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CIBSE Knowledge Series — Indoor air quality and ventilation

Indoor air quality and ventilation

CIBSE Knowledge Series: KS17

AuthorEoin Clancy


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Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Indoor air quality (IAQ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.1 Definition and importance of IAQ . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.2 Why ventilation is required . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.3 Requirements for good IAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.4 Regulations and standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.5 Common pollutants, pollutant sources and related health

issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.5.1 Pollutant types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Gaseous pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Volatile organic compounds (VOCs) . . . . . . . . . . . . . . . . . 6 Odours . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Particulates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Water vapour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.5.2 Pollutants and exposure limits (short/long term) . . . . . . . 9 2.6 Occupant comfort and IAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.7 Sick building syndrome (SBS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.8 External (outdoor) air pollution . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3 Ventilation systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.1 Types of ventilation system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.1.1 Natural ventilation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.1.2 Mechanical ventilation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 3.1.3 Mixed mode ventilation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 3.2 Required ventilation flowrates for good IAQ . . . . . . . . . . . . . . . . 19 3.2.1 Fresh air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Prescribed flow rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 3.2.2 Flows required for thermal comfort (heating) . . . . . . . . . 20 3.2.3 Flows required for thermal comfort (cooling) . . . . . . . . . 22 3.2.4 Flows between spaces and pressurisation . . . . . . . . . . . . . 22 3.2.5 Pollutant removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 3.3 Measuring ventilation and IAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 3.3.1 Measurement of flowrates . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Tracer gas methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Direct measurement methods . . . . . . . . . . . . . . . . . . . . . . . 29 3.3.2 Measuring IAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

4 Achieving optimum IAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 4.1 Reducing the risk of poor IAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 4.1.1 Fresh air supply rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 4.1.2 Well designed air distribution systems . . . . . . . . . . . . . . . . 34 4.1.3 Filtration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 4.1.4 Regular maintenance of ventilation plant . . . . . . . . . . . . . . 36 4.1.5 Selection of materials to minimize pollutant emissions . . 37

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39


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CIBSE Knowledge Series — Indoor air quality and ventilation 1

1 Introduction

This guide presents an overview of indoor air quality (IAQ) in buildings, and outlines how IAQ impacts on occupants’ health and performance. Ideally the surrounding environment and facilities in a particular space should provide healthy conditions in terms of sufficient fresh air, low pollution concentrations, adequate lighting and heating, access to drinking water and catering areas, and satisfactory sanitary installations. Cooling and/or air conditioning may also be needed depending on climate conditions and internal heat gains. The building should also have security and fire/smoke protection systems to protect the occupants and the building fabric in the event of unwanted intrusion and the outbreak of a fire or other undesirable high risk event (e.g. flooding).

Section 2 gives an overview of IAQ, regulations and standards, types of pollutants and allowable exposure limits, their impact on health, and the relationship between thermal comfort and IAQ.

Section 3 provides information on ventilation systems, and on the calculation of required flowrates to ensure good IAQ.

Section 4 explains how control of IAQ might be achieved.


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CIBSE Knowledge Series — Indoor air quality and ventilation2

2 Indoor air quality (IAQ)

2.1 Definition and importance of IAQ

Good IAQ may be defined as air with no known contaminants at harmful concentrations. Common contaminants or pollutants include gaseous pollutants, such as carbon dioxide (produced by occupants and from combustion appliances), volatile organic compounds (released by carpet glues and other materials), odours and particulates.

Good IAQ is essential to ensure the health and comfort of occupants.

2.2 Why ventilation is required

Ventilation is an essential component for the provision of good IAQ and thermal comfort. Specifically, ventilation is needed for:

⎯ providing fresh air for metabolism and for the dilution and removal of pollutants from within a space

⎯ extracting contaminants at source (e.g. extract systems for kitchens, bathrooms, industrial processes and fume cupboards)

⎯ satisfying combustion needs for appliances such as gas cookers, boilers and unvented heaters

⎯ distributing conditioned air (for heating or cooling)⎯ space pressurisation to inhibit the infiltration of pollutants from

outside or from one space to another (e.g. preventing integrated circuits within cleanrooms from being contaminated by dust particles)

⎯ pre-cooling building fabric (e.g. night venting of naturally ventilated spaces).

2.3 Requirements for good IAQ

The requirements for good IAQ are:

⎯ the provision of sufficient fresh air supply rates to dilute and remove pollutants (see section 3.2)

⎯ effective ventilation, i.e. providing ventilation where it is needed and in a form that will most efficiently remove pollutants

⎯ low external pollution concentrations⎯ low pollutant emission rates from internal sources, including

materials.

Where there are no abnormal pollutant emissions, the required ventilation rate can be met by following methods such as those provided in Part F of


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the Building Regulations for England and Wales(1) (see section 3.2.1). For specific pollutant applications ventilation rates may be determined using methods outlined in sections 3.2.2 to 3.2.4 or by using the pollution dilution equation in section 3.2.5. An effective ventilation system will distribute fresh air to the required locations, minimise the dispersal of pollutants and ensure that their concentrations are minimised in occupied areas.

Data on emission rates from internal sources are included in Table 2.2 (CO2), Table 2.3 (Formaldehyde), Table 2.4 (Odours), and Table 2.5 (Moisture emission rates). Basic information on external pollution is detailed in section 2.8; indoor concentrations should be below those provided in Table 2.6.

2.4 Regulations and standards

Building ventilation systems need to comply with the relevant legislative requirements. These vary between countries; therefore it is important to understand the requirements relevant to a particular building location. These include adhering to the particular building regulations that are in force in the UK and Ireland; other statutory documentation and ISO/British Standards, i.e. there are legal and operational requirements on the part of building developers and owners to provide adequate ventilation for occupants, combustion appliances and other pollution generating components.

Table 2.1 lists particular regulations, standards and guidelines.


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Regulation or standard Area covered Requirements

Building Regulation Part F1 (England and Wales)(1)

Provision of adequate fresh air. Size of opening areas for (1) background ventilation and (2) rapid ventilation.Particular extract ventilation rates from kitchens, toilets, etc.

Building Regulation Part J1 (England and Wales)

Provide adequate air for combustion devices.

EH40/2005 Workplace exposure limits (including consolidated amendments Oct 2007) (HSE)

Limit exposure to various pollutants. Provide adequate fresh air, filtration.

Air quality guidelines for Europe (WHO, Copenhagen, 2000) 2nd edition

As above. As above.

Ambient air quality and cleaner air for Europe – EEC Directive 2008/50/EC (May 2008)

Limit exposure to SO2 and suspended particulates.

HSE Approved Code of Practice L24: Workplace health, safety and welfare

Ensure minimal contamination of mechanical systems, including air conditioning systems,

Regular maintenance of systems.

BS EN 13986: 2002 (Emissions from) wood panels. Selection of materials with low emissions.Regular cleaning.Replacement at end of life.Provision of adequate fresh or ‘unpolluted’ air.

BS EN 14080: 2005 (Emissions from) glued laminated timber.

As above.

BS EN 14342: 2005 (Emissions from) parquet flooring. As above.

BS EN 14041: 2004 (Emissions from) vinyl, laminated and rubber floorings, linoleum and carpets.

As above.

BS EN 13964: 2004 (Emissions from) suspended ceiling tiles.

As above.

Table 2.1:Regulations and standards affecting IAQ


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2.5 Common pollutants, pollutant sources and related health issues

2.5.1 Pollutant types

Gaseous pollutants

Common gaseous pollutants found in buildings include carbon dioxide (CO2), carbon monoxide (CO), nitrogen oxide (NO), nitrogen dioxide (NO2), sulphur dioxide, ozone (O3) and (depending on location) naturally occurring radon.

