Natural Ventilation Systems

Thermal chimneys NAB Docklands, Vic (source: AIRAH)

Natural Ventilation Systems This technical information sheet describes an approach rather than a technology. It provides the basic understanding for architects and engineers to be able to design natural ventilation strategies with supporting strategies such as solar chimneys.

Introduction Natural Ventilation systems rely on natural driving forces, such as wind and temperature difference between a building and its environment, to drive the flow of fresh air through a building. Both work on the principle of air moving from a high pressure to a low pressure zone. Natural ventilation systems are usually integrated into building systems where there is some mechanical support; these are called mixed mode or hybrid ventilation buildings. The main benefit of some augmentation by mechanical systems is that there is less unpredictability with indoor environment conditions, though it will result in greater energy use. Natural ventilation can be an appropriate choice when compared to air conditioning in the temperate climate of Victoria, particularly as the nights are cool and this can be used to pre-cool the building (see CH2 for case study of natural ventilation used for night purge www.ch2.com.au). It can save substantial amounts of energy by decreasing or eliminating the need for mechanical cooling. It may also improve the building’s indoor air quality. Buildings with well-designed natural ventilation systems often provide very comfortable and pleasant environments for the occupants.

There are two fundamental approaches to designing for natural ventilation that will be effective in most Victorian situations: > Cross ventilation which uses air-pressure differentials caused by wind > Stack ventilation which uses the increased buoyancy of air as it warms up Depending on the approach when using natural ventilation, a conscious choice may need to be made by the building users to have greater fluctuations in indoor thermal conditions. The temperature will not always be predictable to plus or minus half a degree, as the use of external air will, if it is not tempered using mechanical systems, mean that temperature and humidity vary.

Suitability Most suited to: > Buildings with a narrow plan or atria with floor plate width of 15m or less > Sites with minimal external air and noise pollution (though still possible if they are present) > Open plan layouts Not suited to: > Buildings with a deep floor plan > Buildings that require precise temperature and humidity control > Buildings with individual offices or small spaces > Buildings with continual heat loads above 35–40 W/m2 > Locations with poor air quality

Benefits

> The main savings are due to cooling energy reduction (roughly equivalent to an economy cycle), not needing to run HVAC fans and increased occupant satisfaction.

The flow through a building is related to the size of the openings (both inlets and outlets), restrictions along the flow path, furnishings and the distance between openings.

> A Sydney study showed a 25 – 33% reduction of energy use in a naturally ventilated mixed mode building with high occupant comfort satisfaction scores > Reference 8 .

Basic principles for sizing and placing openings are: > The area of the opening at intake must be equal to or 25% smaller than the area of opening for exhaust.

> International studies in similar climatic regions using natural ventilation only (not mixed mode), show capital costs savings in the region of 10 – 15% > Reference 3 and energy costs that are 40% lower than air-conditioned equivalents > Reference 2 . > Increased fresh air supply to a space may result in higher thermal comfort levels and increased productivity.

Wind driven ventilation

> Natural ventilation systems may have an increased robustness, flexibility and adaptability.

> Air flow will take the line of least resistance so follow the flow line to check for dead spots (areas where fresh air does not go). > Consider security, privacy and noise transfer.

Technical considerations Wind driven ventilation > Driving pressure under natural ventilation is very low, around 10 Pa > Reference 6 . > Design openings so that there is a minimal chance of occupants feeling drafts – > Reference 3 recommends 0.8 m/s with winter rates as low as 0.15 m/s. > Air change rate should provide adequate fresh air, levels stipulated by AS1668.2 1991 are

Cross ventilation Cross ventilation depends on two continuously changing factors: wind availability and wind direction. Consequently, it is a somewhat unreliable source for providing air flow and thermal comfort. In cross ventilation the wind creates a high pressure zone where it impacts the building and a low pressure zone on the leeward side, drawing air through the building > Figure 1 . Pressure is highest near the centre of the windward wall diminishing to the edges as the wind finds other ways to move around the building so air intakes are preferable near the centre or the high pressure zones. To determine the amount of natural ventilation that you will obtain from an opening, in a given space, standard formulas are used. A very worthwhile but technical reference is the CIBSE guideline – Natural Ventilation in Non-Domestic Buildings AM 10 > Reference 2 provides standard formulas to determine the amount of ventilation from an opening in a given space as it is affected by internal fit-out and distances.

