MSc: AEES DL, Module CEM160, Lecture: Principles of Natural Ventilation 1

IntroductionThis lecture will primarily cover the basic principles of ventilation, the main focus being on natural strategies. A good introduction to the topic can also be found in McMullan (2007, pp. 73-77). This lecture will cover the following topics:

Why ventilation is needed ●

How much ventilation is needed, i.e. ventilation rates ●

Approaches to ventilation ●

Energy use ●

Thermal Comfort ●

Wind effects on ventilation ●

Methods of ventilation ●

Infiltration ●

Modelling ventilation ●

Why ventilation is neededVentilation is the movement of air in and out of a building. It should serve two purposes:

To provide fresh air for occupants and to dilute and exhaust 1. pollutants; in other words, to provide an appropriate level of indoor air quality (IAQ).

To provide cooling in buildings which are overheating.2.

Ventilation can result in energy being lost from the building. Warm air lost from a building will be replaced by cool air, which has to be heated to the required temperature. In hot climates the situation may be reversed with ventilation bringing warm air into a building.

Successful ventilation design should provide for good air quality, i.e. low levels of pollutants. It should also ensure occupant comfort via spaces that are ‘airy’ but not draughty and ‘fresh’ but not stuffy. Too little ventilation puts the health and comfort of the occupants at risk, while

Principles of Natural Ventilation Mike Thompson (2000)*

* Revised and edited by Sam Saville (2008), Kelvin Mason (2008), Frances Hill and Kate Millbank (2011).

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2 Lecture: Principles of Natural Ventilation, Module CEM160, MSc: AEES DL

Progressive building designs find better ways of achieving this balance. The Passive House (Passivhaus) concept for example, is based on the principle of achieving a very low energy building with limited air infiltration (0.6 or less air changes per hour). Instead Passivhaus designs rely on a controlled ventilation strategy using mechanical ventilation with heat recovery (MVHR). This system provides sufficient fresh air to the indoor environment whilst also reducing the energy required to heat a property. It does this by pre-heating any incoming fresh air with ‘recovered’ heat energy from exhaust stale air using a heat exchanger.

Ventilation ratesAll occupied spaces need to be ventilated. Ventilation rates depend on the occupancy and type of building. Details for different building types and different occupancy levels can be obtained from the UK Chartered Institution of Building Services Engineers guide (Guide B: Heating,

too much results in an energy loss with consequent cost and possibly carbon emission penalties.

In practice, it is very difficult if not impossible to prevent air movement in and out of buildings, particularly in older designs. This process of unwanted ventilation is termed air infiltration. Many building materials and most constructions are somewhat permeable to infiltration. Though infiltration through building elements may be small, over a large area this this can amount to significant heat loss. Gaps, cracks, chimneys and ill-fitting components are all pathways for air infiltration.

Modern buildings generally have better control over ventilation than older ones. However in the drive towards improving energy performance care must be taken to avoid hermetically sealing indoor spaces. Typically, buildings are designed on the principle ‘build tight’ and ‘ventilate right’ in order to provide building occupants with a balance of good air quality, comfortable conditions and efficient use of energy.

CIBSE have published their Guide B: Heating, Ventilating, Air Conditioning and Refrigeration report, which includes details on ventilation a p p r o a c h e s , ventilation rates and how to assess ventilation requirements.

Table 1: Ventilation rates according to building use. Source: CIBSE Knowledge Series KS6: Comfort

Building type Ventilation rate Air changes per hour

Domestic – habitable rooms 0.4 - 1

Domestic – bathrooms 15 (l/s) 3

Kitchens 60 (l/s)

Offices 10 (l/s per person) 1-2

Retail 10 (l/s per person)

Bars 15 10-15

Part F of the UK building regulations can be downloaded here

The Passivhaus website provides a good overview of Passivhaus design principles and construction standards.

