Design of passive ventilation

A passive ventilation system should be designed to achieve air flow rates that are sufficient to remove pollutants and are comfortable for occupants.

On this page:

• Factors affecting air flow rates
• Calculating air flow rate
• Designing ventilation features.

The optimum air flow rate will depend on temperature and humidity. The higher the temperature and humidity, the more air flow is needed to maintain comfortable temperatures.

Factors affecting air flow rates

Air flow rates depend on:

• prevailing wind direction
• average wind speeds (as a guide, use half the average seasonal speed, as wind speeds rarely fall below this value)
• how the site is influenced by daily and seasonal variations in wind such as onshore/offshore winds and how these may change during the day
• building form – whether it enhances or restricts air flow
• surrounding landforms and planting – will they obstruct air flow
• orientation and position of windows, doors, roof ventilators, skylights and vent shafts
• surface pressure coefficients around the building.

Calculating air flow rate

The air flow rate through a ventilation inlet opening forced by wind can be calculated using the formula Q = Cv x A x v where:
Q     = air flow rate (m³/s)
Cv     = effectiveness of the openings (assumed to be 0.5–0.6 for perpendicular winds and 0.25–0.36 for diagonal winds)
A     = free area of inlet openings (m²)
v     = wind velocity (m/s)

Ventilating features

When designing a natural ventilation system, the long façade of the building should be facing the prevailing wind direction, with doors and opening windows providing the ventilation openings.

Ensure that openings (inlet and outlet) are:

• not obstructed
• the same size
• able to control the flow
• located in opposing pressure zones to increase the potential air flow.

Other ventilating features include:

• maintaining a vertical distance between two openings to create a stack effect, i.e. hot air rising and thereby enhancing air flow
• shafts to promote air flow
• maximising air flow by designing open plan spaces
• maximising air flow by having openings at different levels or near the ceiling on opposite sides of the space
• using architectural and landscape features to direct and control air flow - for example, using casement sashes on the windward façade as these can be more efficient than other types of sashes, and including opening windows on the leeward face.

Passive stack ventilation

A passive stack ventilator is a vertical or near-vertical ventilation shaft where moist warm air is naturally drawn up and expelled outside through a vent above the roofline. (The vent should be near the ridge to reduce the effect of wind gusts.) Temperature differences lead to a natural, continuous movement of air. Cooler fresh air is drawn into the building through open windows, louvres, trickle ventilators or air leakage.

The stack effect can be enhanced by the use of a solar chimney (see drawing). These provide a clear pathway for the air and can help to increase performance. Solar chimneys naturally heat the rising air to increase the temperature difference between the warmer exhaust air and cooler intake air.

Stack ventilation can be a good option to ventilate a bathroom or other wet room.

Stack effect ventilation works particularly well in winter, when indoor temperatures are much warmer than the outside. This approach may not be as effective in summer, when the indoor/outdoor variation in temperature may be less, or indoors may even be cooler than outdoors.

G4/AS1 mentions passive stack ventilation in the kitchen/bathroom/toilet/laundry. Section 1.3.7 of the Acceptable Solution outlines the requirements, which include that these ventilators:

• be designed in accordance with AS/NZS 4740:2000 Natural ventilators - Classification and performance section 3
• do not reduce the performance of the building envelope and partition walls for external moisture, fire and acoustics.
• There are separate requirements for ventilating kitchens and ventilating bathrooms, toilets and laundries.

 

Update: 18 August 2017