Ducts and Fittings

Ducts carry air to and from the occupied space. They are generally hidden in the ceiling space but on occasion they are exposed. Ducts can take various forms and be made of a variety of materials.


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The most popular are rigid galvanised sheet metal and flexible ducts; they can range in size from many meters to only 100mm in diameter. As the supply air ducts carry cooled air to the space they are insulated either internally or externally to stop heat transfer and condensation. Return Air Ducts carry air from the space back to the AHU or FCU for filtering and conditioning. It is often this internal insulation which can become contaminated with Mould. See Anti-Microbial Coatings

HVAC Tip

The first 10 metres of a supply air ducts the most likely place to find Mould or other biological contamination. If there is no access to inspect this area then no one can know what is inside. AS/NZS 366.2 requires annual inspection of moisture producing areas in the system. Clean-Air can install Access Panels for these regular inspections.

There are various fittings within the ducts these include the Registers or Supply Air Grilles we see in the ceiling or on walls. There are also return air grilles or registers in ceilings or walls. There many different types of grilles and registers to suit the architectural style of the building. Within the ducts are various fittings which control the flow of air through the system these may include Guide Vanes, Control Dampers both fixed and actuated, Heaters, and Control & Safety Sensors. These items are all affected by a build-up of dust, debris and mould within the system.


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HVAC Tip

Often around a supply grille there is an area of debris which builds up. This is not actually from dirty air from the air conditioning system but is caused by impingement of dust from the occupied space as it meets the conditioned air being delivered into the space. This is more of a general cleaning issue than a HVAC Hygiene issue. Please contact Clean-Air Aust for further information on this.