For a guide to typical ventilation requirements see E n v i r o n m e n t a l Science in Building (sixth edition) by Randall McMullan. Chapter 4 ‘Air Control in Buildings, pp. 74

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MSc: AEES DL, Module CEM160, Lecture: Principles of Natural Ventilation 3

The ventilation rate is related to the air change rate by the following equation:

Vr = V × ACH × 1000

3600

where

Vr = ventilation rate (l/s) ●

V = volume of indoor space ●(m3)

ACH = air change per hour ●

3600 = seconds in an hour ●

1000 = litres per cubic CM ●

The recommended fresh air ventilation requirements for spaces with high occupancy levels cause problems in naturally ventilated buildings in cold climates in winter. When the air is cold outside, high ventilation rates mean an energy penalty and heat recovery is not usually feasible for naturally ventilated spaces. Moreover, the large volumes of incoming cold air are also likely to cause thermal discomfort. In such cases, controlled mechanical ventilation generally gives a better solution. The inverse parallel problem can occur in hot climates, where the air change rate suggested by high occupancy can result in the building overheating.

Ventilating, Air Conditioning and Refrigeration), although there is a fee for downloading the publication. For a guide to typical ventilation requirements see McMullan (2007, p74). Part F of the Building Regulations also gives guidance for achieving adequate IAQ levels in the UK. Table 1 summarises some of the main ventilation requirements for different spaces. These requirements are for people alone. Additional ventilation may be needed for any process that produces pollutants or heat, as Figure 1 shows.

The units for expressing ventilation rate are:

Air changes per hour (ACH) ●which, logically enough, is simply the number of times the air in a space is changed each hour.

The occupancy ventilation ●rate is measured in litres per second per person, which is used to specify ventilation rates in spaces that may vary in size but have known and relatively high occupancy levels (such as a school) and are largely mechanically ventilated.

Figure 1: Fresh air ventilation requirements

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4 Lecture: Principles of Natural Ventilation, Module CEM160, MSc: AEES DL

Approaches to ventilationBy definition, mechanical ventilation, which can vary from a simple fan to full air conditioning, involves the use of energy, usually by means of electricity. Electricity is generally an expensive form of energy and in most situations its generation results in carbon emissions, contributing to the problem of climate change.

Carefully designed, natural processes can provide ventilation at low or zero running costs. Natural ventilation is often considered to be the most energy efficient and healthy solution. It is a fundamental part of ‘passive design’, which involves the integration of daylight, thermal mass, insulation and solar radiation into the overall concept of a building.

In some situations natural ventilation strategies will need extra consideration due to:

Noise infiltration in urban areas - research into this suggests ●balconies can attenuate some noise from street canyons (see Wilson et al, 2005).

Health and Safety: the height of windows might be an issue for health ●and safety and security. Also smoke will follow ventilation paths, so fire strategies need consideration.

The majority of contemporary approaches to ventilation, particularly in non-domestic buildings, tend to opt for hybrid approaches. Incorporating some mixture of natural and artificial ventilation is known as ‘mixed mode’. This could mean using natural ventilation in summer and mechanical during winter. Mixed-mode often represents the most energy efficient solution to ventilation requirements. Figures 2 and 3 illustrate examples of the different ventilation techniques. The ‘youngest’ air is nearest the supply and, for efficient air distribution, the oldest air should be nearest the exhaust.

Seminar Discussion Point: Is natural or mechanical ventilation the way forward for sustainable buildings of the future that are trying to balance high levels of energy ef fic iency with adequate ventilation and internal air quality?

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MSc: AEES DL, Module CEM160, Lecture: Principles of Natural Ventilation 5

Figure 2: Design detail of natural ventilation system

Figure 3: Mixed mode ventilation

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6 Lecture: Principles of Natural Ventilation, Module CEM160, MSc: AEES DL

Energy and ventilationDepending on the climate characteristics and season, the air supplied to a space may have to be heated or cooled. For mechanical ventilation this heating or cooling is provided by preheating or cooling before delivery. For natural ventilation, preheating is sometimes achieved using solar energy. Alternatively, cool air drawn into the building can exchange heat in some way with warm air being expelled. Mechanical ventilation systems generally allow the possibility of heat recovery in cold climates. Some typical figures of ventilation heat losses for the UK climate are shown in Figure 4 and Table 2.

The heat lost or gained by ventilation can be calculated using a formula, please refer to appendix A(1) ‘Ventilation Calculations’.

BedZED provides an excellent example of pre heating fresh air with its heat recovery wind cowls. See Arup’s report on-line, (especially page 14).