Carbon dioxide is probably the most common pollutant. It is exhaled as part of the metabolic process and emitted from appliances such as gas cookers and boilers. It is non-toxic at normal concentrations, but when present at relatively high concentrations (above 10 000 ppm) causes drowsi-ness and, at much higher values, unconsciousness. The carbon dioxide emitted by occupants and/or appliances can provide an indication of the ventilation rate in a space. In a sedentary occupied zone a concentration of 800 to 1000 ppm typically represents a ventilation rate of about 10 l/s per person. It is for this reason that carbon dioxide monitoring is increasingly being integrated into ventilation control systems. Table 2.2 lists carbon dioxide emission rates for stated processes/appliances.

In contrast, carbon monoxide is highly toxic at low concentrations (above 86 ppm). It is produced when insufficient oxygen is available for combustion and/or when a combustion appliance is faulty. It can also come from outdoor sources, particularly vehicles. Because of its toxicity and risk of formation in highly airtight buildings, Part L of the Building Regulations for England and Wales(2) now requires that carbon monoxide monitors or alarms be placed in rooms containing fuel burning appliances that draw combustion air from the room.

Nitrous oxides (either NO or NO2) is also generated during combustion, particularly at high temperatures. Sulphur dioxide is produced when a fuel containing sulphur is burnt (e.g. fuel oil). Other relatively common gaseous pollutants are ozone and radon.

Ozone is produced by the action of sunlight on nitrous oxides. Nitrous oxides, sulphur dioxide and ozone act as lung irritants.

Radon is released by igneous rocks such as granite; this gas on its own causes limited adverse effects (being stopped by relatively thin materials), but associated isotopes released as a result of the radioactive process can be carcinogenic if inhaled. Special treatment to avoid the seepage of radon into buildings located in areas with high radon concentrations is required.


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This includes installing a low permeability membrane over the entire footprint of a building and an extraction system to remove radon from the substructure.

Emission rates of pollutants can be determined for particular conditions (e.g. according to activity level or rate of combustion in boilers). If the emission rate is known, then the amount of ventilation needed to dilute the pollutant concentration to a safe level can be determined (see section 3.2.5).

Typical safe concentrations of commonly occurring pollutants are presented in Table 2.6.

Process Emission rate Comments

Occupant Seated at rest (1 to 1.5 met)

0.004 to 0.005 l/s P = 4 x 10−5 x M A P = CO2 emission rate (l/s) M = metabolic rate (W · m−2) A = body surface area (m2)(M = ~ 70 W · m−2; A = ~ 1.8 m2)

Flueless liquid petroleum gas (LPG) appliance

0.032 l/s per kW Self standing and discharging directly to ambient air.

Gas cooker (natural gas) approx. 0.1 l/s per kW

Ring in operation, extract fan off.

Volatile organic compounds (VOCs)

Volatile organic compunds include benzene, formaldehyde and trichloroethylene (TCE). Benzene is used as a solvent and is present in paints, plastics, inks and rubber. Formaldehyde is released by laminates (see Table 2.3), paints, and glues. Trichloroethylene is used in the manufacture of inks, paints, lacquers and adhesives. Many VOCs have unpleasant odours and some are carcinogenic.

Process Emission rate (mg · h−1 · m−2)

Comments

Woodchip boards 0.46 to 1.69 Depends on type

Hardboard 0.17 to 0.51 Compressed cellulose boards

Plasterboards 0 to 0.13 Depends on type

Wallpaper 0 to 0.28 As above, type of glue used

Table 2.2:Emission rates of carbon dioxide for occupants and various processes(3)

(Note: 1 met is the heat generation rate

(W · m−2) of an adult male sitting quietly.)

Table 2.3:Flowrates of formaldehyde for various processes(3)

Note that emission rates will depend on

ambient conditions including material

temperature, age (including time after

installation, local ventilation rate and other

variables; manufacturers/suppliers may be able

to provide more accurate data).


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Odours

Sometimes it may be difficult to isolate the cause(s) of complaints relating to unsatisfactory IAQ, i.e. concentrations of known pollutants may be below threshold values or recommended short term exposure limits (see Table 2.6). However, some of these contaminants may result in the presence of odours, some of which may be unpleasant and result in com-plaints from occupants.

Sources of odours include cooking processes, decomposing foods, sanitary appliances (e.g. WCs, sinks), soil and waste water drains, building materials and furnishings and, of course, humans (via sweat, etc). These may be mixtures of VOCs, water vapour and odorous gases. If cooking odours can be contained within hoods via extract ventilation, and other smells prevented from entering a building using vapour barriers/well sealed con-tainment, then the main sources may be due to the presence of humans and those released from building furnishings and materials.

Sensory analysis was pioneered by Fanger(4), who quantified odour emissions by comparing them with a well-known source, i.e. a person sitting quietly and experiencing thermal comfort. A new unit, the ‘olf’ was defined as the release rate of bio-effluents from a ‘standard’ person. A ‘decipol’ is one olf ventilated at the rate of 10 l/s of unpolluted air.

To use the units, Fanger produced a curve that showed the relationship between ventilation rate per olf (units: l/s per olf) and the percentage of dissatisfied persons (see Figure 2.1). The latter is commonly shortened to PPD (%). This could then be used to find the required (fresh air) ventilation rate to achieve a ‘low’ occupant dissatisfaction level (e.g. less than or equal to a PPD of 15%) relative to the presence of odours.

Figure 2.1:Percentage of dissatisfied persons (PPD) versus the ventilation rate per standard person (from ASHRAE Fundamentals 2009; Section 12.6, figure 5(5))

60

50

40

30

20

10

00 5 10 15 20 25 30 35 40 45 50

CATEGORY C

B

A

PERC

ENT

DIS

SATI

SFIE

D, P

D

VENTILATION RATE q, L/s per olf


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For example, to achieve the 15% figure, the flowrate would need to be about 10 l/s per olf. See Table 2.4 (Sensory pollution loads).

Source Sensory load

Adult sitting quietly (1 to 1.5 met) 1 olf

Adult exercising: Low level (3 met) Medium level (6 met)

4 olf10 olf

Child (4 to 16 years) 1.3 olf

Low polluting building 0.1 olf · m−2

Non low polluting building 0.2 olf · m−2

Particulates

Particulates are fine particles released by occupants, generated during combustion or produced by external sources (such as vegetation and via vehicle exhausts). They may also be discharged from clothing, carpets, wallboard, aerosol sprays, dust-mites/insects and moulds. The biogenic pollutants or those produced by biological processes (e.g. fungi, moulds, mites, bacteria, viruses and pollen) cause many of health problems, including lung irritation, bronchial asthma and allergic rhinitis. Other toxic particulates include asbestos fibre and tobacco products.

Typical particles vary in size from 0.1 to 10 000 µm (see Figure 2.2). Critical sizes of particle are considered to be below 10 and 2.5 µm, these are referred to as PM10 and PM2.5 respectively.

Table 2.4:Sensory pollution loads from human sources and buildings(3)

Figure 2.2:Particle types and size ranges(6)

10 0000·01 0·1 1·0 10Particle diameter / µm

100 10002 3 4 5 6 7 89 2 3 4 5 6 7 89 2 3 4 5 6 7 89 2 3 4 5 6 7 89 2 3 4 5 6 7 89 2 3 4 5 6 7 89

Permanent impurities Temporary impurities Heavy industrial dust

Electron microscope Microscope Visible to naked eye

Smokes Fog Mist Drizzle Rain

Polio myelitis Tobacco smoke Staphylococcus

BacteriaViruses

Influenza Pollen

ZnO fumes Alkali fumes

Pulverised fuel ash

Pigments

Mould spores


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Whenever possible, sources of particulate pollution should be identified and eliminated. Where this is not possible, they are removed using filters, as described in section 4.1.3.

Water vapour

Water vapour may be considered a pollutant as its presence increases relative humidities and promotes mould growth. It can also affect the release rate of VOCs. It is discharged by occupants and from washing/drying, cooking and combustion appliances.

Typical moisture release rates are shown in Table 2.5.

