilation?
Can Ventilation Energy Loss Be Avoided or Recovered?
What Is the Relationship Between Ventilation Rate and Odour?
Is There a Relationship Between Ventilation and Health?
How is Ventilation Provided?
How Do Ventilation Needs and Strategies Differ According to Building Type?
How Is the Choice of Ventilation Influenced By Climate and Local Environment?
What Regulations and Standards Govern the Choice and Performance of Ventilation Systems?
What Other Aspects Must be Considered in the Design Process?
Other Issues (Cooling, Efficiency, Calculation techniques, etc.)
Summary and Introduction
Ventilation is essential for the health and comfort of building occupants. It is specifically needed to dilute and remove pollutants emitted from unavoidable sources such as those derived from metabolism and from the essential activities of occupants. Ventilation represents only one aspect of the total building air quality equation, it should not be used in place of source control to minimise pollutant concentrations in a space. Avoidable pollutants should be eliminated.
Air infiltration can destroy the performance of ventilation systems. Good ventilation design combined with optimum air-tightness is needed to ensure energy efficient ventilation. Ultimately, ventilation needs depend on occupancy pattern and building use. No single economic solution to ventilation exists. A full cost and energy benefit analysis is therefore needed to select an optimum ventilation strategy.
It is important to understand the complexities of ventilation systems and how performance is influenced by the building structure itself. The intention of this Chapter is to review the role of ventilation in contributing to a healthy environment in buildings. It is intended to be purely descriptive and is aimed at providing an introduction to current ventilation philosophy. Examples cover the home, workplace and industry.
What is Ventilation?
Definitions covering ventilation and the flow of air into and out of a space include:
Purpose provided (intentional) ventilation: Ventilation is the process by which ‘clean’ air (normally outdoor air) is intentionally provided to a space and stale air is removed. This may be accomplished by either natural or mechanical means.
Air infiltration and exfiltration: In addition to intentional ventilation, air inevitably enters a building by the process of ‘air infiltration’. This is the uncontrolled flow of air into a space through adventitious or unintentional gaps and cracks in the building envelope. The corresponding loss of air from an enclosed space is termed ‘exfiltration’. The rate of air infiltration is dependent on the porosity of the building shell and the magnitude of the natural driving forces of wind and temperature. Vents and other openings incorporated into a building as part of ventilation design can also become routes for unintentional air flow when the pressures acting across such openings are dominated by weather conditions rather than intentionally (e.g. mechanically) induced driving forces. Air infiltration not only adds to the quantity of air entering the building but may also distort the intended air flow pattern to the detriment of overall indoor air quality and comfort. Although the magnitude of air infiltration can be considerable, it is frequently ignored by the designer. The consequences are inferior performance, excessive energy consumption, an inability to provide adequate heating (or cooling) and drastically impaired performance from heat recovery devices. Some Countries have introduced air-tightness Standards to limit infiltration losses (Limb 1994).
Other air losses, e.g. duct leakage: Air leakage from the seams and joints of ventilation, heating and air conditioning circulation ducts can be substantial. When, as is common, such ducting passes through unconditioned spaces, significant energy loss may occur. Modera (1993), for example, estimates that as much as 20% of the heat from typical North American domestic warm air heating systems can be lost through duct leakage. Pollutants may also be drawn into the building through these openings. As a consequence, considerable research and development into the performance of duct sealing measures is being undertaken.
Air recirculation: Air recirculation is frequently used in commercial buildings to provide for thermal conditioning. Recirculated air is usually filtered for dust removal but, since oxygen is not replenished and metabolic pollutants are not removed, recirculation should not usually be considered as contributing towards ventilation need.
Why Is Ventilation Needed?
Ventilation is needed to provide oxygen for metabolism and to dilute metabolic pollutants (carbon dioxide and odour). It is also used to assist in maintaining good indoor air quality by diluting and removing other pollutants emitted within a space but should not be used as a substitute for proper source control of pollutants. Ventilation is additionally used for cooling and (particularly in dwellings) to provide oxygen to combustion appliances. Good ventilation is a major contributor to the health and comfort of building occupants.
How Does Ventilation Work?
Ventilation is accomplished by introducing ‘clean’ air into a space. This air is either mixed with the air already present in the enclosure to give ‘mixing’ or ‘dilution’ ventilation, or is used to ‘displace’ air in the space to give ‘displacement’ or ‘piston flow’ ventilation. These techniques give characteristically different pollutant profiles.