Figure 4: Variation in ventilation heat loss for a typical factory in relation to transmission losses through the fabric and infiltration

Table 2: Typical ventilation heat loss for different building types

Heat loss (KWh/m2/year)

Total heat loss (%)

House 130 25-30

Office 100 30-40

Factory 140 40-50

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MSc: AEES DL, Module CEM160, Lecture: Principles of Natural Ventilation 7

Ventilation and Thermal ComfortThermal comfort is affected by air temperature and air movement. Ventilation obviously has an impact. If the external temperature is lower than the internal, then natural ventilation can be used for cooling if necessary. Another secondary cooling effect results from air movement or draughts. Even if the temperature of the incoming air is the same as that already inside it can feel cool because it is moving. Heat is lost from the body by the consequent evaporation of sweat on the skin. Designers can achieve cooling by utilising this effect even when internal and ambient temperatures are similar.

The air temperature within a space is affected by the temperature of the surfaces with which it comes into contact. Air temperature will vary in a typical building space, creating temperature gradients. These tend to be greater in the vertical plane due to buoyancy effects, i.e. warm air, being less dense, rises. Ventilation air movements can alter this temperature gradient. If air speed in a space is greater than 0.15 to 0.2 m/s it is perceived as a draught, especially if the occupants of the space are engaged in sedentary activity. Draughts can be caused by open windows, vents or fans. In addition, when warm air comes into contact with a cooler surface, for example the glazing of a skylight, it cools and can create a down draught: cool air, being more dense, ‘falls’. In mechanically ventilated spaces, down draughts can be caused by streams of warm air jets coming into contact with, for instance, cooler walls. In order to maintain thermal comfort, designers need to be aware that higher air temperatures will be needed in heated spaces where there is a lot of air movement.

In hot climates higher air speeds become desirable for their cooling effect on thermal comfort. However, air movement at speeds greater than about 0.5 m/sec can cause practical problems such as papers blowing around. Figure 5 shows how draughts can result from both natural and mechanical ventilation.

Figure 5: Draughts from natural and mechanical ventilation

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8 Lecture: Principles of Natural Ventilation, Module CEM160, MSc: AEES DL

Ventilation effects due to windWind flow around buildings is complex but statistical analysis can be carried out to identify the direction and strength of any prevailing wind. This is described by the Wind Rose Diagram, an example of which is shown in Figure 6.

Figure 6: Example of a wind rose

Dynamic and Static wind pressuresStatic Pressure, Ps, is the pressure of the free flowing air stream and is equal to atmospheric pressure. Dynamic pressure, Pd, is the wind pressure exerted on the building and is equal to:

Pd = 0.5ρv2

Where: ρ is the density of air (kg/m) v is the velocity of the wind (m/s)

The total pressure on a building is the sum of the two pressures, Ps + Pd, and is measured in Pascals (Pa), where 1 Pa = 1 kg/ms2.

So the example Wind Rose (shown in Fig 6), illustrates that the strongest and most frequent winds are coming from the West.

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MSc: AEES DL, Module CEM160, Lecture: Principles of Natural Ventilation 9

Figure 9: Wind flow around different forms

Figure 7: Cross ventilation driven by wind pressure differences

Figure 8: The effect of building design on wind pressuresWind effects on buildings and surroundingsThe form of the building can have a significant influence on the way a natural ventilation system is able to work. Pressure differences, as indicated in Figures 7 to 9, can encourage ventilation. Various devices for protection against wind may be used. These generally operate best if about 50% of the screen is permeable; solid screening produces highly turbulent recirculation regions. Note the + indicates an area of positive pressure while - indicates negative pressure that ‘sucks’ at the building structure.

Note that forms such as trees and walls can be used to provide shelter, creating areas of positive or negative pressure. Pressure gradients across the buildings can facilitate natural ventilation if required.

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10 Lecture: Principles of Natural Ventilation, Module CEM160, MSc: AEES DL

Pressure coefficients We have seen that the pressure at any point on a building envelope depends on building form and local topography. It can be calculated for a given wind speed and direction by using the pressure coefficient (Cp). This is defined as the ratio of the dynamic pressure (Pd) on the envelope of a building, to the pressure at the wind speed reference height, effectively the static pressure (Ps). The pressure coefficient is normally only derived from wind tunnel tests. The value of Cp. will be positive on the windward side of the building and negative on the leeward side, as shown in Figure 10. The pressure difference ΔP across a building due to wind can be calculated using:

ΔP = 0.5ρv2 (Cp1 - Cp2)

Where Cp1 and Cp2 are the pressure coefficients for the windward and leeward sides of the building. For an example of this please refer to appendix A (2) ‘Ventilation Calculations’.