Process Type Emission rate

Adult occupation Sleeping 0.04 kg · h−1

Active 0.05 kg · h−1

Cooking Electricity 2.0 kg/day

Natural gas 3.0 kg/day

Washing clothes 0.5 kg/day

Bathing/washing 0.2 kg/person/day

Dishwashing 0.4 kg/day

Unvented tumble drier 1.5 kg/person/day

Flue-less combustion Natural gas 0.16 kg · h−1 · kW−1

Kerosene 0.10 kg · h−1 · kW−1

Liquid petroleum gas 0.13 kg · h−1 · kW−1

2.5.2 Pollutants and exposure limits (short/long term)

The adverse effect of a particular pollutant depends on the exposure time to the contaminant. It also depends on the age(s) and sex(es) of persons or groups occupying a space. Current data apply mainly to people in good health and, in the case of working environments, to those between the ages of approximately late teens (16 to 19) and normal retirement age (65 in the UK and Ireland).

Assessment of the toxicology/occupational exposure for a particular substance can be primarily determined from the exposure concentration at which no unfavourable adverse health effects would be expected to occur. This is based on the known/predicted effects of the contaminant.

Table 2.5:Typical moisture emission rates from occupants, from washing and via fuel combustion(3,7)


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Table 2.6:Indicated exposure limits for selected airborne pollutants(3,8)

Pollutant Type Source(s) Effects Short term Long term

Conc Averaging time (hours)

Conc Averaging time (years)

Benzene VOC Solvents, fuel combustion

Carcinogen 5 ppb 1

Carbon dioxide

Gas Combustion appliances, occupants

Causes loss of concentration

5000 ppm 8

Carbon monoxide

Gas Combustion appliances

Lethal at low concentrations

26 ppm86 ppm

10.25

Formaldehyde VOC Insulation, product binders, particle board

Strong irritant, carcinogen

80 ppm 0.5

Hydrogen sulphide

Gas Decaying organic waste

Strong odour, irritant

5 ppb 0.5

Nitrogen dioxide

Gas Combustion appliances

Lung irritant 150 ppb 1 21 ppb 1

Ozone Gas Electrical equipment (e.g. motors), UV light sources

Lung irritant 60 ppb 8

Particles (non-biological)

Combustion appliances, aerosol sprays, clothing, carpets, wallboard

Allergen; cause of bronchial asthma and allergic rhinitis and may aggravate eczema symptoms

150 µg/m3 24 50 µg/m3

1

Particles (biological)

Humans, pets, insects, moulds, air conditioners, plants

Allergen; cause of bronchial asthma and allergic rhinitis

Radon Gas Building materials (e.g. various rocks), soil

Risk of lung cancer 400 Bq/m3 2160 200 Bq/m3

1

Sulphur Dioxide

Gas Traffic exhaust,Combustion appliances

Lung irritant 100 ppb46 ppb

0.2524

19 ppb 1

Tetrachloro-ethylene

VOC Solvents 250 µg/m3 24

Toluene VOC 68 ppb 168

Water vapour Washing, cooking, respiration

Mould/fungi growth(relative humidity should be maintained below 60 %). High and very low concentrations cause thermal discomfort


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Table 2.6 details short and long term maximum exposure concentrations for common pollutants. Note: For radon, the concentrations are expressed in Becquerels per cubic meter (Bq/m3).

2.6 Occupant comfort and IAQ

Thermal comfort is achieved when a person feels neither too hot nor too cold; in other words thermal ‘well-being’ is neutral with respect to the surrounding environment. The defining variable which governs a thermal comfort level is the dry resultant temperature. This is usually based on the average of air and mean radiant temperature, where the mean radiant temperature is approximately equivalent to the area weighted average fabric surface temperature. It may be measured by recording the temperature at the centre of a blackened sphere and the local air velocity.

For comfortable conditions to be maintained within occupied spaces, variables such as dry resultant temperature, relative humidity and local air speed must be kept between specified limits. Increasing the ventilation rate usually improves indoor air quality but may cause the local air speed to increase. thus bringing about discomfort (if the supply air temperature is significantly greater or lower than local air temperatures in the occupied zone). Modifying a ventilation system to improve air distribution may also increase local air speeds, thus adversely affecting comfort.

CIBSE Guide A, chapter 1(9) gives recommended internal conditions for a variety of room types (i.e. dry resultant temperatures (winter and summer), relative humidities, lighting levels and noise ratings). Suitable internal conditions for offices are given in Table 2.7.

Variable Value Effect on IAQ of exceeding these values

(winter) (summer)

Dry resultant temperature(Tdr, °C)

21–23 22–24 Increasing Tdr may increase release of VOCs, possible reduction in IAQ

Relative humidity (%)

40–70 40–70 As above. High values may result in condensation; mould formation

Local air speeds (m/s)

~ 0.1 ~ 0.3 Increasing air speeds may improve IAQ but increase the risk of discomfort (ducted air supply; cooling)

Figure 2.3 illustrates the radiant and convective heat exchange that occurs between a person and his/her surroundings

Table 2.7:Approximate thermal comfort variables and their affects on IAQ (offices)(10)


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2.7 Sick building syndrome (SBS)

Sick building syndrome (SBS) is a condition that may be experienced by occupants of buildings. The symptoms include headaches, eye strain and lung irritation. Usually the symptoms disappear when the affected person leaves the ‘mal-functioning’ building and are of short duration. Sick building syndrome occurs in buildings which may have a number of defects, including inadequate ventilation, low light levels and/or flickering light fittings, excessive odours, high ambient noise levels, lack of local controls and other factors.

Typically inadequate ventilation may be due to insufficient supplies of fresh air, excessive recirculation of air, poorly maintained air handling units (i.e. filters not replaced when needed) or a poorly designed and/or installed ductwork system. Low light levels are the result of light fittings not being replaced, the wrong type of fitting being used, poor lighting controls and/or insufficient numbers of light fittings being installed. Poor levels of daylighting can have the same effect. Excessive odours may be due to a poorly

Radiant and convective heat exchange from /to occupant and heat sources to/from fabric surfaces and surrounding air

Solar heat gain

Fabric heat loss

Air supply (cooling in summer)

Figure 2.3:Heat exchange between a person and his/her surroundings in a mechanically ventilated space


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designed/installed kitchen ventilation system and inadequate ventilation rates. High noise levels may be present due to the proximity of a space to an adjacent busy road and/or a plant room, insufficient sound insulation and lack of sound absorption materials used on wall and ceiling surfaces. No local controls may be available for an occupant not adjacent to an openable window or an adjustable thermostat (in many cases local thermostats may be disabled to prevent excessive adjustment by occupants)(8). Typical symptoms with remedies are summarized in Table 2.8.

Symptom Variables affecting symptom

Reducing adverse effects of variables

Lung irritation Inadequate fresh air flowrates, poor air distribution, poorly maintained ventilation plant.

Provide sufficient fresh air (minimum 10 l/s per person); carry out regular maintenance of air handling units; use well designed ventilation systems.

Eye strain Insufficient lighting levels. Provide sufficient number of light fittings, install correct type of fitting; use high frequency units;increase daylight levels.

Headache See above.High external and local noise sources.

As above.Relocate occupied areas; use adequate acoustic insulation;acoustically isolate plant.

The existence and extent of SBS may be established using a questionnaire. Occupants are asked to score their satisfaction level for ambient variables including air temperature, perceived humidity level, local (fresh) air flow, noise, task lighting, glare and odours, and their ability to control some of these values. Analysis is then performed on the data to determine whether conditions are satisfactory for the occupants in each assessed area. Results should indicate what modifications might be required to achieve acceptable thermal, lighting and acoustic conditions. In relation to ventilation, this may include increased fresh air flowrates, adjustment of diffusers, more frequent maintenance of ventilation plant and improved occupant access to controls.

An example of a simple questionnaire dealing with thermal, lighting and acoustic comfort variables is shown in Figure 2.4.

