Depth of space relative to the size and position of ventilation openings
Ceiling height
Thermal mass exposed to air movement
Location of ingress vents in respect to external pollution sources
Heat gain within the building
Climate including wind speeds, diurnal range.
Single sided ventilation (figure 9), in some respects the simplest form of venting a space, has been shown to be effective for room depths up to two-and-half
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times the height of the opening (CIBSE AM10). As it is a wind pressure driven mode of ventilation it is dependent on external climate which means a low level of operational control. Single sided ventilation can be increased in effectiveness by high level and low level openings such as facilitated by the traditional sash window.
This can increase effective ventilation depth to three times the height of the opening with incoming air usually entering at lower level and warmed air exiting at higher level. Useful for summer cooling and odour removal but, in a temperate maritime climate, this strategy will require winter heating of incoming air to avoid discomfort and subsequent energy losses.
Figure 9: single sided and cross ventilation strategies – effective depth to height (author)
Cross ventilation (figure 9) is again largely wind pressure driven but will also have some element of thermal buoyancy where there is a change in height between
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the inlet and outlet. This strategy is usually effective for room depths up to four times the height of the opening which has traditionally given building depths around 12 to 13m. Awbi (2003) claims effectiveness up to five times the height of the opening thus giving potential plan depths of in excess of 15m in standard commercial room heights (note however that this plan depth would be the maximum for useful day lighting which is facilitated by a room depth of two-and-half times the opening height). Like single-sided ventilation, it is dependent on external climate so a level of control is lacking and likewise, winter heating of incoming air is required.
Cross ventilation is usually conducted via windows and ducts but wind scoops can be used when there is a dominant prevailing wind direction. To improve air distribution into deeper spaces ducted or underfloor vent paths can be used – these supply ducts must be designed for very low pressure.
Stack ventilation covers those strategies where driving forces direct an outflow from the building and thus draw in cool air at low level and uses the density differential between cooler and warmer air. For equal ventilation rates the openings at ground floor need to be smaller than those nearer the top of the building. If using chimneys for stack ventilation then essential that the air in the chimney is warmer than the ambient air which might mean insulation required. The chimney outlet should be in the negative pressure zone of the building.
62 Figure 10: wind pressure and thermal buoyancy
Stack ventilation (figure 10) can be used when cross or single-sided ventilation strategies don’t provide enough air or may be considered too unreliable. This can be because the building plan is very deep or because a high ventilation rate is required for example in auditoria. Thermal buoyancy is the main ventilation driver, but depending on positions of inlet and outlet, wind pressure may assist or indeed, counteract the ventilation effectiveness. Large enclosures and high-ceilinged spaces such as atria experience temperature stratification with the warm air at higher levels helping to drive stack ventilation.
Air flow in stack ventilation is due to:
Stack pressure –this is proportional to the vertical distance between inlet and outlet; it also requires, and is driven by, the temperature difference between air inside and outside.
The wind pressure at the discharge end of the stack.
Stack effect is due to density differences: warm air rising with cooler air replacing the rising warm air. The level within the building where the inflow changes to an outflow is the neutral pressure level. The position of the neutral
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pressure level is a function of the density difference between cold and warm air and the vertical distribution of the openings. The stack pressures are a function of the temp difference and the height between the opening and the neutral pressure level.
(CIBSE AM10, 2015)
Awbi further notes that the performance of the stack is most reliable in colder weather and high wind speeds when the temperature differential between inside and outside is greater and suction higher. The minimum height of the stack above roof level, to avoid back flow into the building, is dependent on the position of the stack above the roof and also the roof pitch. It is important that the vent stack is sized correctly in order to minimise pressure drops due to friction and dynamic pressure loss at outlets, bends, grilles and cowls.
Figure 11: passive stack ventilation, design rules (Short A. 2017)
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There are several factors which can be utilised in order to increase the effectiveness of natural driving forces in ventilation:
Vertically spacing intake and extract openings as far apart as possible to increase thermal buoyancy.
Optimising the use of wind conditions on site by using for example wind towers
Using large room heights and volumes to even out variations in ventilation flow rates (Heiselberg P. 2008).
When utilising low level openings in the façade for natural ventilation then it becomes important to preheat the incoming air in winter to avoid cold draughts in winter. A combination of low and high level openings can work successfully: low level windows in the winter heating season, preheating the air and utilising the higher wind pressures; high level windows for the cooling season with the lower natural driving pressures, night cooling of exposed ceilings and larger airflow rates.
(Heiselberg P. 2008)
Heiselberg’s work examines pressure losses and some of his conclusions are important rules of thumb in the design of natural ventilated buildings:
Use as few ventilation components as possible i.e. keep it simple
Use low pressure-loss components for example by using an aerodynamic form
Use components that are easy to use and clean
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Minimise the need for ventilation channels by using façade air intakes and direct air transfer between rooms
Use air paths of large dimensions and aerodynamic sections in which air speed is less than 1ms-1
Natural ventilation responds to the prevalent site conditions and, when well designed, is a measured interaction between external and internal environmental conditions. Site and climate data such as temperature and humidity, prevailing wind speed and direction, solar radiation, and external noise and pollution sources are all critical factors in assessing the role of natural ventilation in building design.
2.11.4 Selecting the strategy. CIBSE AM10 (2015) manual contains a