4 Types of Ductwork For Central Air Conditioner

Ductwork is a network through which heated or cooled air is circulated in your home or building. Supply ducts carry treated air, which means it has been heated or air conditioned, to air vents. Return ducts bring untreated air to the air handler or furnace to be heated or cooled again.

You have multiple options for what material to use for ductwork. This article will focus on the various types of ductwork available today – Rigid and flexible. Types of rigid and flexible ductwork are explained, and we briefly highlight how they are made, some of their features, and pros and cons of each.

2 General Categories – Rigid and Flexible

This is the easiest distinction between two types of materials. Sometimes both are used.

Rigid Ductwork

For many decades, all ductwork was rigid – meaning that it could not be bent or flexed under any normal conditions. Over the years, several variations have come into being and are available to the builder and homeowner today. We will look at the three most common types to consider.

Sheet Metal Ductwork

Sheet metal ductwork is made of galvanized steel or aluminum and is the most common type of ductwork. It is the most durable because of the strong material from which it is made. The galvanized steel has a zinc coating that is resistant to rust. Aluminum, of course, is lighter and will not rust at all, but it is not nearly as strong. More care must be taken during installation as well as whenever any work is being done near it after installation.

The interior surface of galvanized ductwork has tiny irregularities that will trap dust and other particles over time, so the ducts should be professionally cleaned occasionally to prevent excess impurities in your household air. Some experts recommend cleaning ducts every 3 to 5 years. Because the surface of sheet metal ductwork is non-porous however, it is the least likely of all types of ductwork to have mold or biological growth, even in warm, humid climates.

Sheet metal ductwork comes in several different shapes, which each have their own advantages over others in certain applications.

Rectangular sheet metal ductwork is often used as the main “trunk” of an HVAC system. It is connected directly to the plenum of the air handler and is the largest (volume-wise) duct in the system since it must carry the largest quantity of air. Rectangular ductwork is often custom-made in a local shop to the exact dimensions needed for each job. Then it is transported to the job site where it is assembled for installation.

Round ductwork is mainly used as the “branches” coming off the rectangular “trunk” of a typical HVAC system. The round ducts distribute the air to various rooms in a building and normally end at a register (air vent or grate), which may be in a floor, ceiling or wall. Various kinds of elbows, “T’s and junctions are available to make all the connections involved in a complete system. This is an area where the expertise of those installing the ductwork will shine. A job well done will produce a tight system that operates efficiently. When the installers do a poor job, the resulting system of ducts will have leaks that lose air pressure and cause hot and cold spots in your home. And all that leaked air is like leaking money. Imagine heated air leaking into the attic or basement when you want it in the living room or bedroom. Similar can be said for cooled, dehumidified air leaking from gaps in poorly fitted ductwork. It wastes money with high energy bills. This is true of all ductwork types when poorly installed.

Flat Oval ductwork is often found between studs in walls to carry air from one level to another. Flat oval has been a more recent replacement for small rectangular ducts for this purpose. Also made from galvanized steel, they offer a slightly greater efficiency over their rectangular counterpart.

Spiral Ductwork

Spiral Ductwork is one of the newer offerings in rigid ductwork that is an improvement over the traditional sheet metal style. It is a spirally wound product with all seams on the outside. This makes the inside surface very smooth, with no longitudinal seams or square corners to restrict airflow or cause turbulence inside the duct. The result is that the air flows with less hindrance, which means higher efficiency and less strain on the HVAC equipment. Spiral ductwork is most commonly made of galvanized steel when it is hidden behind walls and flooring or used in the attic. When the ductwork is exposed, then stainless steel, aluminum and copper spiral ducts are considered.

A trending use of spiral ductwork is that of installing it inside the living space of a home or office so that it becomes part of the interior design. Stainless, aluminum and copper can give a very upscale accent, while painted galvanized duct surfaces can either blend with or provide a contrast to other interior colors.