Table 2.8:Factors affecting SBS, and how their effects might be reduced


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2.8 External (outdoor) air pollution

Poor outdoor air quality will have a significant impact on indoor air quality. External pollutants include vehicle exhausts, discharges from combustion appliances, industrial process and power station exhausts. Fungal spores and pollen from vegetation may also cause air quality problems. Vehicle exhausts include CO, CO2, NOX and SO2, particulates (especially from diesel engines) and VOCs (e.g. from petrol and oil). Ozone is generated by the action of sunlight on nitrous oxides (present in vehicle exhausts). Power stations release similar pollutants to vehicles, but are usually located in rural or semi-rural areas, so the impact of their pollutants is much reduced. Typical pollutants are described in section 2.5.1.

In urban areas, traffic is a significant contributor to external pollution, with additional pollutant emissions coming from building exhausts and industrial processes. Although traffic is a low level source, local aerodynamic effects can cause considerable variations in concentrations. For example, trapped vortices can accumulate pollutants on the side of a building. Because of emission control legislation, there have been major reductions in vehicle

Figure 2.4:Questionnaire to assess thermal, lighting and acoustic comfort conditions in a room(11)

Assessment of comfort levels

Date & Time

Location (see sketch below)

Too low OK Too high1 2 3 4 5

Variable: (Please tick one of the boxes below)Air temperatureRelative humidityDust levelsAir speedFluctuations in air speed Location; ankle height or head height (siting)Lighting Daylighting available? Yes N o

Other conditions (please provide comments)

Occupant name(Leave blank if desired)

Plan of building


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emissions per vehicle mile. These reductions will continue as older vehicles are replaced with newer ones with lower emission rates and fitted with catalytic converters. Although emissions from vehicles are reducing, the greater number and usage of vehicles is resulting in greater outflows of pollutants.

Vehicle exhaust contaminants can be reduced by regular engine maintenance, the replacement of older vehicles with newer models and the use of particulate collection devices (e.g. filters and cyclones). Similarly, emissions from power plant and industrial processes can be controlled by particulate filtration and gas absorption.

The control and containment of industrial pollutants comes under the requirements of the Environmental Protection Act(12). Additionally, in the UK, local authorities are required to collect and maintain pollutant concentration data(13). Areas with particularly poor air quality must be designated Air Quality Management Areas and a remedial plan established to improve air quality in these zones. Additional information on external pollutant concentrations as measured by UK local authorities can be found in reference 13.

Table 2.9 lists examples of characteristic levels of perceived external air quality (typical daily values) and concentrations of typical outdoor pollutants (annual average concentrations).

Perceived air quality (decipol)

Air pollutants

CO2

(mg · m−3)CO(mg · m−3)

NO2

(μg · m−3)SO2

(μg · m−3)Particu-lates(μg · m−3)

Excellent 0.0 680 0–0.2 2 1 <30

In towns, good air quality

<0.1 700 1–2 5–20 5–20 40–70

In towns, poor air quality

>0.5 700–800 4–6 50–80 50–100 >100

Table 2.9:Examples of outdoor levels of air quality(14) (Note: there is no direct relation between

perceived air quality and the pollutants listed

in this table)


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3 Ventilation systems

3.1 Types of ventilation system

Ventilation may be driven by natural forces (natural ventilation) or by the use of a fan (mechanical ventilation). Natural ventilation uses buoyancy and/or wind to provide the driving force(s) for air movement, while in the case of mechanical ventilation air flow is driven by a fan.

Mechanical ventilation is required when external temperature, solar radiation and relative humidity are ‘high’ and/or if internal heat gains are excessive (causing uncomfortable internal conditions). It may also be needed to give known and controlled ventilation rates and to provide a particular pressure regime within a building.

3.1.1 Natural ventilation

Natural ventilation flowrates depend on a number of factors including:

⎯ inside and outside (air) temperatures⎯ local wind speeds and pressure coefficients⎯ location, size and nature of openings (infiltration and purpose

provided openings)⎯ nature of flow paths within a space ⎯ the flow regime.

Internal air temperatures will be influenced by the surrounding fabric temperatures, outdoor conditions and heating/cooling loads. Since both the indoor and outdoor temperatures will vary throughout the day and across the seasons, temperature-driven flows are highly variable. Varying wind speeds and directions will also generate fluctuating flowrates. Thus it is generally difficult to calculate accurate average flowrates. However, design flowrates can be found if various assumptions are made. Typically, if the outside temperature is low (e.g. less than about 5 °C in the UK), flowrates will more likely to be influenced by wind speed. On the other hand, if the internal minus external temperature difference is low (approx. 3 to 5 °C) and external temperatures are high, then ‘summer’ design flows may be calculated using the above temperature difference (assuming all other variables are constant). Since the ventilation contribution due to stack and wind pressures are largely independent, an approximate estimate of the actual flowrate can be made by calculating the value due to wind and temperature difference separately, and taking the largest of the two values. More guidance is provided in CIBSE AM10(15) and CIBSE Guide B(6).


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Prescribed methods of establishing infiltration rates can be found in CIBSE Guide A(9). Empirical formula may be used to check the relative effects of wind speed and/or temperature difference on bulk flowrates (see Awbi(3)).

Calculation methods for stack and wind driven flowrates are provided in CIBSE Guide B, chapter 2 (ventilation)(6) and also in Awbi(3). The effective inlet areas depend on flow path directions and the flow regime. Areas are added for inlet/outlet flows moving in the same direction; the inverse of the square of areas is used for flows travelling in series. For example a room has two inlets and two outlets on opposite walls, each with an area of 0.01 m2. With wind induced flow, the total effective inlet area is 0.02 m2, whereas with stack flow the equivalent area is 1.41 × 0.01 or 0.141 m2 (i.e. √2 × area of opening).

Natural ventilation may be provided via openings on one side of a room or via openings on both sides; see Figures 3.1 and 3.2. Calculation methods and formulae for finding flowrates in spaces ventilated via openings or windows on one or both (opposing) facades are provided in CIBSE Guide A(9), chapter 4, Tables 4.22 and 4.23, and in CIBSE Applications Manual AM10(15).

h approx1·5 m

W

H

W ≤ 2·5 H

Figure 3.1:Single sided (natural) ventilation(15)

W

H

W ≤ 5 H

Figure 3.2:Crossflow (natural) ventilation(15)


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It may be assumed that wind driven flowrates will predominate in winter and buoyancy driven flowrates in summer.

Figure 3.1 shows flow paths for entering and exhaust air for an office space with known occupant positions and pre-defined space geometry. It is ventilated using a window with top and bottom openings assuming that air flow is driven largely by temperature differences between inside and outside. Figure 3.2 illustrates flow directions assuming air movement is driven by wind pressures (with a positive and negative pressure areas being created on the windward and leeward sides of the building façade).

3.1.2 Mechanical ventilation

Mechanical ventilation systems (see Figure 3.3) consist of supply and extract air handling units, and ductwork connecting the units to inlet and outlet units. This type of system is described as balanced because of the dual network. Sometimes a system may be supply only, when the flow is dominated by a supply fan, with extract by natural ventilation, or extract only, when the flow is dominated by an extract fan, with supply by natural ventilation. The balanced system may be configured to use full fresh air with heat recovery (where no extract air is recirculated) or to provide ventilation with recycled air.

A domestic ventilation unit is shown in Figure 3.4; this consists of a loft mounted system which includes a heat recovery device or air-to-air heat exchanger. Warm air is extracted from kitchen and bathroom areas, passed through the heat exchanger and then discharged to the outside. The heat recovered is transferred to incoming fresh air which is then supplied to living and bedroom spaces.

Figure 3.4:Domestic mechanical ventilation system with heat recovery

Centralised plant area

Chiller

Boilers

Large ductwork and air handling unit

Occupied floors

VAV terminalunits

Figure 3.3:Mechanical ventilation/air conditioning system with variable flows(16)


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An overview of mechanical ventilation systems is provided in the BSRIA’s Illustrated guide to mechanical services(16). Calculation procedures are detailed in A practical guide to HVAC Building Services calculations(17). A full description of mechanical ventilation systems is given in CIBSE Guide B(6).