Mixing ventilation: Mixing is stimulated by natural turbulence in the air and (in the case of mechanical ventilation) by the design of the air supply diffusers. Mixing ventilation is especially important when recirculation is used to provide thermal conditioning. If mixing is perfect, the pollutant concentration is uniform throughout the space. The relationship between ventilation rate and concentration of pollutant (assuming a constant emission rate) is illustrated in Figure 1.1(a).
Displacement ventilation: Displacement ventilation methods are becoming popular in some Countries for offices and other non domestic buildings. In principle they are more effective at meeting ventilation needs than the equivalent mixing approach, however air cooling or heating capacity is limited by nature of the need for careful thermal control of the supply air temperature. Additional conditioning is typically met by radiative ceiling panels. Unlike mixing ventilation, the spatial concentration of pollutant within the space is non-uniform, with air upstream of the pollutant source being uncontaminated while the air downstream of source may become heavily contaminated. Good design is aimed at ensuring the separation of occupants from polluted air. A typical pollutant profile is illustrated in Figure 1.1(b). In this example, pollutant build-up (e.g. metabolic carbon dioxide) is kept above the occupant breathing zone. In practice some mixing inevitably occurs. Very careful air flow and temperature control is needed to inhibit mixing. Contaminants upstream of the occupied space or ‘breathing’ zone must be avoided. Examples of such pollutants include floor level contaminants and emissions from floor coverings and Good ventilation is a major contributor to the health and comfort of building occupants.
Figure 1.1 Characteristics of (a) Dilution
Ventilation and (b) Displacement Ventilation
Interzonal ventilation: In dwellings, it is common to extract air from ‘wet’ rooms such as kitchens and bathrooms. Fresh ‘make-up’ air is then drawn through air inlets or mechanically supplied to living areas and bedrooms. This induces a flow pattern that inhibits the cross-contamination of air from ‘polluted’ spaces to ‘clean’ spaces. Similar examples apply to clean room and hospital applications.
Short circuiting: If a ventilation system is poorly designed, ‘short circuiting’ may occur in which fresh ventilation air is extracted from the building before it has mixed with or displaced stale air. This can occur if air diffusers and outlets are positioned too close to each other or, in the case of displacement systems, the supply air temperature is higher than the room air temperature.
How Much Ventilation Is Needed?
The quantity of ventilation needed depends on the amount and nature of pollutant present in a space. In practice an enclosed space will contain many different pollutants. If the emission characteristics of each is known, then it is possible to calculate the rate of ventilation needed to prevent each pollutant from exceeding a pre-defined threshold concentration. When identical pollutants are emitted from more than one source, then the ventilation rate must be based on the total emission rate from all sources. To determine the overall ventilation need, it is useful to identify the dominant pollutant. This is the pollutant that requires the greatest amount of ventilation for control. Provided sufficient ventilation is achieved to control the dominant pollutant, all the remaining pollutants should remain below their respective threshold concentrations (see Figure 1.2). The minimum acceptable ventilation rate is that which is required to dilute the dominant pollutant to an acceptable concentration. Pollutants from localised sources should be enclosed or extracted at the point of source to avoid contamination of occupied spaces.
Figure 1.2 Controlling the Dominant Pollutant
It is useful to classify pollutants in terms of unavoidable and avoidable sources. Unavoidable sources are associated with metabolism and the essential activities of occupants. On the other hand, avoidable sources are associated with excessive emissions from materials and poorly designed appliances. If the dominant need for ventilation is from an avoidable source, then the reduction or elimination of the pollutant source will provide the most effective and energy efficient method of air quality control.
Unfortunately, acceptable safety and comfort concentrations of many pollutants are presently unknown. There is, therefore, currently much debate on how to address the ventilation requirements for such pollutants. On the other hand, recommended safe concentrations are available for several of the most common pollutants. Provided these known pollutants represent the dominant need for ventilation and emissions from avoidable sources are minimised, then any risk to health and comfort can be avoided.