High pressure zone

Windward side

10 l/s per person, projects such as CH2 provide 22.5 l/s per person (mechanically aided). > Use Computational Fluid Dynamics (CFD) modelling to optimise performance and ensure minimisation of potential dead spots. > When modelling natural ventilation, take into account other buildings around it as they will affect the effectiveness of the wind hitting the building to create the pressure differentials needed. Again, > Reference 2 is an excellent source of the calculations needed to effective design the natural ventilation system. See pages 51– 52.

Design issues to consider with cross ventilation Using cross ventilation will have a strong influence on building aesthetics and site planning. To maximize the effectiveness of openings, narrow buildings with open plans and well placed openings work best (particularly if the longest faces of the building are perpendicular to the typical wind direction). Furthermore, single-loaded corridors (rooms only on one side of a corridor) will provide better airflow than double-loaded ones as it makes it easier to provide openings on opposite walls. Building elements like fins, wing walls, parapets and balconies may be designed to enhance wind speeds and should be an integral part of cross-ventilation design though caution needs to be taken that they do not cause turbulence and block air flow > Reference 1 . Below are some considerations that should be made when designing for cross ventilation:

Leeward side

> Will work well if the room is up to 5 times the width of the ceiling height > Figure 2 . Low pressure zone

> Figure 1 Pressure effect from wind > Reference 1

> If cross ventilation is not possible placing windows on adjacent walls, at 90 degrees to each other, will work but limit room size to 4.5 m x 4.5 m. > In a standard office, partitions should not be higher than 1200mm, but this will depend on opening sizes.

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x

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> Figure 2 Cross ventilation rule of thumb – effective for up to five times the ceiling height

> Partitions should not obstruct air path. Design spaces so that they are parallel to main ventilation path > Figure 3 .

> Elements such as overhangs can help or hinder air flow and should be carefully integrated into CFD modelling.

> Place high emission equipment such as printers in separate mechanically ventilated rooms on the east or west facades as these are the areas of highest heat load from the sun with least benefit from windows.

> Openings should be fully operable by occupants if a manual operational strategy is chosen. This requires easy access to opening mechanisms as well as training and appropriate information to work well. > Provide inlet openings on the windward side (pressure zone) and outlets on the leeward side (suction zone). > The inlet location affects airflow patterns far more significantly than outlet location. Inlet location should be a higher priority (if faced with a choice) as a high inlet will direct air toward the ceiling and may bypass the occupied level. > Ensure windows have effective seals to avoid unwanted infiltration. > Orient the building and openings to maximize exposure to prevailing winds. > Consider designing cross-ventilation openings that are secure enough to be left open at night, so that natural ventilation can provide additional night time cooling benefits.

> Figure 3 For effective cross ventilation ensure partitions are designed to minimise resistance

> Concentrate ventilation openings in spaces most likely to require cooling.

Integration with windows

> Minimize solar gain use shading devices like overhangs, awnings and fins to control solar gain.

The apertures for cross ventilation can also serve as windows for views and daylighting. All architectural elements intended to enhance one strategy should also work for the other. However, an orientation that works for ventilation (the windward side) may not be ideal for daylighting for which north-and south-facing are best. In Victoria, windows ideally should take advantage of the cool south westerly winds in summer. Prioritise the needs of the space based on function and climate > Reference 1 . Summary of considerations: > Openings should not be obstructed. > Openings should be staggered and the maximum vertical distance apart possible to increase pressure differences – this will depend on ceiling height but research shows 1.5 m or more is best.

> Awning windows work for air intakes, hopper windows for outflow. Double hung windows are ideal for single sided ventilation because they provide high and low openings in the one unit. Casement windows are suitable to catch breezes when the prevailing wind direction is consistent. > Window insect screens decrease the flow rate of slow breezes more than stronger breezes (60% decrease at 2.4 km/h, 28% decrease at 9.7km/h) > Reference 4 . > Only the clear opening area of a window can be used to calculate capacity.