3.1.3 Mixed mode ventilation

Mixed mode systems use both natural and mechanical ventilation. When external temperatures are low, a mechanical system is operated to provide pre-heated air to a building. In ‘mid-season’ (when outside temperatures are approximately 10 to 15 °C) natural ventilation is used. The mechanical ventilation system is again operated to increase air velocities and for cooling when high external temperatures and/or excessive internal heat gains are experienced. Further information is available in the CIBSE Applications Manual AM13: Mixed mode ventilation(18).

3.2 Required ventilation flowrates for good IAQ

In England and Wales, ventilation requirements for health are covered by Part F of the Building Regulations(1) and, for minimising energy consumption, by Part L of the Building Regulations(2). Through Part F minimum needs are specified for all common buildings including dwellings, offices, schools, public buildings and retail premises. Extract and whole building ventilation rates are specified for domestic and non-domestic buildings; see Table 3.1.

3.2.1 Fresh air Table 3.1(a):Dwelling extract ventilation rates(1)

Room Min intermittent extract rate (l/s)

Continuous extract (l/s)

Min. high rate Min. low rate

Kitchen 30 (adjacent to cooker)60 (elsewhere)

13 Greater than whole building

Utility room 30 8 Ventilation rate given in Table 3.1(b)

Bathroom 15 8

Sanitary 6

Table 3.1(b):Whole dwelling ventilation rates(1)

Minimum ventilation rate should be not less

than 0.3 l/s per m2 internal floor area. Tabular

data assume 2 occupants in main bedroom

and a single occupant in all other bedrooms. If

greater numbers of people are present, add

4 l/s per person.

No of bedrooms in dwelling

1 2 3 4 5

Whole building ventilation rate (l/s) 13 17 21 25 29


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3.2.1 Fresh air

Prescribed flow rates

In an office environment (which, as with most other enclosed spaces, is now free of tobacco smoke within the UK), the fresh air ventilation rate is 10 l/s per person(1). Thus the total fresh air flowrate for a room is given by the flowrate per person times the number of occupants. Other air flowrates are recommended for specific areas such as bathrooms (see Tables 3.1(a) to (c) and Part F of the Building Regulations(1). Minimum ventilation rate should be not less than 0.3 l/s per m2 internal floor area. Tabular data assumes two occupants in the main bedroom and a single occupant in all other bedrooms. Tabular data assumes two occupants in the main bedroom and a single occupant in all other bedrooms.

3.2.2 Flows required for thermal comfort (heating)

In a mechanically ventilated space, ventilation is also commonly used as part of the heating and cooling distribution system.

The amount of ventilation to maintain an internal room air temperature of Tr is given by:

�VQ

C T T=

-( )htg

p s rr (3.1)

where �V is required warm air ventilation rate (m3 · s−1), Qhtg is the room heating load (W), r is the air density (kg · m−3), Cp is the specific heat capacity of air (1020 J · kg−1 · K−1) and Ts and Tr are the supply and room air temperatures respectively (°C). Note: the above equation gives an

Table 3.1(c):Extract rates for non-domestic buildings(1)

Room Extract rate (l/s)

Rooms containing printers and photocopiers in substantial use (more than 30 min/h)

20 per machine during use. If room continuously occupied, use greater of extract rate and whole building ventilation rates

Office washrooms and sanitary accommodation

(intermittent)15 per shower/bath; 6 per WC/urinal

Food and beverage preparation areas (not commercial kitchens)

(intermittent; during food/beverage preparation)15 microwave and beverages only30 adjacent to cooker60 elsewhere (cooker)

Specialist buildings and spaces (e.g. commercial kitchens, sports centres)

See Building Regulations Approved Document F(1), Table 2.3 and/or CIBSE Guide B(6), chapter 2


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approximate flowrate only; it is assumed that the moisture contents (i.e. mass of water to that of air) of the supply and room air are the same.

Heat losses occur via the external fabric and by air leakage; to maintain the internal air temperatures at comfortable values when the outdoor temperature is low heating needs to be provided.

Room heating loads are found by calculating the steady state fabric and infiltration losses. These do not take into account contributions from solar and internal heat gains. Air density at 20 °C is about 1.2 kg · m−3. For other temperatures, air densities are given in Table 3.2.

Temperature (°C) Density (kg · m−3) Specific heat capacity (kJ · kg−1 · K−1)

0 1.29 1.006

10 1.24 1.011

20 1.20 1.018

30 1.16 1.030

40 1.11 1.050

Usually the airflow required for heating is much greater than that needed for the supply of fresh air, therefore recirculation or heat recovery is applied. It is essential to ensure that the requirement for fresh air is maintained throughout the entire heating range. This requires that the fresh air damper will have a minimum opening position so that the required flowrate is always provided when a building is occupied. The actual flow will depend on the fan rating, and total system pressure drop including that across ventilation plant, ductwork and dampers. Further information is available in CIBSE Guide B, chapter 2(6).

Example:

The heating load for a room is 10 kW. The room is to be maintained at a temperature of 21 °C and the supply air condition is 30 °C dry-bulb and 30 % RH. Find the required warm air flowrate.

From a suitable table (thermodynamic properties of fluids), the specific volume is 0.87 m3 · kg−1 at 30 °C dry bulb and 30 % RH. Hence the air density is 1/0.87 = 1.149 kg · m−3.

Hence ventilation rate, �V =-

100001 149 1020 30 21. ( ) ( )

= 0.948 m3 · s−1.

Table 3.2:Density and specific heat capacity of (dry) air at various temperaturesSupply air temperatures are typically 30 to

35 °C while the room air temperature is

commonly 21 °C.


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If a full fresh air ventilation system is used to supply the above air flow, the fresh air flow is equivalent to the ventilation rate (i.e. 0.948 m3 · s−1). If recirculated air is used, the fresh air flow will depend on the number of occupants in the space.

3.2.3 Flows required for thermal comfort (cooling)

A similar calculation approach is applied to cooling and again, the requirement to meet fresh air needs is essential. The flowrate for cooling is thus:

�VQ

C T T=

-( )clg

p r sr (3.2)

where �V is the required cool air ventilation rate (m3 · s−1), Qclg is the room cooling load (W), ρ is the air density (kg · m−3), Cp is the specific heat capacity of air (~ 1020 J · kg−1 · K−1), Tr is the room air temperature (°C) and Ts is the supply air temperature (°C). Note: the above formula gives an approximate flowrate only. It is assumed that the moisture contents (i.e. mass of water to that of air) of the supply and room air are the same.

Cooling needs to be provided to maintain comfortable internal temperatures when solar and/or internal gains are high. The cooling load can be found by estimating solar and internal heat gains. The room air temperature to be maintained in summer is commonly 22–24 °C (offices), while the supply air temperature may be about 15 °C.

Ideally the same ventilation rate should apply in summer and winter so that the same ventilation system can be used.

3.2.4 Flows between spaces and pressurisation

Ventilation is often used to maintain pressure differences between spaces (e.g. in kitchen areas to prevent odours and water vapour from dispersing into other rooms). Extract ventilation drives the contaminated air through the fan and out of a roof or wall mounted exhaust vent. The resultant under-pressure causes make-up air to come from adjacent spaces. It is important to ensure that the exhaust vent does not contaminate adjacent air supply intakes.

The flowrate to be maintained for pressurisation may be obtained by assuming that the sum of the flowrates into and out of a space is equal to zero. Air is lost or gained by leakage from openings around doors, windows, gaps between surface interfaces and via ‘leaky’ surfaces and to/from a ventilation system via diffusers and grilles. Pressurised spaces include operating theatres and cleanrooms (as used for the production and testing of integrated circuits), which need to be kept free of contaminants.


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20 Pa 10 Pa10 Pa

5 Pa

10 Pa

Calculating flowrates due to pressurisation are found using:

ri i1

0�Vi

==

N

Â (3.3)

where ri is the air density (kg · m−3), V̇i is the flowrate via path i (m3 · s−1) and N is the number of flowpaths.

