The design of controlled natural ventilation systems requires identification of the prevailing wind direction, the strategic orientations and positions of openings on the building envelope. These openings include windows, doors, roof ventilators, skylights, vent shafts, and so forth.

3.1  Ventilation rates

When designing a ventilation system, the ventilation rates are required to determine the sizes of fans, openings, and air ducts. The methods that can be used to determine the ventilation rates include:

(a)  Maximum allowable concentration of contaminants

A decay equation can be used to describe the steady-state conditions of contaminant concentrations and ventilation rate, like this:
 

Ci = Co + F / Q

(3)

where	Ci	= maximum allowable concentration of contaminants
	Co	= concentration of contaminants in outdoor air
	F	= rate of generation of contaminants inside the occupied space (l/s)
	Q	= ventilation rate (l/s)

(b)  Heat generation

The ventilation rate required to remove heat from an occupied space is given by:
 

(4)

where	H	= heat generation inside the space (W)
	Q	= ventilation rate (l/s)
	cp	= specific heat capacity of air (J/kg.K)
		= density of air (kg/m3)
	Ti	= indoor air temperature (K)
	To	= outdoor air temperature (K)

(c)  Air change rates

Most related professional institutes and authorities have set up recommended ventilation rates, expressed in air change per hour, for various situations. The ventilation rate is related to the air change rate by the following equation:
 

(5)

where	Q	= ventilation rate (l/s)
	V	= concentration of contaminants in outdoor air
	ACH	= air change per hour

Table 1 gives some recommended air change rates for typical spaces. Table 2 provides some examples of outdoor air requirements for ventilation.

Table 1  Recommended air change rates

Space	Air change rates per hour
Carparks	6
Kitchen	20 - 60
Lavatory	15
Bathrooms	6
Boiler rooms	15 - 30

Table 2  Outdoor air requirements for ventilation

Application	Estimated maximum occupancy (persons per 100 m2 floor area)	Outdoor air requirements (l/s/person)
Offices
- office space	7	10
- conference room	50	10
Retail's Stores
- street level	30	5
- upper floors/arcades	20	5
Education
- classroom	50	8
- auditorium	150	8
- library	20	8
Hospitals
- patient rooms	10	13
- operating rooms	20	15

Note:  Data source: ASHRAE Standard 62-1989, Ventilation for Acceptable Indoor Air Quality.

3.2  Flow caused by wind

Major factors affecting ventilation wind forces include:

• average wind speed;
• prevailing wind direction;
• seasonal and daily variation in wind speed and direction;
• local obstructing objects, such as nearby buildings and trees;
• position and characteristics of openings through which air flows; and
• distribution of surface pressure coefficients for the wind.

The following equation shows the air flow rate through ventilation inlet opening forced by wind:
 

(6)

where	Q	= air flow rate (m3/s)
	A	= free area of inlet openings (m2)
	v	= wind velocity (m/s)
	Cv	= effectiveness of the openings (assumed to be 0.5 to 0.6 for perpendicular winds and 0.25 to 0.36 for diagonal winds)

3.3  Flow caused by thermal forces

If the building's internal resistance is not significant, the flow caused by stack effect may be estimated by:
 

(7)

where	Q	= air flow rate (m3/s)
	K	= discharge coefficient for the opening (usually assumed to be 0.65)
	A	= free area of inlet openings (m2)
	h	= height from lower opening (mid-point) to neutral pressure level (m)
	Ti	= indoor air temperature (K)
	To	= outdoor air temperature (K)

3.4  Guidelines for natural ventilation

The following guidelines are important for planning and designing natural ventilation systems in buildings:

• a natural ventilation system should be effective regardless of wind direction and there must be adequate ventilation even when the wind does not blow from the prevailing direction;
• inlet and outlet openings should not be obstructed by nearby objects;
• windows should be located in opposing pressure zones since this usually will increase ventilation rate;
• a certain vertical distance should be kept between openings for temperature to produce stack effect;
• openings at the same level and near the ceiling should be avoided since much of the air flow may bypass the occupied zone;
• architectural elements like wingwalls, parapets and overhangs may be used to promote air flow into the building;
• topography, landscaping, and surrounding buildings should be used to redirect airflow and give maximum exposure to breezes;
• in hot, humid climates, air velocities should be maximised in the occupied zones for bodily cooling;
• to admit wind air flow, the long façade of the building and the door and window openings should be oriented with respect to the prevailing wind direction;
• if possible, window openings should be accessible to and operable by occupants;
• vertical shafts and open staircases may be used to increase and generate stack effect;
• openings in the vicinity of the neutral pressure level may be reduced since they are less effective for thermally induced ventilation;
• if inlet and outlet openings are of nearly equal areas, a balanced and greater ventilation can be obtained.

A successful application of natural ventilation strategies is only possible when there are no problems in many areas at various levels from the design stage to actual operating demands placed on the building users (Allard, 1998). These potential barriers include:

• Barriers during building operations
• Safety concerns
• Noise from outdoor
• Dust and air pollution
• Solar shading covering the openings
• Draught prevention
• Knowledge of the users about how to take the best advantage of natural ventilation
• Barriers during building design
• Building and fire regulations
• Need for acoustic protection
• Difficult to predict pattern of use
• Devices for shading, privacy & daylighting may hamper the free flow of air
• Problems with automatic controls in openings
• lack of suitable, reliable design tools
• Other barriers
• Impact on architectural & envelope design
• Fluctuation of the indoor conditions
• Design a naturally ventilated building requires more work but could reduce mechanical system (design fee on a fixed percentage of system's cost)
• Increase risk for designers
• Lack of suitable standards