Integration with HVAC

Case study – Single sided ventilation – State Emergency Services HQ South Melbourne

Natural ventilation can replace air conditioning entirely or may be integrated with mechanical systems in a hybrid/mixed mode. There are two options with this approach. The first, such as used at Reservoir Civic Centre where there was a highly climatically controlled sound studio, is to divide the building into separate zones for natural ventilation and mechanical ventilation. The second is to have a system that switches to natural ventilation when the weather conditions are favourable such as Kangan Batman Tafe.

Cost

Another option is a hybrid system, where natural ventilation is used when the weather is mild and the HVAC when external conditions are beyond comfort parameters. This can work with a changeover system in which windows are shut when the air conditioning is on. Carefully designed changeover controls can be used to automatically shut off the air conditioning if windows are open, if mistakes are made in design this will lead to dissatisfaction > Reference 1 . Crucial for the use of natural ventilation is that it is designed to work on still days, regardless of wind direction. This will require modelling and potentially the use of mechanical systems such as fans.

This building uses single sided natural ventilation to all the open plan office areas. Air comes in via floor level attenuated grills and is expelled through high level attenuated louvers. Being near a busy roadway the air is drawn through a system to minimise air and noise pollution. The natural ventilation system is integrated with the nearby tunnel controls so that air intake closes when tunnel venting occurs and wind directions are unfavourable. Sophisticated design and integrated modelling was requires to ensure sufficient air pressure could be achieved.

> Avoid locating heat retaining ground surface treatments on the windward side such as large asphalt parking lots.

Design issues to consider with single sided ventilation Using single sided ventilation is less effective than cross ventilation and will need careful design and modelling to work effectively. The summary below outlines the general issues that should be considered: > Will work effectively at a distance into the room which is two times the ceiling height. > If openings are designed to be around 1.5 m apart vertically then the effective distance will be 2.5 times ceiling height. > The openable component needs to be 1/20 of the floor area.

Limitations

Site considerations: > Use good site planning, landscaping and planting strategies to cool the incoming air. A body of water or fountain on the windward side will precool the incoming air through evaporative cooling. Tall deciduous trees on the windward side will lower the temperature of the inflow and shade the openings though careful choice of species must be made to minimise such as pollen infiltration.

Capital costs for a fully naturally ventilated building may be 10–15% lower than air-conditioned equivalents > Reference 3 . Costs for cross ventilation are low to moderate. Buildings that use natural ventilation may have higher initial costs because operable windows typically cost 5% to 10% more than fixed glazing, but the savings from not using air conditioning will offset this added cost. Hybrid systems will be more expensive because of the higher cost for operable windows and interlocking controls for the HVAC system. Costs vary greatly depending on installation of HVAC system – key criteria that determine cost are the complexity of the system and if it is manually or automatic controlled. This cost can vary between $50–$150 per square metre (2006/7 prices).

The benefits of natural ventilation are threatened by poor management and operation. Problems associated with building openings are: > Safety – people may fall out > Security and fire > Outdoor Noise > Internal Acoustics > Insects, odours, dust and air pollution > Fluctuation of internal temperature

Stack driven ventilation Outlet

Design considerations with Stack ventilation

In stack ventilation, air movement is created by cool air being drawn into the space low down to replace warmer air that naturally rises as it is warmed and is then exhausted at the top see > Figure 4 . To obtain cooling, the incoming air must be cooler than the ambient internal temperature. The cool air may be drawn from a shaded or landscaped space or from over a body of water, a thermally massive labyrinth or other source of cooling. Stack ventilation works best in spaces with high ceilings and where cross ventilation is not feasible.