The flowrate (V̇i) may be found using:

�V A Pi = 0 775. D (3.4)

where V̇i is the leakage rate through openings (m3 · s−1), A is the leakage area (m2) and ∆P is the pressure drop across the opening (Pa).

Note that: 0.775 = 0.6 × 2

rair

with rair (air density) = 1.2 kg · m−3 at 20 °C

Sample calculation:

A cleanroom is to be kept at elevated pressures of 10 and 15 Pa relative to adjacent spaces. Air is supplied to the room at a rate equivalent to 100 air changes per hour. Leakage occurs via gaps around a double door to a corridor (maintained at 5 Pa) and through openings around three other single doors to other areas (kept at 10 Pa). Air is supplied via a ceiling mounted diffuser. Assume no air movement occurs across floor, wall or ceiling surfaces. Determine the room extract ventilation rate. Assume air density is constant. The room dimensions are 5 m × 6 m × 3 m (width × length × height). See Figure 3.5.

Figure 3.5:Room pressurisation

The leakage rate is found using:

�V A Pleakage = 0 775. D (3.5)

where �Vleakage is the leakage rate through openings (m3 · s−1), A is the leakage area (m2) and ∆P is the pressure drop across the opening (Pa).


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Flowrate across single door opening: �Vsd

= 0.775 × 0.018 × 10 = 0.0441 m3 · s−1

Flowrate across double door opening: �Vdd

= 0.775 × 0.030 × 15 = 0.090 m3 · s−1

Hence total outflow = 3 × 0.0441 + 0.090 = 0.2223 m3 · s−1

For flow balance, total supply flowrate = total extract flowrate

Ventilation supply rate = 100 × volume/3600 = 2.5 m3 · s−1.

Hence outflow rate = 2.5 – 0.2223 = 2.28 m3 · s−1.

3.2.5 Pollutant removal

Rooms may have one or more pollutant sources/types, different ventilation systems with specific supply and extract locations, given ambient conditions (e.g. temperatures, relative humidities, local air speeds) and normally it may be difficult to calculate transient and steady-state pollutant concentrations. However, if some simplifying assumptions can be made, then various formulae can be used.

For a single room or space, the pollutant concentration for one pollutant source in one room can be found from the following dilution equation(9):

C tQ C q

Q qe C e

Q q

Vt

Q q

Vt

( )( )

=+( )

+-

È

ÎÍÍ

˘

˚˙˙

+- +È

ÎÍ˘˚̇

- +ÈÎÍ

˘˚̇e

o1 (3.6)

where Q is the flowrate (m3 · s−1), Ce is the pollutant concentration in the outside air, q is the pollutant release rate (m3 · s−1), V is the room volume (m3), t is the time (seconds or hours) and Co is the initial concentration of pollutant. Equation 3.6 is shown in graphical form in Figure 3.6.

The above formula is based on the following assumptions: (1) single pollutant source, (2) constant flowrate, (3) constant external pollutant concentrations, inside and outside pressures, and (4) pollutant and air completely mixed.

If there is no contaminant source (i.e. q = 0), then the concentration will decay depending on the ventilation rate, i.e.

C t C e

Q

Vt

( ) =-È

ÎÍ˘˚̇

o (3.7)


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0

50

100

150

200

250

300

350

400

450

500

0 0.02 0.04 0.06 0.08 0.1

Time (h)

Gas

con

cent

rati

on (

ppm

)

C(t) (ppm)C(t) with CO2 (ppm)C(t) (decay) (ppm)

1

2

3

If the initial concentration of the pollutant is zero, then the concentration is calculated using:

C tQ C q

Q qe

Q q

Vt

( )( )

=+( )

+-

È

ÎÍÍ

˘

˚˙˙

- +ÈÎÍ

˘˚̇e 1 (3.8)

As the time, t becomes large or greater than four time constants*, the transient term becomes small and equation 3.8 simplifies to the following (equation 3.9):

C tQ C q

Q q( )

( )=

+( )+e (3.9)

The above can be rearranged to give a value for flowrate, i.e.

Q = q (1 − C)/(C − Ce) (3.10)

This can be simplified to (as C << 1)

Q = q / (C – Ce) (3.11)

The above formula can be modified to calculate a ventilation supply flowrate if the pollutant emission rate, the required room plus outside air contaminant concentrations and the ventilation system efficiency are known(8), i.e.:

�V q

E C Csupply

v room fa

=-( ) (3.12)

Figure 3.6:CO2 gas concentration versus time assuming (1) initial concentration CO2 = 0, (2) CO2 > 0, (3) no pollutant source (decay in concentration with time)

* The time constant is the time taken to reach 63% of the steady state value.


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where �Vsupply is the fresh air supply per person (m3 · s−1), q is the pollutant release rate (m3 · s−1 per person), Ev is the ventilation system efficiency (fraction), Croom is the room pollutant concentration to be maintained (ppm) and Cfa is the fresh air pollutant concentration (ppm).

The ventilation efficiency fraction (Ev) can be established using Table 3.3.

Room ventilation system Temperature difference between supply and room

air (Ts – Tr)

Ventilation effectiveness

Mixing ventilation; air supplied and extracted at high level

< 02 to 5> 5

0.9 to 1.00.8

0.4 to 0.7

Mixing ventilation; air supplied at high level and extracted at low level

< −5−5 to 0

> 0

0.90.9 to 1.0

1.0

Displacement ventilation; air supplied at low level and extracted at high level

> 20 to 2< 0

0.2 to 0.70.7 to 0.91.2 to 1.4

Thus, for example, if the supply and room air temperatures are 15 and 22 °C, respectively, for a mixing system with the air supplied and extracted via ceiling mounted diffusers/grilles, then the ventilation effectiveness is between 0.9 and 1.0. A displacement system with supply and room temperatures of 18 and 22 °C has an effectiveness value between 1.2 and 1.4.

Example (calculation of flowrate)

The carbon dioxide (CO2) pollutant emission rate for one person is 5 × 10−6 m3 · s−1. The room CO2 pollutant concentration to be sustained is 1000 ppm while the fresh air CO2 concentration is 350 ppm. Find the required supply flowrate per person to keep the internal pollutant concentration at 1000 ppm. Assume there are no other occupants or pollution sources, the air is well mixed in the room, and that Ev = 0.80.

The required flowrate is:

�Vsupply = ¥-( ) ¥

-

-5 10

0 8 1000 350 10

6

6. = 5 / 520 = 9.62 × 10−3 m3 · s−1 or 9.62 l · s−1

Table 3.3:Ventilation efficiencies for simple mixing and displacement systems(7)


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y = -0,3388x + 6,5749R 2 = 0,9683

6100

6200

6300

6400

6500

6600

6700

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Time (h)

Ln g

as c

once

ntra

tion

Figure 3.7:Plot of the natural logarithm of tracer gas concentrations versus time. The slope of a best fit line indicates the average infil-tration rate during the test period (for this example the air change rate was 0.34)

3.3 Measuring ventilation and IAQ

3.3.1 Measurement of flowrates

Tracer gas methods

In naturally ventilated spaces, the average ventilation rate may be found by using tracer gas measurement. This involves dispersing a readily detectable, inert and non-toxic gas into the space and observing its concentration value over a given time period. Gases commonly used include sulphur hexafluoride and carbon dioxide. The easiest method is the decay approach, in which the gas is thoroughly mixed in the space and left (under normal ventilation operating conditions) to decay in concentration as it disperses from the space. In a mechanically ventilated room, the overall ventilation rate is found by monitoring the tracer gas concentration in the room exhaust duct. This gives a measure of the average ventilation rate in a space; a high rate of decay indicates a high mean ventilation rate. For a constant ventilation rate and perfect mixing the decay curve is exponential (see Figure 3.7).