Figure 10: Windward and leeward pressures

Homebui ld ing and Renovation Magazine provide a useful summary of wind assisted passive stack ventilation at BedZED, whilst BDSP (environmental consultants for London’s Iconic ‘Gherkin’ building designed by Norman Foster), feature some useful modelling images of passive stack ventilation and air flow (click on Projects → Highrise → 30 St Mary Axe).

Another good description of the passive stack effect can be found on the Green Building Advisor website

Here is a YouTube clip showing airflow modelling for Norman Foster’s Swiss Re building (otherwise know as the Gherkin or 30 St Mary Axe).

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MSc: AEES DL, Module CEM160, Lecture: Principles of Natural Ventilation 11

Methods of natural ventilation

The Stack EffectReiterating what we know: As air is heated it becomes less dense and rises; as air cools it becomes denser and falls. The stack effect occurs when there is a difference between the inside and outside temperature, as in Figure 11. If the inside air temperature is warmer than the outside air, it will be less dense and more buoyant and will rise through the space. This creates a relatively high pressure zone in the upper level of the building. The warm air will then tend to exhaust from the space through leakage and openings. It will be replaced by cooler, denser air drawn into the relatively low pressure zone in the bottom floor of the building.

Figure 11: The Stack effect. Source HUD

The stack effect increases with increasing internal/external temperature difference. It also increases as the height of the building increases, i.e. the difference in height between air inlet and exhaust. A neutral plane, where internal and external pressure are equal, will exist somewhere between. Above the neutral plane the internal air pressure will be positive relative to the neutral plane and air will exhaust through any available openings. Below the neutral plane, the internal air pressure will be relatively negative and external air will be drawn into the space. The further away a space is from the neutral plane, the greater the pressure difference and the greater the airflow in that space.

To know how to calculate the pressure difference due to a stack effect, please see appendix A (3) ‘Ventilation Calculations’.

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12 Lecture: Principles of Natural Ventilation, Module A3/ CEM 160, MSc: AEES DL

Figure 12: Forces that give rise to internal air movement

Mechanisms for internal air movementAs shown in Figure 12, internal air movement is caused by:

Pressure difference due to winds and the stack effect, ●

Internal air jets from windows or generated by mechanical fans, ●

Downwards draughts from cold surfaces, ●

Heat given off from people or radiators. ●

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Air movement and building formFigure 13 illustrates natural ventilation strategies for various building forms. If natural ventilation is the preferred choice, this has a large influence on the building form. Mechanical ventilation does not have this direct influence. However, the space needed in the building to accommodate the various devices together with the pipes and ducts to deliver heat or cooling can be very considerable.

Figure 13: Natural ventilation strategies in relation to building form

MSc: AEES DL, Module A3 /CEM 160, Lecture: Principles of Natural Ventilation 13

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14 Lecture: Principles of Natural Ventilation, Module A3/ CEM 160, MSc: AEES DL

Solar ChimneysFigure 14 shows an example of a solar chimney. Solar chimneys can be constructed in a narrow, vertical passage, such as a conventional chimney previously used for expelling smoke. An absorber, typically metal painted with a selective black coating, is situated behind a glazed front that is exposed to solar radiation. The chimney space is well insulated from the house. For the device to work, the chimney must terminate above the roof level. A rotating metal turbine is constructed with an integral opening that faces away from the prevailing winds. Thus, heated air can exhaust without being blown back down the chimney. The

Figure 14: Solar chimney. Source: Greenbuilder

rotating turbine increases the flow of air due to the stack effect.

A similar principle is adopted by the evaporative downdraft cooling tower (Figure 15) which is closely related to the solar chimney. Rather than rely on the process of hot air rising to create passive stack effect, evaporative downdraft cooling uses the process of evaporation from water (present at the top of the cooling tower) to cool incoming air and draw it down into the building. This can offer a good cooling strategy and alternative to air conditioning in hot climates.