Too often it falls upon ventilation to accomplish tasks for which it is not appropriate. The prime role of ventilation is to dilute and remove pollutants from unavoidable sources. In essence these are those generated by occupants themselves and by their essential activities. All other pollutants should be controlled by elimination or source containment. Some pollutants are chemically reactive, adsorbed on to surfaces, or have emission characteristics which are stimulated by the ventilation process itself. Such pollutants may not respond to the basic principles of ventilation, in which case ventilation may not be an entirely suitable control mechanism. Examples may include certain volatile organic compounds (VOC’s), soil gases and moisture. Again source avoidance or containment are the best control strategies. Ventilation cannot in itself deal with contaminants introduced into the supply air upstream of the point of delivery. Typical examples include outdoor contaminants, contamination of the ventilation system itself or contaminant sources located between the point of air supply and the ‘breathing’ zone. Filtration techniques combined with careful air intake placement may be necessary to cope with outdoor sources.
How Polluted Can a Building Become?
Steady state pollutant concentration: The pollutant concentration in a space depends on the rate of pollutant emission and the rate at which the space is ventilated. Provided the emission rate remains constant, then a steady state concentration is eventually reached which is independent of the enclosure volume. Under conditions of uniform mixing, the concentration throughout the space will be uniform, whereas if mixing is non uniform (e.g. displacement ventilation), the pollutant concentration will vary throughout the space.
Transient pollutant concentration – the building as a ‘fresh air’ reservoir: The time it takes for the steady state concentration to be reached depends on the rate of ventilation and the volume of enclosed space. Thus it may sometimes be possible to avoid immediate air quality problems by taking advantage of the fresh air already stored in a room. The capacity of a building to act as a reservoir is useful for absorbing the impact of transient pollution emissions and variations in ventilation rate.
It may also be used to advantage if the outdoor air becomes polluted for a short period (e.g. rush hour traffic) by temporarily restricting the rate of ventilation. Older naturally ventilated buildings are typically constructed with high ceiling heights to provide an air quality reservoir (see Figure 1.3).
Figure 1.3 The Building as a ‘Fresh Air’ Reservoir
What is the Energy Impact of Ventilation?
Approximately 30% of the energy delivered to buildings is dissipated in the departing ventilation and exfiltration air streams. In buildings constructed to very high Standards of thermal insulation, the proportion of airborne energy loss can be much higher. This loss has important implications both at the consumer level, where the cost must be met, and at the strategic level, where it impacts on primary energy need and environmental pollution. The amount of energy consumed is dependent on the flow rate of ventilation and the amount of conditioning of the air that is necessary to achieve thermal comfort (see Figure 1.4). Additional energy is needed to drive mechanical ventilation systems, cool air by refrigeration or evaporation and maintain acceptable humidity levels.
Figure 1.4 The Energy Impact of Ventilation
Can Ventilation Energy Loss Be Avoided or Recovered?
Ventilation energy demand can be reduced considerably by adopting a variety of energy efficient ventilation techniques. These include:
Minimising the need for ventilation: Energy demand may be curtailed by ensuring that the need for ventilation is reduced. This means minimising emissions from avoidable pollutant sources. Any extra ventilation needed to dilute and remove avoidable pollutants can be equated directly against conditioning load.
Avoid uncontrolled air infiltration losses: Poor building air-tightness results in excessive air infiltration and resultant uncontrolled energy loss. In many Countries building air-tightness can be improved considerably without detriment to indoor air quality. Infiltration driven by stack effect is particularly high when the difference between inside and outside temperature is at its greatest. This often corresponds to periods of maximum thermal conditioning need.
Demand controlled ventilation: If the dominant pollutant can be identified and measured, then the ventilation rate can be automatically adjusted to respond to need by means of demand controlled ventilation. This is especially successful at tracking metabolically produced carbon dioxide in densely and transiently occupied buildings (e.g. offices, schools and theatres). In dwellings, moisture sensors are used with varying success to control the rate of ventilation in bathrooms and kitchens.
Heat recovery: As much as 70% of the energy lost through mechanical balanced or extract ventilation can be recovered by the use of ventilation heat recovery systems. However potential savings must be equated against capital cost, ongoing maintenance needs and electrical (fan and/or heat pump) load. Their performance can also be destroyed by poor building air-tightness. The cost effectiveness of heat recovery systems is largely dependent on the severity of outdoor climate, the quality of the building envelope and the ventilation need.