Initially all openings are closed (including doors) and the space is injected with a suitable gas such as carbon dioxide. Desk mounted fans are used to promote mixing of the gas with room air. Measurements are made at various locations to ensure that a uniform concentration of gas has been achieved. Vents are then opened or the ventilation system is activated. Records are made of the gas concentrations at selected locations and the corresponding times. It may be assumed that an exponential decay in gas concentration will occur. The graph is made linear by plotting the natural log of the average concentration against time (hours); the slope of a best fit


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line via the experimental data gives the air change rate. By making local measurements (e.g. in the vicinity of a workspace or desk) it is possible to assess the local ventilation rate and the ventilation effectiveness in a particular zone. Figure 3.8 shows the apparatus used to carry out tracer gas injection and sampling in a large naturally ventilated space.

Figure 3.9 shows the variation in CO2 concentrations with time in a room ventilated using infiltration only. Inside and outside temperatures are also illustrated. The room had been occupied up to about 14:45, and the decay in gas concentration occurred after this time.

S

C

PC

GGGA

Injection line

Tracer Gas

Sample line

Figure 3.8:Schematic diagram of tracer gas injection and sampling system (S – sampling, A – analyser, C – control system hardware and interface unit, G – gas supply)

0

200

400

600

800

1000

1200

12 13 14 15 16 17 18

Time (h)

5

10

15

20

25

CON(3)CON(4)

CON(5)Tai

TaoCO

2 co

nc

(pp

m)

Tem

p (

°C)

Figure 3.9:CO2 concentrations (on the lhs (CON(3)) and rhs (CON(4)), and at high level lhs (CON(5))), internal and external air temperatures versus time (maximum occupancy level equalled 50). Infiltration only


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(1) (2)

h

(1) (2) Figure 3.10:Flow measurement device (orifice plate).

Direct measurement methods

Measurement of flowrates for mechanically ventilated systems can be obtained using a calibrated orifice plate mounted in the supply duct (see Figure 3.10)(3). The orifice plates flow coefficient has been found by experiment (i.e. the relationship between flowrate and pressure drop across the plate is known), and if the pressure drop is recorded, then the flowrate can be calculated.

Flowrates using an orifice may be found using:

�V C AP= d E 2

D

r (m3 · s−1) (3.13)

where Cd is the discharge coefficient (~ 0.6 for a sharp edged plate or obstruction), AE is the area of plate opening (m2) = πd2/4, DP is the pressure drop across plate (Pa) and r is the air density (kg · m−3).

A manometer or differential pressure gauge is connected to tubing located one diameter upstream and 0.6 diameters downstream of the plate (for d/D = 0.5 where d is the diameter of orifice plate opening and D is the internal diameter of duct).

Flowrates may also be determined by measuring the air velocity at selected positions within the supply duct. Velocities are generally measured using a pitot tube connected to a pressure gauge or manometer, or using a hot wire probe and meter.

Instantaneous velocities will typically fluctuate, so an average value using (say) five or more readings needs to be calculated. The flowrate is then the


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Figure 3.11:A CO2 sensor and multi-sensor devices for measuring CO2, relative humidity, temperature and air speeds

average velocity times the duct area. An approximate value of flowrate can be established by recording the air velocity at the centre of a duct. The average velocity is about 0.9 times this value, and this can be multiplied by the duct area to find the flowrate.

3.3.2 Measuring IAQ

To measure IAQ, the concentrations of gases such as CO2 need to be determined.

A good indication of IAQ is the concentration of the dominant pollutants (e.g. CO2, CO, VOCs, moisture and particulates). Spot checks may be made using a hand-held device (which may also record temperature and relative humidity), see Figure 3.11. A CO2 sensor placed in the main extract duct of a mechanical ventilation system may be used to monitor and control flowrate. A record of the CO2 concentration can indicate if fresh air flowrates are adequate. Carbon monoxide should always be monitored close to combustion appliances. This is now a requirement in many cases. Relatively high CO concentrations point to problems with combustion appliances and are extremely dangerous.


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Method Description Typical contaminant detected

Gas chromatography(using following detectors)

Flame ionization

Flame photometry

Photoionization

Mass spectroscopy

Separation of gas mixtures by time of passage down absorption column.

Change in flame electrical resistance caused by ions of pollutant.

Measures light produced when pollutant ionized by flame.

Measures ion current for ions created by UV light.

Pollutant molecules charged, passed through electrostatic magnetic fields in vacuum; path curvature depends on mass of molecule allowing separation and counting of each type.

Volatile non polar organics; sulphur; phosphorous; most organics excluding CH4; halogenated and nitrogenated organics; VOCs with boiling points below 65 °C.

Infrared spectroscopy Absorption of infrared light by pollutant gas in a transmis-sion cell; a range of wavelengths is used, allowing identification and measurement of individual pollutants.

Acid gases; carbon monoxide; many organics; any gas with an absorptionband in the infrared.

High performance liquid chromatography

Pollutant is captured in a liquid, which is then passed through a liquid chromatograph (analogous to a gas chromatograph).

Aldehydes; ketones; phosgene; nitrosamines; cresol; phenol.

Colorimetry Chemical reaction with pollutant in solution yields a coloured product whose light absorption is measured.

Ozone; oxides of nitrogen; formaldehyde.

Electrochemical Pollutant is bubbled through reagent/water solution, changing its conductivity or generating a voltage.

Ozone; hydrogen sulphide; acid gases.

UV absorption Absorption of UV light by a cell through which the polluted air passes is measured

Ozone; aromatics; sulphur dioxide; oxides of nitrogen; carbon monoxide.

Chemiresistor (metal oxide) Contaminant interacts with coated metal oxide surface at high temperature, changing the resistance to electrical current

Carbon monoxide; hydrogen sulfide;organic vapours

Table 3.4:Methods of measuring gaseous and VOC concentrations(5)

Particle concentrations should be monitored if particles are thought to be a problem. Commercial particle counters are expensive, but are usually available for hire.

Table 3.4 provides a summary of the various methods used for detecting gases and VOCs and for measuring pollutant concentrations.


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Table 3.5 gives a listing of variables to be measured when completing a routine IAQ assessment in office spaces.

Factor Recommended measurements

Always Additional If applicable*

Temperature Air temperature √

Temperature gradient √

Daily temperature rise √

Operative or radiant temperature

√

Relative humidity

√

Air velocity Mean √

Turbulent intensity √

Organic compounds

Total VOC √

Main individual VOC √

Formaldehyde √

Other aldehydes √

Methane √

Nitrogen dioxide

√

Carbon dioxide √

Carbon monoxide

√ √

Ozone √

Radon √

Table 3.5:Factors to be measured for routine IAQ assessment (offices)(19)


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Table 3.5:Continued

Factor Recommended measurements

Always Additional If applicable*

Airborne particles

Asbestos √

PM10 √

PM2.5 √

Bacteria √

Fungi √

Deposited dust (furniture)

Allergens √

Deposited dust (floor)

Allergens √

Organic component √

Bacteria √

Outdoors Air temperature √

Relative humidity √

Wind direction √

Wind velocity √

Any chemical measured internally

√

* If variables significantly different from expected values


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4 Achieving optimum indoor air quality

4.1 Reducing the risk of poor IAQ

Optimum IAQ is achieved by:

⎯ minimising sources and flows of pollutants, e.g. by replacing certain materials by equivalent items with lower emission rates

⎯ using the recommended fresh air supply rates (see section 3.2)⎯ ensuring a well designed air distribution (ductwork) system⎯ filtration⎯ ensuring that air intakes are not adjacent to local outdoor pollutant

sources⎯ complying with best practice, i.e. using good operating procedures

and allowing adequate time and resources for commissioning and maintenance of ventilation plant

⎯ selecting materials to minimize the emission of avoidable indoor pollutants such as VOCs.

Pollutant concentrations can also be minimised by removing pollutant sources, e.g. by replacing certain materials with equivalent items with lower emission rates.

4.1.1 Fresh air supply rates

Adequate ventilation provides enough ‘treated’ fresh air (at least 10 l/s per person). The volume per person will depend on the pollution loads within a space. See section 3.2.1 for information on prescribed flowrates for dwellings and non-domestic buildings.

4.1.2 Well designed air distribution systems

Well designed ventilation systems will effectively distribute supply air and remove stale air. Local extract systems should be used to extract contaminants from specific areas.