Figure 15: Passive Downdraft Cooling. The Zion National Park Visitor Center. Source: National Renewable Energy Laboratory

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MSc: AEES DL, Module CEM160, Lecture: Principles of Natural Ventilation 15

LouvresLouvres are used to provide a weather protected ventilation opening. They are used as the air intake/exhaust for mechanical ventilation systems, for natural ventilation in some industrial buildings (for example where there is a process generating a lot of heat that requires high rates of natural ventilation) for offshore platforms, and on ventilation turrets. Louvres are usually calibrated by their manufacturer in terms of a pressure flow relationship.

WindowsDifferent window designs will produce different air flows and ventilation rates, as shown in Figure 17. Heiselberg et al (2001) provide a detailed analysis of the characteristics of airflow from open windows.

The air flow through windows can either be one-directional or, as in the case of stack flow, air can enter at the lower zone and leave at the upper zone. There are a number of theoretical methods for predicting the flow through a large opening, including the de Gids and Phaff method. These can be found

Figure 16: Turret with Louvres for high exhaust level

Figure 17: Air flow patterns for different window types

in appendix A (4) ‘Ventilation Calculations’.

However, as Tine Larsen and Per Heiseberg (2008) point out, calculating the flow through even a single window is more complicated than it seems:

‘Even though opening a window for ventilation of a room seems very simple, the flow that occurs in this situation is rather complicated. The amount of air going through the window opening will depend on the wind speed near the building, the temperatures inside and outside the room, the wind direction, the turbulence characteristics in the wind and the pressure variations caused by e.g. wind gusts. Finally, it also depends on the size, type and location of the opening. Many of these parameters are unsteady which makes the calculation of air-change rates even more complicated...’

(pp.1031)

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16 Lecture: Principles of Natural Ventilation, Module CEM160, MSc: AEES DL

InfiltrationAs already discussed infiltration is the uncontrolled flow of air through openings or cracks in the building envelope. This occurs when there is a pressure differential due to wind, temperature difference or mechanical ventilation. The extent of infiltration will depend on the air tightness of the building, the greater the air leakage the larger the infiltration rate.

The flow through cracks and openings can be predicted given the size and nature of the crack or opening and the pressure difference across it. Not all cracks and openings allow air to pass through them with equal ease.

Typical Infiltration and air leakageTable 3 shows some typical infiltration coefficients of windows and Table 4 shows some effective air leakage areas of building components.

Figure 18: Typical air leakage and infiltration in a building. Source: Infrared Visions

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MSc: AEES DL, Module CEM160, Lecture: Principles of Natural Ventilation 17

Figure 19: CIBSE crack length model of infiltration

Window Type Infiltration coefficient for pressure difference of 1Pa

Horizontal or vertically pivoted, weather stripped 0.05

Horizontally or vertically pivoted, non-weather stripped 0.25

Horizontally or vertically sliding weather stripped 0.125

Horizontally or vertically sliding , non-weather stripped 0.25

Table 3: Infiltration coefficients for window systems

For tall buildings: CIBSE crack length modelThis model can be used to predict the ventilation rate for tall buildings, given an effective perimeter crack length of the window framing. It allows for wind and stack effects, building height, location and window system. The method uses a chart technique. The sequence for using the chart (Figure 19) is as follows:

Enter the building height on the left-hand horizontal axis;1.

Plot a line vertically until it intersects with the sloping line appropriate to the general 2. terrain in which the building is situated;

Plot a line horizontally until it intersects with the sloping line which is appropriate to 3. the window system;

Plot a line vertically to read off the infiltration rate on the top right horizontal axis. This 4. is the rate per unit length of open window joint.

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18 Lecture: Principles of Natural Ventilation, Module A3/ CEM 160, MSc: AEES DL

Building components Unit Best estimate RangeCeiling

general cm2/m2 1.8 0.79 - 2.8drop cm2/m2 0.19 0.046 - 0.19recessed lights cm2/each 10 1.5 - 21surface-mounted lights cm2/each 0.82

Doors single, not weatherstripped cm2/each 21 12 - 53single, weatherstripped cm2/each 12 4 - 27double, not weatherstripped cm2/m2 11 7 - 22double, weatherstripped cm2/m2 8 3 - 23interior (stairs) cm2/lmc 0.9 0.25 - 1.5mail slot cm2/lmc 4

