Ducts work better together—here’s how they’re joined
When it comes to a properly functioning HVAC system, ductwork is something of an unsung hero. Even with the best air conditioner or furnace in the business, you’d be out of luck without the series of metal pipes or tubing that runs through your home, regulating airflow and ensuring every space is evenly cooled or heated.
Of course, ducts don’t just arrive pre-assembled and perfectly sized for your home. They must be constructed and retrofitted to meet your home’s needs, which usually means that separate pieces of ductwork need to be joined together—that’s where duct joints come in.
To understand how duct joints work, we must understand what ducts are made of. There are a few different types of ductwork out there, including rigid, semi-rigid, and flexible ductwork.
Rigid ductwork is most common and is usually made of sheet metal, like galvanized steel or aluminum (which is also what semi-rigid ductwork is made of). This type of ductwork may also be made of fiberglass or incorporate fiberglass insulation.
Flexible ductwork is made of flexible plastic and metal wire and can be bent and shaped to suit a home’s needs.
Different types of ductwork use different types of duct joints, though all these joints have the same purpose: to link together ducts and help air move efficiently and properly through the system. Duct joints may be screwed or riveted into place, sealed with a sealant, or welded.
Let’s dig into the different ways in which ducts are held together.
Also known as a Pittsburgh lock or Pittsburgh seam, Pittsburgh joints occur longitudinally along the duct—that is, the long way down, not the short way across. These are less of a joint holding multiple ducts together, and more part of the way each individual duct is constructed.
Pittsburgh joints are specifically designed using overlapping layers of metal that can handle pressure increases and decreases, which ducts must allow as air flow and temperature expands and contracts the metal.
S cleats and drive cleats, also known as D cleats, are the components that make up the connective fittings that can be slipped together along the transverse portion of the duct—which is the wide part rather than the long one, i.e., where the ducts connect to one another.
The S cleat runs along the top of the duct, while the drive cleat runs along its height from top to bottom, and the two are molded in such a way to fit neatly into each other. They are then hammered into place. This is one of the most common ways ducts are fitted together in commercial settings or for HVAC systems that put out low to medium air pressure.
Ducts can also be fitted together using a transverse duct flange (TDF) joint, which refers to a lifted metal piece at the end of the duct that can be fitted onto a similar TDF joint on another duct. In other words, rather than the ducts fitting neatly together with cleats, the transverse duct flange joint allows them to be riveted together along the flanges themselves, which join up.
A slip joint, in ductwork, is basically what the S and D cleats create together: a system where the ducts can be simply slipped onto each other, and then hammered and perhaps riveted into place.
In some cases, especially in industrial or commercial capacities with greater range of air flow and temperature, expansion joints can be used in ductwork, which incorporate a fabric section that allows the joints to more fully expand and contract as needed.
In some cases, you may need your ducts to bend in order to fit into your home’s space—which is where duct elbows come in. Prefabricated of sheet metal and available from home improvement stores, duct elbows can help you get your ducts moving around corners or through vertical space.
In some cases, you may also need to make a shift between one size of duct and another within your ductwork system in order to regulate air flow or meet your HVAC system’s requirements. A duct reducer is a metal joint that’s wider on one end and narrower on the other, allowing you to easily make this shift.
Finally, your ducts are pointless unless they actually lead to an air vent, or register, in your house—which is where duct boots come in. With one circular end (to fit your ducts) and one rectangular end, duct boots work to connect your ducts to your air registers, where air flows out into your home. That means that your HVAC system can do its job and keep your home nice and warm (or cool, depending on the season).
Still have questions about the types of duct joints that’ll get your home’s air flowing properly—or ready to get started on your project? Your local duct installer can answer your questions and help you get going.
Duct installation or replacement costs $1,250 on average, though most homeowners will spend between $450 and $2,180 on the project. Duct installation costs depend on the duct material you choose, the complexity of the ductwork, and the size of your home.
Flexible, non-insulated duct material costs $1 to $ per square foot, whereas flexible, insulated duct material costs more—$2 to $4 per square foot.
Duct installation is not a DIY-friendly project, and in many areas, it’s not legal to do this work on your own. A professional is often required to install your ducts, due to local codes. Furthermore, improper installation could lead to leaks, damaged ducts, poor energy efficiency, and warranty invalidation. It’s best to call a duct professional to handle installation.
The most common way of joining ductwork depends on the application and strength required, as there are several ways to join them. Duct installation pros will often use a slide and drive, also known as “S and Drive,” to join HVAC ducts. Other methods include the Pittsburgh seam, a snap-lock, duct boots, frames, pocket locks, bar or angle cleats, and screws.
The most common type of ducts are flexible ducts and sheet metal ducts. Flexible ducts can be insulated or non-insulated, and work well in tight spaces. They’re also lightweight and easy to install. Sheet metal ducts are popular for their durability and longevity. Other less-common types of ducts are fiberglass and fiberboard.
You can hire a professional to splice ductwork, but the method they’ll use will depend on the type of ductwork you have. A pro can join two pieces of flexible ductwork together using a male connection piece that fits the size of your duct. They can also use a splice collar. A pro can also connect two rectangular duct sections using a Flat S Lock and Drive Cleat.
