Energy

Air extraction systems remove moist or polluted air, typically from kitchens and bathrooms, to the outside.

On this page:

• Extraction rate
• Fans
• Ductwork
• Extract location
• Exhaust location

With an air extraction system, replacement air is drawn in due to reduced indoor pressure through gaps, and open doors and windows. Air extraction systems must not vent into a ceiling or roof space.

Extraction rate

While overall house ventilation rates are expressed as the number of air changes per hour, mechanical ventilation systems move particular volumes of air per unit of time – usually the number of litres per second (l/s) or m³/hour.

NZS 4303:1990 Ventilation for acceptable indoor air quality, Table 2 sets out the mechanical extract air flow rate requirements. In houses, the minimum extract air flow rate is:

• for kitchens – 50 litres per second (l/s) intermittent, 12 l/s continuous
• for bathrooms and toilets – 25 l/s intermittent, 10 l/s continuous.

Rates of 50 l/s and 25 l/s are also the minimum flow rates required in kitchen and bathroom extract fans in G4/AS1.

Fans

Mechanical air extract ventilation is generally driven by a fan. To minimise energy use, fans should be sized and controlled to move only the amount of air required for the time required – i.e. air should only be extracted while pollutants or moisture is being produced.

For rooms used infrequently or intermittently, an extract fan linked to the operation of the light switch means the fan will only run while the room is occupied. A timer switch can ensure that it runs for a short time afterwards. Alternatively, a sensor can switch the extract fan on when steam or excessive moisture is detected.

If the fan noise is a problem, it can be mounted outside the room and connected via ductwork.

Ductwork

Ductwork may be required if the extract air:

• is coming from an internal room
• is to be discharged via the ceiling space out through the roof or soffit
• cannot be discharged through a suitable space in the external wall
• is to go through a heat exchanger.

If ductwork is needed, it will add to the demand on the fan, so the fan size should be increased accordingly.

Ductwork that passes through a cold air space should be insulated to reduce the condensation created in the duct.

Extract location

The best location for the extract grille or fan where moist air is being extracted is:

• as high as possible, e.g. ceiling or high on the wall
• as close as possible to the source of the moisture
• opposite the point in the room where replacement air will enter so that the maximum amount of contaminated air is replaced by fresh air.

Exhaust location

Stale, moist or polluted air must be discharged outside and not into another building space.

Locate the exhaust air outlet so that the discharged air:

• does not re-enter the home’s fresh air supply
• is not drawn inside through a window or other passive ventilation route
• does not enter another dwelling.

Prevent water from entering through the air outlet by installing:

• louvres – although some rattle in the wind, and airflow is reduced by around 30% for fixed louvres and 50% for gravity louvres
• automatic shutters that open when the fan is switched on
• backflow flaps that are fitted with inline ducted fans.

Updated: 06 October 2021