An Introduction to Ductwork

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Ductwork is a system of tubes or conduits carrying air around a building for ventilation purposes. This could be a basic duct from a fireplace chimney or a complex air conditioning system. The pipes used to carry gas or water are not categorised as ductwork.

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Designing ducting is amongst the first considerations when planning a new building, as the routing of ducts will need to pass through structural elements and other design features.

What materials are used?

Ducts are generally fabricated from galvanised mild steel, aluminium, polyurethane and phenolic foam, fibreglass or metal coils overlaid with plastic (flexducts). The materials used will depend on the requirements of each design, for example, galvanised steel is popular because its zinc coating stops rust forming, and aluminium is chosen because it is lightweight and easy to install.

Factors affecting layout

The routing of ductwork can be extremely complex, including a combination of supply and return ducts branching to cover all areas of a building. Generally, Heating Ventilation & Air Conditioning (HVAC) ducts will pass through voids over a suspended ceiling to supply and remove air through diffuser grilles. The air can be drawn through or blown from the ducts using fans or other air-handling units.

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Duct size is dependent on the relationship between volume, air velocity and resistance to flow, and exact requirements for this are set out in building regulations Part F, Ventilation. There is plenty of guidance material on this available online.

Air handling units can generate a lot of noise and vibration, and this must be reduced by the inclusion of flexible sections and by staying clear of building structure.

System components

Numerous additional components can be incorporated into ducting and there are many companies who can advise on specific ductwork supplies, such as Examples of components that may be found in typical systems include:

• Distribution boxes which redirect the airflow as necessary.
• Take-offs, which divert small amounts of the main flow to branch ducts.
• Volume control dampers allowing manual or automatic airflow adjustment.
• Fire and smoke dampers which are needed in ducting routed around fire compartments.
• Turning vanes which guide air to reduce turbulence or resistance due to changes of direction.
• Maintenance and cleaning access points.

All ductwork must be tested before use to check for correct sealing.

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