Ductwork

Ductwork, ducts, or ducting, are conduits, or tubes, that typically form part of a ventilation system, used to convey air throughout a building. An example of a simple elementary duct is a fireplace chimney, used to convey smoke to the outside. Hard pipes used to transfer water or gas are not classed as ductwork.

Duct design involves planning (laying out), sizing, optimising, and detailing. Ductwork should be among the first items to be considered when designing a new building because of its importance in the overall utility of the building, and the need to integrate complex duct routes with other elements of the overall design. This can be particularly difficult where structural elements pass through building services spaces, such as the downstands of beams, or where ducts have to pass through other elements of the building.

Ducts can be fabricated from a range of materials:

• Galvanised mild steel: This is the most common material used as the zinc coating prevents rust forming.
• Aluminium: This is lightweight and quick to instal. Custom shapes can be easily fabricated as required by the particular design.
• Polyurethane and phenolic foam panels: These are manufactured with aluminium facings on both sides.
• Fibreglass: This provides built-in thermal insulation and sound is absorbed by the interior surface.
• Flex ducts: These are typically made of flexible plastic over a metal wire coil.

Ductwork layouts may be very complex, involving a number of supply and return ducts, that branch to all parts of a building. Typically Heating Ventilation and Air Conditioning (HVAC) ducting will run through voids above a suspended ceiling, supplying and extracting air from the occupied spaces below through diffuser grilles. Air may be drawn or blown through the ducts by fans or by air handling units.

Duct sizes are calculated based on the relationship between air volume, size, air velocity and resistance. The requirements for the provision of air within buildings is set out in Part F of the building regulations, Ventilation.

Air handling units generate noise (and vibration) which can be disruptive, and this can be compounded where ductwork passes between acoustically separate spaces. In this case, acoustic attenuators might be included in the air handling unit or ductwork. In addition, vibration can be generated. This vibration can be isolated by inserting flexible sections between air handling units and ductwork and by isolating the unit from the building structure.

Other components that can be incorporated in ducting systems include:

• Distribution boxes to redirect airflow as required.
• Take-offs, which allow a small portion of the main duct’s flow to be diverted into branch ducts.
• Volume control dampers, which allow for the adjustment of air flow, and may be manual or automatic.
• Smoke and fire dampers where the ductwork passes between fire compartments.
• Turning vanes where there are changes of direction and help minimise turbulence and resistance by guiding the air.
• Access points for cleaning and maintenance.

Before approval, ductwork should be tested to confirm it is adequately sealed.

NB The Scottish Building Standards, Part I. Technical Handbook – Domestic, Appendix A Defined Terms, defines a duct as: ‘…the structure, trunking, or casing, with any apertures, enclosing a passage, other than a flue, used solely for conveying air, gases, or refuse.’

[edit] Related articles on Designing Buildings Wiki

• Air conditioning.
• Air handling unit.
• Building services.
• Cooling systems for buildings.
• Electrical control systems.
• Heating.
• HVAC.
• HVAC balancing.
• Mechanical, electrical and plumbing MEP.
• Mechanical ventilation of buildings.
• Pipework.
• Plant room.
• Raised floor.
• Safe ductwork - Loss Prevention Standard (LPS 2084).
• Thermal comfort.
• Underfloor air conditioning at London Grade II listed landmark.
• Valves.
• Ventilation.
• Vertical riser.

Comments

Air duct cleaning units produce commotion which can be problematic, and this can be aggravated where ventilation work goes between acoustically separate spaces. For this situation, acoustic attenuators may be incorporated into the air taking care of unit or ventilation work.