Walls (exterior) cast-in place concrete cm2/m2 0.5 0.048 - 1.8clay brick cavity wall (finished) cm2/m2 0.68 0.05 - 2.3precast concrete panel cm2/m2 1.2 0.28 - 1.65low-density concrete block (unfinished) cm2/m2 3.5 1.3 - 4low-density concrete block (painted) cm2/m2 1.1 0.52 - 1.1high-density concrete blk. (unfinished) cm2/m2 0.25

Windowsawning, not weatherstripped cm2/m2 1.6 0.8 - 2.4awning, weatherstripped cm2/m2 0.8 0.4 - 1.2casement, not weatherstripped cm2/lmc 0.28 casement, weatherstripped cm2/lmc 0.24 0.1 - 3double-hung, not weatherstripped cm2/lmc 2.5 0.86 - 6.1double-hung, weatherstripped cm2/lmc 0.65 0.2 - 1.9single-hung, weatherstripped cm2/lms 0.87 0.62 - 1.24single horizontal slider, weatherstripped cm2/lms 0.67 0.2 - 2.06single horizontal slider, wood cm2/lms 0.44 0.27 - 0.99single horizontal slider, aluminium cm2/lms 0.8 0.27 - 2.06storm inside, heat shrink cm2/lms 0.018 0.009 - 0.018window sill cm2/lmc 0.21 0.139 - 0.212

Electrical outlets/switchesno gaskets cm2/each 2.5 0.5 - 6.2with gaskets cm2/each 0.15 0.08 - 3.5

Piping/plumbing/wiring penetrationsuncaulked cm2/each 6 2 - 24caulked cm2/each 2 1 - 2

Vents bathroom with damper closed cm2/each 10 2.5 - 20bathroom with damper open cm2/each 20 6.1 - 22

Notes: 1. lmc = linear metre of crack; lms = linear metre of sash. 2. Data based on a pressure difference of 4 Pa and CD = 1. 3. Data source: 1997 ASHRAE Fundamental Handbook, Ch. 25.

Table 4: Effective air leakage areas of building components. Source: Hong Kong University

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MSc: AEES DL, Module CEM160, Lecture: Principles of Natural Ventilation 19

Modelling and VentilationThere are models that enable the whole building ventilation or infiltration rate to be predicted, such as the British Standard ventilation prediction model. These models combine empirical knowledge and simple theory. They can be used to predict ventilation rates in response to the location and distribution of air leakage and openings over the building envelope. Some these models include: the CIBSE single-sided model, the crack length model (Figure 19) and de Gids and Phaff (Appendix A(4)).

Computational Fluid Dynamics (CFD) is now widely used and can quickly provide accurate, visual results.

ConclusionThis lecture has covered natural ventilation along with some types of mechanical ventilation. We have seen that ventilation can provide clean air and cooling. Ventilation rates depend on the type and function of the building, occupancy and the nature of that occupancy: a small hotel kitchen in which a team of chefs are at work will require more ventilation than a domestic sitting room. Natural ventilation can provide cooling and clean air with no energy input, as required for mechanical systems. Running costs and carbon emissions are therefore likely to be reduced. Where natural ventilation cannot meet the needs of building occupants, hybrid (mixed-mode) systems can be considered.

Ventilation of any sort is likely to exhaust warm air from a building and so has an ‘energy penalty’. Thermal comfort and ventilation are closely related and both air temperature and air movement (draughts) must be considered in this regard. We have considered wind effects and how these can cause areas of positive and negative pressures in the envelope of a building. Methods of natural ventilation we have considered include the stack effect and, of course, windows. Infiltration has been considered as unwanted and uncontrolled ventilation. Finally, we have looked very briefly at modelling ventilation and infiltration. The numerous equations we have considered throughout (and in Appendix A) should help with getting a quantitative feel for the theory. It is not the intention that they should be committed to memory, however!

The Journal of Ventilation website might be a useful place to start further research

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Appendix A ‘Ventilation Calculations’The following appendix provides calculations and formula relating to highlighted areas within the lecture.

1. To calculate heat lost or gained by ventilation: Q (Watts) = M.cp. ΔT

Where: M = mass flow rate of air (kg/s) cp = specific heat capacity of air at relevant temperature (J/kgK) ΔT = Temperature difference Ti – Ta where Ti is the internal temperature and Ta is ambient temperature (K)

For a first estimate, the relevant temperature of the air can be taken as (Ti + Ta)/2.