The design considerations with the integration of stack ventilation are similar to those for wind driven ventilation discussed above. The most important issue is ensuring that the pressure within the stack remains high, so that air is always travelling upwards and out and therefore drawing fresh air in. The main points to consider when designing stack ventilation are: > Effective across a width of 5 times the vertical distance from inlet to exhaust. > Chimneys can be used as the exhaust point if the temperature inside can be kept warmer than the ambient internal building temperature. > Design chimneys to minimise pressure drops to allow movement of air, for example through techniques discussed below. > The function of solar chimneys is improved by techniques to increase the air temperatures at the top of the chimney such as glass panels and dark coloured surfaces to capture solar radiation > The exhaust must be placed on the leeward side to take advantage of negative wind pressures to draw the air out and not block flow with positive pressure. It may require operable vents on all faces to respond to different wind directions. > The function of solar chimneys is improved by shaping the roof to aid air flow over it, in the manner of an aerofoil, to increase the negative pressure on the exhaust side. > Design of chimney to ensure that pressure remains low (i.e. air inside is warmer than outside) to maintain correct flow direction in cool weather, for example use glass or dark heat-absorbing material near the top of the chimney. > In unpredictable climatic areas fans could be used, these could be run by solar panels on the solar chimney. Caution needs to be used when using fans that they do not interfere with natural air flow.

Solar Chimney Solar chimneys are a method of enhancing stack ventilation. Additional height and well designed air passages increase the air pressure differential. Chimneys should be constructed to capture solar radiation to increase the heat of the air at the top and increase the difference in temperature between incoming and out-flowing air. The increase in natural convection that occurs from these measures enhances the draw of air through the building > Figure 5 > Figure 6 .

Low pressure zone

Inlet

High pressure zone

> Figure 4 Pressure effect from stack ventilation, make sure height difference between openings is maximised > Reference 1

Exhaust air

Thermal mass High temperatures increase updraft

Rotating turbine

Insulation Damper

Fresh air

> Figure 5 Cross section of Victoria Barracks, Sydney, showing air flow path of cross flow natural ventilation incorporating a solar chimney.

Metal absorber with black selective coating

Replacement air drawn from cool side of house

> Figure 6 Thermal chimney > Reference 4

Rules of thumb Controlling ventilation Manual or occupant: > Occupants only affect the control of their own space with minimal effect on the whole ventilation strategy. > The BMS lets user know when to open and close windows. > Building users are trained. > Controls are intuitive and accessible. > Any night cooling is automatically controlled to avoid over cooling and to restrict operation to unoccupied times. Automatic: > In heating mode, ventilation rate should be controlled by CO2 levels or occupancy sensors. > In cooling mode, ventilation rate should be controlled by internal temperatures and some anticipatory control to increase ventilation rate before temperatures rise. > Include night cooling strategies if temperatures are high at the end of the day. Mixed mode > Reference 8 : > Planning for mixed mode control systems needs to be carefully undertaken. > Ensure sufficient commissioning over all seasons to allow for tuning. Operational considerations: > If manual operation is required, train occupants so that they understand how the system works. > The mechanisms for operable inlets and outlets should be well maintained and clean. > Periodically clean windowsills, panes, fins and louvres to ensure healthy air intake for the space. > Careful consideration needs to be taken of how the system will function in winter, ensure that the system is not creating heat when the windows are open.

Sustainability Victoria Level 28, Urban Workshop 50 Lonsdale Street Melbourne 3000 T > 03 8626 8700 E > [email protected] W > www.sustainability.vic.gov.au

References and Tools > Reference 1

Hawaii Commercial Building Guidelines for Energy Efficiency www.archenergy.com/library/ general//chapter2_nat_vent_030604.pdf > Reference 2 Natural Ventilation in Non-Domestic Buildings CIBSE AM 10 – 1997 and 2005 > Reference 3

Making natural ventilation work, Andrew Martin and Jason Fitzsimmons, BSRIA GN 7/2000. > Reference 4 Passive Solar Guidelines (Residential) www.greenbuilder.com/sourcebook/PassSolGuide3.html > Reference 5 Gage S. A., Hunt G.R. and Linden P.F. (2001) Top down ventilation and cooling. Journal of Architectural and Planning Research 18:4, p.286. > Reference 6 Chilengwe N. and Sharples S. (2003) Low and High Pressure Experimental Analysis of Ventilators for Natural Ventilation in Buildings. International Journal of Ventilation 2:2, pp. 149-158. > Reference 7 Heiselberg P (2002) Principles of Hybrid Ventilation. Hybrid Ventilation Centre, Aalborg University. http://hybvent.civil.auc.dk/puplications/report/ Principles%20of%20H%20V.pdf > Reference 8 Rowe, D. (1996) Mixed mode climate control AIRAH Vol 50, Issue 12.