Ground pre-conditioning of the supply air: Tempering of the supply air is possible by passing the supply air duct underground. Thermal gain must be equated against additional pressure loss introduced into the ventilation system. This approach has been applied to both single family and multi family (apartment) buildings.
Odour can be regarded as a ‘pollutant’ or as an indicator of the presence of pollutant. Sometimes it may alert the occupant to a potential health risk, although this need not always be reliable since some highly toxic pollutants, such as radon and carbon monoxide, are odourless.
More generally, odour causes discomfort, especially in sedentary environments such as the office or home. A difficulty with odour analysis is that many odours cannot be measured by instrumentation. Evaluation, therefore, has to rely on subjective testing by ‘panellists’ , thus making the interpretation of results difficult. A comprehensive study of odour and the control of odour by ventilation has been made by Fanger (1988). These results are summarised in Chapter 2.
Is There a Relationship Between Ventilation and Health?
Poor ventilation can be associated with unhealthy buildings. Miller (1992), for example, highlights the association of increasing bacteriological concentration with decreasing ventilation rates, while Billington (1982) has produced an historical review of the role of ventilation in improving health and reducing the spread of illness. Studies reported by Sundell (1994) and others have shown that symptoms of building sickness can occur at all ventilation ranges. However, any link between the rate of ventilation and the occurrence of symptoms becomes very weak at ventilation rates above approximately 10 litres/s for each occupant (l/ s.p). It would be incorrect, therefore, to associate all building health related problems with inadequate ventilation. Health problems in buildings may often have much more to do with the character and source of pollutant present in the space rather than the adequacy of ventilation.
Natural ventilation: Traditionally, ventilation needs have been met by ‘natural’ ventilation in which the flow process is driven by wind and temperature. In mild climates, design has often relied on no more than the natural porosity of the building, combined with window opening. In colder climates, natural ventilation designs tend to be more specific and incorporate carefully sized air inlets combined with passive ventilation stacks. Other climates might take advantage of a prevailing wind to drive the ventilation process.
The main drawback of natural ventilation is lack of control, in which unreliable driving forces can result in periods of inadequate ventilation, followed by periods of over ventilation and excessive energy waste. Good design can provide some measure of flow control but normally it is necessary for the occupant to adjust ventilation openings to suit demand. Despite the difficulty of control, natural ventilation is still relied upon to meet the need for fresh air in many types of building throughout the world.
Mechanical ventilation: In principle, the shortcomings of natural ventilation can be overcome by mechanical ventilation. These systems are capable of providing a controlled rate of air change and respond to the varying needs of occupants and pollutant loads, irrespective of the vagaries of climate. Some systems enable incoming supply air to be filtered while others have provision for heat recovery from the exhaust air stream. In some countries, especially in parts of Canada and Scandinavia, mechanical systems are being incorporated into virtually all new building construction and included in many building refurbishment programmes. In milder climate, however, the potential advantages of mechanical ventilation, especially for smaller buildings, can often be outweighed by installation and operational cost, maintenance needs and inadequate return from heat recovery.
Regardless of climate, mechanical ventilation is often essential in large, deep plan office buildings where fresh air must penetrate to the centre of the building and high heat gains can cause over heating.
Several configurations of mechanical ventilation are possible with each having a specific range of applications. The basic options are:
• supply ventilation,
• extract (or exhaust) ventilation,
• balanced supply extract systems,
Ventilation needs and strategies differ according to occupancy patterns and building type. Main considerations are:
Dwellings: The ‘dominant’ pollutant in dwellings is often moisture which is best extracted directly at source from wet zones using mechanical extract ventilation or ‘passive’ stacks. Fresh supply air is needed in living rooms and bedrooms to meet the needs of metabolism. Additional airing may be necessary if smoking takes place. Further air supply in the form of vent openings is necessary to meet the combustion needs of ‘open’ flue and ‘flueless’ combustion appliances. Special care is needed to avoid flue down-draughting resulting from the use of extract systems. In high radon areas, special attention to sealing the foundations is necessary, combined with sub floor venting.
Offices and other non-domestic buildings: Important pollutants in non domestic buildings include metabolic carbon dioxide, volatile organic compounds from furnishings and fittings, and ozone and carbon emissions from printers and photocopiers. In many of these buildings, metabolic carbon dioxide may represent the dominant source of pollutant. High heat gains may affect the choice between minimum ventilation combined with mechanical cooling or maximum ventilation for passive cooling. Industrial processes require special ventilation provision to prevent the discharge of contaminated air both internally to the occupants and externally to the atmosphere. Other special applications include provision for hospital and clean room ventilation design to avoid contamination.
