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Ducting Facts

An overview of the various ducts and components of a duct system

| | Apr 15, 2010 | 1:44 pm
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Ducts are traditionally made of sheet metal, which is installed first and then lagged with insulation. If a ductwork is done from rigid insulation panels it need not be insulated further and is installed in a single fix. Pre-insulated aluminium ductwork is light and easy to install.

The following are some of the various ducts:

Fibreglass duct board
This has built-in thermal insulation and its interior surface absorbs sound, thereby helping the HVAC system to operate quietly.

Flexible ducting
Also known as “flex”, this duct has a variety of configuration. For HVAC applications, it is a flexible plastic over a metal wire coil – becoming a round, flexible duct. This is convenient for attaching supply air outlets to the rigid ductwork, but the pressure loss through flex is higher than for most other types of ducts.

Fabric
Also known as air socks, duct socks or textile ducts, fabric ducts give even air distribution throughout the entire length. These are made of special permeable polyester material. When fitted above a ceiling, fabric ducts must be PVC coated, so as to sustain high-performance without a regular maintenance.

Here are the various components of a duct system:

Vibration isolators
These are inserted into the duct immediately before and after the air handler, whose blowers create vibration, hence, noise. These are made of rubberised, canvas-like materials that mute the vibration to the attached ducts.

Take-offs
When a main duct branches into many subsidiary ducts, the fittings called “take-offs” divert a small portion of the flow from the main duct to each branch duct. The fittings, stuck into openings cut into the wall of the main duct, have small bent metal tabs that retain the take-off on the main duct. Their round versions are called “spin-in fittings”.

Stacks, boots and heads
These are vertical ducts of very wide and thin rectangular or oval sections. A “stack boot” at the bottom provides a transition from a rectangular duct to a thin wall-mounted duct while a “stack head” provides a transition back to ordinary ducting. A “register head” provides the transition to a wall-mounted air register.

Volume and control dampers
This provides the adjustment of the volume of air flow to various parts of the system.

Smoke/Fire dampers
These are found in ductwork, where the ductwork passes through a firewall. A probe in the run of duct detects smoke within the duct system which is supplied from the air handing unit, for instance.

Plenums
These are the central distribution collection units for an HVAC system. The supply plenum directs air from the central unit to the intended rooms while the return plenum carries the air from large vents to a central air handler.

Terminal units
These are fan-powered mixing boxes or VAV “boxes” of either single or dual duct. There is usually one terminal unit per thermal zone.

Air terminals
These refer to supply air outlets and “return” or “exhaust air inlets”. These are mainly used for appearance purposes, but some also carry air filters (thus known as “filter returns”).


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