Using values from engineering tables for the density of air at the relevant temperature, it is possible to convert flow rates of air measured in either litres per second or ACH into mass flow rates, and thence calculate the heat loss or gain.

2. A worked example of Pressure coefficients: What is the pressure difference across a building at the windows, if the pressure coefficients at the windward and leeward windows are 0.5 and -0.4 respectively, the wind speed is 5m/s and the density of air at ambient temperature is = 1.2 kg/m3?

ΔP = 0.5ρv2 (Cp1 - Cp2) = 0.5 x 1.2 x 52 x [5 – (-4)] = 0.5 x 1.2 x 25 x 0.9 = 13.5 Pa

3. The pressure difference due to a stack effect can be calculated using the following equation: ΔPs = P2 – P1

Where: P1 is the pressure (Pa) at a height, h1, say downstairs P2 is the pressure (Pa) at a height, h2, say upstairs ΔPs = ρ2V2g/A2 – ρ1V1g/A1

Where: ρ2 is the density of air (kg/m3) at temperature T2, the upstairs temperature (K)

ρ1 is the density of air (kg/m3) at temperature T1, the downstairs temperature (K)

The Engineering Tool Box provides tables for the density of air

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MSc: AEES DL, Module CEM160, Lecture: Principles of Natural Ventilation 21

V2 is the volume the upstairs space (m3)

V1 is the volume the downstairs space (m3)

A2 is the combined area of all the internal surfaces in the upstairs space (m2)

A1 is the combined area of all the internal surfaces in the downstairs space (m2)

g is the acceleration due to gravity, assumed constant on this scale (9.81 m/s2)

So:

ΔPs = (ρ2V2/A2 – ρ1V1/A1)g

We can see that in a building such as that in Figure 11, where upstairs and downstairs room areas and volumes are equal, the pressure difference is a function of the difference in air density, which is directly proportional to air temperature.

The stack effect is driven by a vertical internal pressure gradient in relation to external pressure. For calm conditions the external pressure will be uniform and equal to the neutral plane value. When we are considering the difference in pressure between two points h1 and h2 in the same room (space) we can employ a different formula to obtain a reasonable approximation:

ΔPh = ρTAgh(1/Te – 1/Ti)

Where: ρ is the density of air (kg/m3) at the average temperature TA TA (K) is the average temperature (Te + Ti)/2 g is the acceleration due to gravity (9.81 m/s2) h is the height difference (h2 – h1) between the two points (m) Ti is the internal temperature (K) Te is the external temperature (K)

Worked Example

What is the pressure difference between two openings 8m apart (vertically) in an atrium, when the internal air temperature is 26°C and the external air temperature is 8°C?

Ti = 273 + 26 = 299K

Te = 273 + 8 = 281K

The density of air ρ at 290K (TA) is approximately 1.2 kg/m3

ΔPh = 1.2 × 293 × 9.81 × 8 × (1/281 - 1/299) = - 5.91 Pa

N.B. The negative value merely indicates the pressure at h2 is the higher.

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22 Lecture: Principles of Natural Ventilation, Module CEM160, MSc: AEES DL

4. For predicting air flow through open windows the de Gids and Phaff method employs the formulae:

v1. e = (C1vr2 + C2HΔT + C3)

Q = 0.5Av2. e

Where: ve is the effective air speed through the window (m/s) vr is the reference wind speed at the (local) weather station H is the window height (m) ΔT is difference between internal and external temperatures (K) Q is the volume flow rate of air through the window (m3/s) C1, C2 and C3 are the regression constants 0.001, 0.0035 and 0.01 respectively

For large openings, such as open windows and doors, it is possible to model the air flow using the CIBSE single sided model. This can predict the flow due to wind effects or the inflow and outflow through one or two vertically displaced openings due to the stack effect. Where wind and stack effects are similar the larger of the two may be selected.

The effective equivalent area of openings in parallel (AP) can be obtained by summing the individual areas:

AP = A1 + A2 ... + An

The equivalent area of openings in series AS, meanwhile, can be obtained from:

1/AS2 = 1 / (A1 +A2 + ... + An)

2

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MSc: AEES DL, Module CEM160, Lecture: Principles of Natural Ventilation 23

ReferencesAwbi, H. (2003) Ventilation of Buildings, Taylor & Francis.