The severity of climate influences the degree of heating or cooling that is necessary to condition the incoming air. Greater potential exists for the use of complex ventilation strategies combined with heat recovery when ventilation heating or cooling loads are high. A system that may be cost effective in one climatic zone may not be appropriate in another. Building location further influences the choice of ventilation strategy. Locations in urban and city areas, for example, can suffer from poor outdoor air quality derived from traffic fumes and industrial pollutants, while outside noise from passing traffic can be excessive, thus restricting the potential for window opening. Adjacent buildings could create conflict in relation to pre-existing air intakes and exhaust points. Rural locations might be subjected to high pollen concentrations and fungal spores resulting in a need for filtration for hypersensitive individuals.
What Regulations and Standards Govern the Choice and Performance of Ventilation Systems?
Numerous Standards relate to the needs and operation of ventilation systems. These vary between countries but Standards are regularly reviewed by the AIVC (Limb 1994). Typically they cover the minimum ventilation rate needed for health and safety, requirements for comfort, the operational performance of ventilation systems, requirements for building and component air-tightness, provision for maintenance, component durability and requirements for ventilation heat recovery.
What Other Aspects Must be Considered in the Design Process?
The designer is faced with many and, sometimes, apparently conflicting requirements in the task of delivering fresh air to occupants. In meeting the design need it is necessary to consider a wide range of criteria, varying from complying with the needs of Building Regulations to planning for maintenance and replacement. It is also necessary to integrate the ventilation system itself into the overall design of the building, especially in relation to air-tightness, room partitioning and accessibility.
Since such a wide range of parameters is involved, there is rarely a unique solution to a particular ventilation design. Instead the designer must base a judgement on the individual needs of each building. Ultimately a robust solution is needed which ensures the health and comfort of occupants. Ventilation needs must be based on criteria that can be established at the design stage of a building. To return afterwards in an attempt to mitigate problems as they arise may lead to considerable expense and failure. Design criteria are considered in the Guide to Energy Efficient Ventilation with special emphasis on the necessity of an integrated approach, design constraints, specifying ventilation needs and design variables.
Frequently, the dominant pollutant is ‘heat’ itself. Particularly in large commercial office buildings, high heat loads are developed through lighting, computing and other electrical sources. Further heat gains are derived from occupants, solar radiation and high outdoor temperatures. These factors make cooling of the indoor air essential. The choice is either to introduce refrigerative cooling or to introduce ventilation cooling. In either case heat gains should be minimised by good building design and reduced power consumption. Refrigerative cooling is energy intensive and contributes to peak power loads. Often, however, climate conditions dictate no other choice especially when the humidity level must be controlled. When refrigerative cooling is needed, ventilation must be minimised to prevent the unnecessary loss of conditioned air. Cooling is sometimes possible by introducing cooler, outdoor air (cooling by ventilation). This may be through window opening or by mechanical means. Ventilation rates for cooling will normally be well in excess of that needed to meet the basic fresh air requirements of occupants but may, nevertheless, accomplish dramatic energy savings over refrigerative cooling. The choice between ventilation cooling and refrigerative cooling is a function of heat gains, humidity loads and outdoor climate. Reducing heat gains by good building design ( e.g. minimising solar gains and introducing thermal mass) and by introducing low energy lighting and night cooling can often bring the threshold in favour of cooling by ventilation or reduce the periods in which refrigerative cooling is necessary.
Can Outdoor Air be Cleaned?
Outdoor air may be ‘cleaned’ by filtration. This is a method by which particulates and, sometimes, gaseous pollutants are removed from the air. Pollutants are intercepted by a filter while allowing clean air to pass through. This method of air cleaning is especially necessary when high concentrations of particulates are present or when the source of pollutant is derived from outside the building. Potential benefits can include improved air quality, reduced dependence on ventilation and improved energy efficiency. Filtration is not, however, a substitute for the ventilation needed to meet the metabolic requirements of occupants. Neither can filtration be used in leaky or naturally ventilated buildings. A review of particulate contamination and filtration methods is presented in the Guide to Energy Efficient Ventilation.