Spiral ductwork can be found in round and flat oval shapes. Both types are available in standard sizes and sections that can be fitted together, and cut if needed, to form your ducts. Custom can also be made to any size. An advantage of some brands of spiral ductwork is that they have a method of connecting end-to-end joints that is far superior to that of sheet metal ducts. Joints are sealed by a rubber gasket and are virtually airtight; they will not deteriorate over time as some mastics and tapes are prone to do.

All varieties of rigid ductwork can be wrapped in insulation.

There are products available to insulate either the inside or the outside of the ducts, regardless of their shape and length.

Here are a few more advantages of rigid ductwork:

  • More resistant to insects and pests entering the system
  • More durable than fiberboard or flexible ductwork
  • Very resistant to tears or punctures
  • Most easily cleaned of all types of ductwork without damaging it or creating separations in the ducts

Rigid ductwork disadvantage are:

  • Difficulty in cutting and fitting together ducts
  • Requires more tools and precision to build than other types
  • Cut metal edges are very sharp, so caution must be taken when cutting it and fitting ducts together – gloves are recommended

Fiberboard Ductwork

Fiberboard ducts are made by compressing fiberglass strands which are bonded with resin and formed into 4’ x 10’ boards. They are then covered with a foil laminate sheet on the outside for moisture protection. The long side edges are designed so that they form an “overlap” joint when mated with another side edge. They can be found in thicknesses of 1”, 1 ½” and 2”. Fiberboard is almost always made into square or rectangular ducts, because it starts as a flat board.

Some of the advantages that fiberboard ductwork offers are:

  • Typically, the least expensive option – both for purchase and installation
  • Practically unlimited options for size of the ducts, because the board is easily cut to size and fitted by a trained installer
  • Insulated to provide both thermal insulation and sound benefits

Some disadvantages are: 

  • The fiberglass strands on the inside surface have been known to break down over time, and the ductwork loses its insulation value and causes fibers to be blown around the house.
  • The texture of the internal surfaces provides more opportunity to trap dust and debris, and this will cause higher static pressure over time – and your HVAC equipment will work harder, causing a rise in energy costs and a decrease in indoor comfort control.
  • As a result of these disadvantages, fiberboard ductwork might require replacement earlier than sheet metal ductwork

Flexible Ductwork

Flexible ductwork is made from a strong steel spring spiral coil covered in a tough and durable, yet bendable, polymer or plastic. Because they are flexible, they can be installed in tight places and can be bent around framing elements – like in an attic. Other types of ductwork would have to use elbows and involve more end-to-end joints to do the same, but plastic flex duct, as it is often called, is very workable.

Most round flexible ductwork used for HVAC applications is insulated with fiberglass on the outside and covered with a thin foil jacket. Some manufacturers are also offering flexible ductwork made from aluminum, stainless steel and PVC plastic, both with and without insulation on the outside. Flex duct is quite popular because of its ease of installation and the fact that it tends to be quieter than metal ducts. It can also be found in rectangular and square shapes, but this is usually made from metal foil and is non-insulated.

Some of the advantages of flexible ductwork are:

  • Inexpensive, lightweight, and easy to install
  • Great for areas with existing framing and hard to reach smaller spaces
  • Can be used in areas that require longer lengths of ductwork without end joints

Some disadvantages are:

  • Improper installation will usually mean kinks and tight bends that restrict airflow
  • Care must be taken to firmly support and secure to prevent sagging, kinks, or tight bends
  • Punctures easily and can be penetrated by some rodents
  • Can be easily crushed
  • May leak at joints between flexible ductwork and sheet metal ductwork

Final Thoughts

HVAC ductwork has come a long way since it was first used in homes almost 100 years ago. Manufacturers now offer a large variety of materials, shapes and styles for you to choose from. Hopefully, this article will have given you some food for thought the next time you consider either installing ductwork in a new building or replacing or repairing some that already exists.

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