Duct (flow)

This will usually have an inner perforated liner, then a 1–2" (2.5-5 cm) layer of fiberglass insulation contained inside an outer solid pipe.

Round duct is made using a continuous spiral forming machine which can make round duct in nearly any diameter when using the right forming die and to any length to suit, but the most common stock sizes range evenly from 4" to 24" (10-60 cm) with 6"-12" (15-30 cm) being most commonly used.

The ductwork construction starts with the tracing of the duct outline onto the aluminium preinsulated panel.

The parts are then typically cut at 45°, bent if required to obtain the different fittings (i.e. elbows, tapers) and finally assembled with glue.

Traditionally, air ductwork is made of sheet metal which was installed first and then lagged with insulation.

Both polyurethane and phenolic foam panels are manufactured with factory applied aluminium facings on both sides.

The thickness of the aluminium foil can vary from 25 micrometres for indoor use to 200 micrometers for external use or for higher mechanical characteristics.

A rigid phenolic insulation ductwork system is listed as a class 1[clarification needed] air duct to UL 181 Standard for Safety.

The knife automatically trims out a groove with 45° sides which does not quite penetrate the entire depth of the duct board, thus providing a thin section acting as a hinge.

The determination which fabric is appropriate (i.e. air-permeable or not) can be made by considering if the application would require an insulated metal duct.

If so, an air-permeable fabric is recommended because it will not commonly create condensation on its surface and can therefore be used where air is supplied below the dew point.

The material can also be fire retardant, which means that the fabric can still burn, but will extinguish when the heat source is removed.

Fabric ducts require a minimum of certain range of airflow and static pressure in order for it to work.

The growth of low-profile ducting has grown significantly due to the reduction of available space in ceiling cavities in an effort to reduce cost.

Since the Grenfell Tower fire in 2017 there has been a rise in the discovery of non-compliant building materials; many PVC low-profile ducting manufacturers have struggled to gain or maintain compliance, and some building projects have had to resort back to using the more expensive steel option.

To avoid this, vibration isolators (flexible sections) are normally inserted into the duct immediately before and after the air handler.

The same flexible section can reduce the noise that can occur when the blower engages and positive air pressure is introduced to the ductwork.

Other take-off designs use a snap-in attachment method, sometimes coupled with an adhesive foam gasket for improved sealing.

Besides the regulation provided at the registers or diffusers that spread air into individual rooms, dampers can be fitted within the ducts themselves.

A fire damper's most important feature is a mechanical fusible link which is a piece of metal that will melt or break at a specified temperature.

This allows the damper to close (either from gravity or spring power), effectively sealing the duct, containing the fire, and blocking the necessary air to burn.

The supply plenum directs air from the central unit to the rooms which the system is designed to heat or cool.

For supply, diffusers are most common, but grilles, and for very small HVAC systems (such as in residences) registers are also used widely.

"[4][needs update][dubious – discuss] A thorough duct cleaning done by a professional duct cleaner will remove dust, cobwebs, debris, pet hair, rodent hair and droppings, paper clips, calcium deposits, children's toys, and whatever else might collect inside.

Cleaning of the duct system may be necessary if: In commercial settings, regular inspection of ductwork is recommended by several standards.

Building codes and UL standards call for special fire-resistant tapes, often with foil backings and long lasting adhesives.

Automated technology exists that can seal a duct system in its entirety from the inside out using a patented process and specialized sealant.

The cost associated with automated duct sealing often makes it impractical for the average homeowner to implement in their own house.