Indices of ventilation efficiency characterise the mixing behaviour of air and the distribution of pollutant within a space. These two aspects may be subdivided into indices of air change efficiency and pollutant removal effectiveness respectively. Ventilation efficiency is based on an evaluation of the ‘age’ of air and on the concentration distribution of pollutant within the air. Some indices are based on room averaged values, while others refer to specific points or locations. This has important consequences because while room values provide some guidance to the overall performance of a ventilation system, point values indicate regions where localised poor ventilation might occur. The concepts of ventilation efficiency are described in the Guide to Energy Efficient Ventilation. These concepts may be applied to entire buildings, single zones or locations within a single zone.
What Provision Should be Made for Maintenance?
Maintenance is needed to ensure the reliability of the ventilation system and to secure the economic operation of the ventilation plant. Evidence suggests, however, that maintenance is often inadequate and that the need for maintenance may even be ignored in the course of building design. Typical problems include worn gaskets, dirty fans and grilles, and ill-fitting and clogged filters. This concern has resulted in much more specific guidelines being developed for the maintenance of ventilation systems, some of which are discussed in the Guide to Energy Efficient Ventilation. Only by correct functioning can a ventilation system be relied upon to meet the indoor air quality needs of a building.
Measurements are needed to verify the performance of ventilation systems and to test the air-tightness of the building shell. They are essential for commissioning, diagnostic analysis, design evaluation and research. In addition, measurement results provide the fundamental means for understanding the mechanics of ventilation and air flow in buildings. Measurement data are also needed to provide background information for parametric studies on building air leakage characteristics, indoor air quality and ventilation system performance. Many measurement techniques have been developed with each having a specific purpose. An analysis of principal methods and applications is presented in the Guide to Energy Efficient Ventilation.
Techniques include:
• tracer gas testing for ventilation rate and ventilation efficiency evaluation,
• pressurisation measurements to determine building and component air-tightness,
• anemometry techniques to measure air flow velocity and turbulence throughout a space,
• sheet light and laser methods to visualise air flow patterns,
• flume models to design and predict ventilation performance,
• wind tunnel techniques for pressure distribution evaluation.
What Calculation Techniques are Available?
Calculation techniques and numerical models are essential for any design process. They provide the means by which the designer can develop and investigate an idea before being committed to the final product. Typical design aspects cover system sizing, performance evaluation, indoor air quality prediction, energy impact assessment, and cost benefit analysis. A calculation technique or model is used to analyse the interaction of design options with fixed constraints. Such a process is necessarily iterative, with adjustments made to parameters over which control is possible, until an optimum design solution is achieved.
A wide range of methods of varying complexity have been developed with no single method being universally appropriate. Selection varies according to the required level of accuracy, the availability of data and the type of building under investigation. As designs have become more complex and performance tolerances more demanding, it is increasingly important for the designer to be able to understand and use calculation techniques. This need has resulted in the development of improved algorithms and wider availability of design data. In addition, sufficient guidance and data are provided to enable basic calculation methods to be performed.
Techniques cover methods to determine:
• air change rates in buildings and rooms,
• the flow rate of air through infiltration and purpose provided flow openings (network methods), see Figure 1.5,
• air flow pattern in a space (computational fluid dynamics).
Figure 1.5 Representing the Building as a Flow Network (Courtesy C-A Roulet, Switzerland)
Various units are used to describe the rate of ventilation. These include:
Volumetric flow rate: Ventilation and air infiltration is commonly expressed in terms of a volumetric air flow rate e.g. litres/s (l/s) or m³/s.
Per occupant air flow rate: Sometimes the volumetric flow rate is divided by the number of occupants in a space to give a flow rate for each occupant. This is commonly expressed in terms of litres/second for each occupant, i.e. l/s.p.
Unit area flow rate: Alternatively, the air flow rate may be divided by the floor area of an enclosure to give a unit area value, i.e. litres/second.m².
Air change rate: Air flow is also often expressed in terms of hourly ‘air change rate’ (ach). This is the volume flow rate of air into an enclosure (e.g. a room or the entire building) divided by the room (or building) volume.
Mass flow rate: Sometimes air flow rate is expressed in terms of the mass flow rate of air, e.g. kg/s. Mass flow is needed to determine the thermal energy carried by the air stream. It is also widely used in ventilation and air flow calculation techniques (see Chapter 12).
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