The effectiveness of a particular ventilation system can be defined in terms of the ratio between the pollutant concentration in the exhaust and the average room pollutant concentration. The local ventilation effectiveness is the ratio of the exhaust contaminant concentration and the corresponding value at a given location. Values for ventilation effectiveness are given in Table 3.3.


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Figure 4.1:Mixing ventilation; supply air is completely mixed with room air

Figure 4.2:Displacement ventilation; supply air is introduced near the floor

Mixing systems (see Figure 4.1) will introduce heated or cooled air at high velocities and aim to achieve a uniform air temperature in a space. Displacement systems (see Figure 4.2) generally provide air at temperatures a few degrees below room temperature at low level and extract stale air at ceiling height. Lowest temperatures will occur at floor level and highest values at ceiling level. Generally displacement (ventilation) systems have greater ventilation efficiencies than mixing systems for typical supply and room air temperatures.

Mixing ventilation systems (with air supply and exhaust at high level) have a ventilation effectiveness of 0.4 to 0.7 assuming a supply/room air temperature differential of more than 5 °C whereas displacement systems have ventilation effectiveness of 1.2 to 1.4 assuming the supply air temperature is below the room air temperature.


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4.1.3 Filtration

Filters will generally be installed to remove particles of size 0.1 to 10 µm from external and recirculated air. Diffusion and electrostatic precipitators are used to remove smaller sized particles from air streams. Large particles can be extracted using viscous film filters, cyclones and settling chambers.

Most filters will not generally reduce gaseous pollutant levels. Activated carbon filters can be used to remove some odours and VOCs.

Filters have classes G1 to G4 (coarse filtration, for ‘general’ building applications), F5 to F9 (fine dust filters with efficiencies from 40 to 95 %), H10 to H14 (HEPA or high efficiency particulate air filters with very high efficiencies, for use in cleanrooms) and U15 to U17 (ultra high efficiency filters, again for cleanroom applications). For most buildings, class G and F panel or bag filters are used. See Figure 4.3.

10 0000·01 0·1 1·0 10Particle diameter / µm

100 10002 3 4 5 6 7 89 2 3 4 5 6 7 89 2 3 4 5 6 7 89 2 3 4 5 6 7 89 2 3 4 5 6 7 89 2 3 4 5 6 7 89

Settling chambers

Low pressure-drop cyclones

High efficiency cyclones

Viscous film filters

Impingement filters (dry)

Electrostatics precipitators

Diffusion filters

Nom

inalaperture

335328122411

167614051003

853699599500422

295

211

15210489766653

567810121618222530365272

100150170200240300

Mesh

Figure 4.3:Range of particle sizes dealt with by different types of filter(6)

Further information on filters is provided in the CIBSE Guide B, chapter 2, Ventilation and air conditioning(6).

4.1.4 Regular maintenance of ventilation plant

Indoor air quality levels can be increased by regular maintenance of ventilation plant. This can include replacing filters at the required intervals and/or when filter pressure drops become excessive, cleaning of heating/cooling coil surfaces and ducts and the cleaning/ chemical treatment of humidifiers.

The changing of filters and the cleaning processes outlined above will have the added benefit of reducing electrical (fan) energy consumption(6).


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4.1.5 Selection of materials to minimize pollutant emissions

Materials used should be carefully selected such that they emit little or no VOCs and/or particles. See Table 4.1. Suitable materials include untreated wood, stainless steel, plastic coated steel (as used in cleanrooms) and ceramic tiles.

Material Pollutant Minimising pollutant emissions

Carpets with PVC backing

Alcohols, alkanes, esters, other VOCs

Limit use of adhesives.Fresh air ventilation.Cleaning/vacuuming.

MDF partitions Formaldehyde, methylpentane, alkanes, aldehydes, ketones, esters

Use unpainted natural woods.Fresh air ventilation.Cleaning.

Plywood partitions Formaldehyde, aldehydes, ketones, esters, alcohols, other VOCs

Use unpainted natural woods.Fresh air ventilation.Cleaning.

Timber beams, frames and studs

Aldehydes, other VOCs Use untreated timber, use water based paints.Fresh air ventilation.

Vinyl wall coverings Alkanes, toluene, ketones, alcohols, other VOCs

Use paper wall coverings.Fresh air ventilation.Cleaning.

European standards and product directives have been developed for various materials and products (including those materials used for internal floors, within and on walls/ceiling surfaces and points/solvents). A European Standard for measuring VOC emissions from new building products, BS EN ISO 16000-10, was published in 2006. This provides a means for testing VOC emissions from materials and can be used as a basis for certification of a product(20).

Table 4.1:Some materials which emit pollutants and how the release of pollutants might be reduced(20)


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Further reading

Awbi HB Ventilation of buildings(3)

CIBSE Comfort CIBSE Knowledge series KS06(10)

CIBSE Natural ventilation in non-domestic buildings CIBSE AM10(15)

CIBSE Ventilation and air conditioning ch. 2 in CIBSE Guide B(6)

Edwards R Handbook of domestic ventilation(7)


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References

1 HM Government Building Regulations Approved Document F: Ventilation (2010 Edition) (London: RIBA) (2010)

2 HM Government Building Regulations Approved Document L1A: Conservation of fuel and power

– New dwellings (2010 edition); Approved Document L1B: Conservation of fuel and power –

Existing dwellings (2010 edition); Approved Document L2A: Conservation of fuel and power –

New buildings other than dwellings (2010 edition); Approved Document L2B: Conservation of fuel

and power – Existing buildings other than dwellings (2010 edition) (London: RIBA) (2010)

3 Awbi H B Ventilation of Buildings (London: Spon) (2003)

4 Fanger P O Thermal comfort: analysis and applications in environmental systems (New York: McGraw Hill) (1972)

5 American Society of Heating, Refrigerating and Air-Conditioning Engineers ASHRAE Handbook

– Fundamentals (Atlanta, GA: ASHRAE) (2009)

6 Chartered Institution of Building Services Engineers CIBSE Guide B: Heating, ventilation, air

conditioning and refrigeration (London: CIBSE) (2005)

7 Edwards R Handbook of domestic ventilation (Oxford: Elsevier: Butterworth-Heinemann) (2005)

8 Chartered Institution of Building Services Engineers CIBSE TM40: Health issues in building

services (London: CIBSE) (2006)

9 Chartered Institution of Building Services Engineers CIBSE Guide A: Environmental design (London: CIBSE) (2006)

10 Chartered Institution of Building Services Engineers CIBSE Knowledge Series KS06: Comfort (London: CIBSE) (2006)

11 Clancy E Analysis of energy consumption with a factory (Internal report for private client) (2006) (unpublished)

12 Chartered Institution of Building Services Engineers CIBSE TM21: Minimising pollution at air

intakes (London: CIBSE) (1999)

13 DEFRA http://uk-air.defra.gov.uk/ (Accessed 15 June 2011)

14 European Committee for Standardisation (CEM) CR 1752 Ventilation for buildings – Design

criteria for the indoor environment (Brussels: CEN) (1998)

15 Chartered Institution of Building Services Engineers (CIBSE) AM10: Natural ventilation in

non-domestic buildings (London: CIBSE) (2005)

16 Building Services Research and Information Association BSRIA illustrated guide to mechanical

building services (Bracknell: BSRIA) (2002)

17 BSRIA A guide to HVAC Building Services Calculations (2nd Edition) (Licensed CD: BG 30/2007CD) (Bracknell: BSRIA) (2007)

18 Chartered Institution of Building Services Engineers CIBSE AM13: Mixed mode ventilation (London: CIBSE) (2000)

19 Crump D, Raw G J, Upton S, Scivyer C, Hunter C and Hartless R A protocol for the assessment

of indoor air quality in homes and office buildings BR450 (Watford: BRE) (2002)

20 Crump D and Yu C VOC emissions from building products, Parts 1 and 2 BRE Digest 464 (Watford: BRE) (2002)


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