BS 5925 (1991) Code of practice for ventilation principles and designing for natural ventilation, BSI.

CIBSE (2005) Handbook of Domestic Ventilation, BSE.

CIBSE (2005) Natural Ventilation in Non-Domestic Buildings, CIBSE.

CIBSE (2000) Mixed Mode Ventilation, CIBSE.

CIBSE (2006) Knowledge Series KS6: Comfort, CIBSE, January.

Heiselberg, P, Svidt, K and Nielsen, P (2001) ‘Characteristics of airflow from open windows’, Building and Environment, 36, pp. 859-869.

Infrared Visions, Typical air leakage and infiltration in a building. Available at: http://www.infraredvision.co.uk/pages/content.php?subID=24&catID=8 (Accessed Oct 2010).

Greenbuilder (2007) ‘Passive Solar guidelines’, in A Sourcebook for Green and Sustainable Building. Available at: http://www.greenbuilder.com/sourcebook/PassSolGuide3.html (Accessed October 2010).

Larsen, T. S. and Heiselberg, P. (2008) ‘Single-sided natural ventilation driven by wind pressure and temperature difference’, Energy and Buildings, 40(6), pp. 1031-1040.

National Renewable Energy Laboratory, Passive Downdraft Cooling. The Zion National Park Visitor Center. Available at: http://www.nrel.gov/docs/fy02osti/32157.pdf (Accessed Oct 2010).

Asfour, O. S. and Gadi, M. (2007) ‘A comparison between CFD and Network models for predicting wind-driven ventilation in buildings’, Building and Environment, 42(12), pp. 4079-4085.

Parker, J. and Teekaram, A. (2005) Wind-driven Natural Ventilation, BSRIA.

Allard, F. and Santamouris, M. (1998) Natural Ventilation in Buildings: A Design handbook (BEST: Buildings, Energy and Solar Technology), Earthscan.

Wilson, M., Nocol, F., Solomon, J. and Shelton, J. (2005) ‘Noise Level and natural ventilation Potential in Street Canyons’, in Natural Ventilation in the Urban Environment: Assessment and Design, London: Earthscan, pp. 103-123.

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24 Lecture: Principles of Natural Ventilation, Module CEM160, MSc: AEES DL

Additional ResourcesGhiaus, C, and Allard, F. (2005) Natural ventialtion in the Urban Environment: Assessment and Design. Buildings, Energy and Solar Technology Series. London: Earthscan.

The Whole Building Design Guide (WBDG) gives some introductory notes and design guidance for the U.S.A. http://www.wbdg.org/resources/naturalventilation.php (Accessed October 2011)

Mahdavi, A. and Doppelbauer E-A. (2010) ‘A performance comparison of passive and low-energy buildings’, Energy and Buildings, 42(8), pp. 1314-1319.

Links within the lectureAll last accessed October 2011.

The passivhaus website: http://www.passivhaus.org.uk/ (accessed Oct 2010)

CIBSE’s Guide B: Heating, Ventilating, Air Conditioning and Refrigeration report. http://www.cibse.org/index.cfm?go=publications.view&item=305

Environmental Science in Building (sixth edition) by Randall McMullan. Chapter 4 ‘Air Control in Buildings, pp. 73-74.

Part F of the UK building regulations: http://www.planningportal.gov.uk/uploads/br/BR_PDF_ADF_2010.pdf

BedZED case study by Arup: http://www.arup.com/_assets/_download/download68.pdf

Homebuilding and Renovating Magazine: http://www.homebuilding.co.uk/feature/passive-ventilation

BDSP: http://www.bdsp.com/ (click on Projects → Highrise → 30 St Mary Axe)

YouTube clip of the Gherkin: http://www.youtube.com/watch?v=uZKd31Gx5wo&feature=related

A description of passive stack ventilation on the Green Building Advisor website: http://www.greenbuildingadvisor.com/stack-effect-when-buildings-act-chimneys

The Journal of Ventilation website: http://www.ijovent.org.uk/

The Engineering Tool Box: http://www.engineeringtoolbox.com/air-properties-d_156.html

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