Natural ventilation

Natural ventilation is commonly classified into three types:

1 crack ventilation;

2 stack ventilation; and

3 window ventilation.

Crack ventilation (or infiltration/exfiltration) occurs through all leakage of a building envelope, mainly through the joints and slits of windows and doors. It is uncontrolled and usually does not correspond to actual ventilation needs. Especially for older buildings with only limited air tightness during winter time, considerable heat losses can occur with the additional appearance of uncomfortable draught. This is one reason why new buildings have to meet a level of air tightness that restricts the air change rate due to infiltration/exfiltration to less than about 0.1 h-1 to 0.2 h-1.

Even small driving forces (pressure difference from the wind passing over a building) can move substantial volumes of air through windows or vent openings. If the outdoor air is colder than the indoor air, it flows across the lower part of the window into the building. Warmer indoor air exits via the upper part of the window, even if there is no wind present. In winter, the resulting draughts pose a comfort problem and, in the case of permanently tipped windows, a serious heat loss. Ideally, windows should be opened fully, briefly, resulting in a flushing of the house air. Table 10.2.1 lists typical ranges of resulting air change rates for different states of window openings.

Table 10.2.1 Estimated ranges of air change rates for different states of window openings

Status of window opening

Range of air change rates (h-1)

Windows and doors closed

0.1-0.4

Windows tilted

0.5-4

Windows open

5-15

Cross-ventilation/opposite windows

up to 40

Stack ventilation can be achieved with ducts that penetrate a room at ceiling level and terminate at roof level. In high-rise buildings, the resulting buoyancy can provide a strong air extraction rate - for example, from kitchen, bathroom or washing rooms that are often without windows. Every room type needs a separate stack.

Window ventilation offers the following advantages:

• An 'open window' environment is popular.

• Natural ventilation costs less than mechanical systems.

• There is almost no maintenance required (work, costs).

• Natural ventilation needs no additional space for machinery.

• If there are enough openings, high air flow rates for cooling and purging are possible. On the other hand, natural ventilation also shows some major disadvantages:

• Air flow rates and the patterns can be excessive or inadequate, varying unpredictably during the day and season. The result can be poor indoor air quality and high heat losses.

• Natural ventilation is unsuited for noisy and polluted locations.

• Filtration or cleaning of incoming air is usually not feasible.

• Occupants must constantly adjust openings to suit the prevailing demand.

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