When to Choose Spiral Ductwork (Pricing, Pros and Cons)

Spiral ductwork is functional and better-looking than traditional sheet metal or flexible ductwork. It’s an ideal choice, if a little expensive, for architectural design with open ceilings. Often used in commercial settings, spiral ducts have their place in loft apartments and other residential settings where a contemporary or urban vibe is desired.

But what does spiral ductwork cost? When does it make sense to use spiral ductwork vs rectangular ducts?

Fun Fact: The Romans were using a series of tubes connected to a lower-level wood furnace to warm the tile floors in their homes as early as 300 A.D. That may have been the first true ducted home heating system.

About Spiral Ductwork

In 1956 a manufacturer in Switzerland invented a machine to produce spiral ductwork. Spiral duct is a rigid spirally wound tube usually made of galvanized steel, but can also be stainless steel, aluminum or copper. It has its seams on the outside, making the inside surface very smooth and having low resistance to airflow. Spiral ducts can be round or flat oval and are commonly produced in sizes from 3” to 63” in diameter. One manufacturer advertises flat oval sizes from 3” x 12” to a whopping 36” diameter x 124” (12-foot) length.

Where to Use Spiral Ductwork

As noted, spiral ducts are most often found in commercial settings. Warehouses, factories, schools, large churches, movie theaters and athletic venues are a few places where they are often found – but you might have to look for them. When the architect or builder wants them to “disappear,” they are often painted to match the ceiling and walls and also makes them blend in so that we just don’t notice them.

There are two common reasons to use spiral ducts – looks and functionality.

Appearance:  In residential settings, spiral ductwork can be attractive when ceiling height is 10 feet or higher and the space has contemporary or urban design. This is true in commercial settings where an “industrial chic” aesthetic is desired.

Function: Spiral ductwork is attractively made, so that it can be “out in the open” if necessary, as we’ve said. This can assist in functionality. When there is no room in the ceiling and you don’t want to install ductwork in unheated or uncooled space (outside, in the attic or garage, for example), spiral ducts work well. They are hung from the ceiling, in most cases. This approach can be cheaper than framing in space for the ducts and creating openings for air vents.

Spiral Ductwork Pros and Cons

Here are spiral ducts advantages and disadvantages.

Pros

When compared to either standard rectangular or round ductwork, spiral ductwork has a number of definite pluses.

• Better Sealed Joints. One of the biggest issues with most HVAC ductwork is getting a good air seal at connections and joints. Anywhere from 25-40% of the air in forced air systems is usually lost through leakage in ducts that are in attics or crawl spaces. Spiral duct systems not only have no joints along the length of each section, which isn’t true of sheet metal ductwork, but have a seal at the joints that meets or exceeds the highest industry air-leakage standards.
• More Even and Efficient Air Flow. The round spiral-wound duct provides an inner surface with almost no air drag or resistance. Because of its design, there is less of a pressure drop in systems composed of spiral ducts. This provides equal and consistent pressure inside the whole system and allows the air to flow evenly and efficiently, which also results in lower energy use.
• Technical note on pressure drop: Air leaves the air handler or furnace at a specific pressure in pounds per square inch, or PSI. When there is friction – yes, even with air – inside the duct, the PSI drops. This means it might come out of the air grate/vent with less force than it should – and that means less heated or cooled air gets into your living space, especially in rooms that are furthest from the air handler.
• Better Air Quality. Tighter joints mean that less dust, pollen and other contaminants are able to infiltrate the system. This will improve the quality of the air that you will breathe in your home or commercial building.
• Lower Cost to Operate. With leaks at a minimum and air flow at its maximum, the cost of moving either heated or cooled air is less than when other types of ducts are used.
• Quieter.  When air is moved past the sharp edges and corners of rectangular ductwork, it makes noise. This is attested by the fact that “silencers” are available from HVAC equipment manufacturers to help quiet these systems. Spiral ducts have no sharp edges to produce noise. And when that air gets to a gap in traditional ductwork, expect a whistling noise. Fewer seams in spiral ducts reduces this annoyance too.
• Simple to Clean. The approved tools and methods for cleaning spiral ducts are easily obtainable, economical, and effective.
• Lighter Weight. With enhanced computer engineering used to design spiral HVAC systems, up to 40% of the total weight of material can be saved over rectangular sheet metal ductwork.

Cons

There are potential downsides to spiral ducts too.

• Inexperienced installers: Some contractors are unaccustomed to working with spiral ductwork, because it isn’t as common. As a result, installation issues with wrong sizing or connection problems can occur.
• Hard to get: Spiral ductwork is not as widely available and is sometimes more expensive than standard metal duct, especially in the smaller sizes used in residential applications. This difference, however, could be more than compensated by savings over time in other areas as mentioned above.

Cost

Spiral duct up to about 12” is used for main lines in residential settings – commercial spiral ducts are often much larger, up to 36”. The most common thickness for either setting is 26 gauge metal. The price of galvanized steel spiral ductwork follows. 95% of spiral ductwork is galvanized.

Spiral Duct 26 gauge 4″ to 12″ in diameter costs $2.87 – $8.77 per linear foot.

Spiral Ductwork in Homes – Residential Use

We’ve touched on the aesthetics of spiral ducts. Here’s more information that will assist you in deciding if they are the right choice for you.

There are two main applications for spiral ductwork in the home. The first is the same as it is for other types of ductwork – that is, out of sight. The second is very different from other types – in plain sight.  Let’s look at both.

A. Ducts Out of Sight

Spiral ducts can be used to replace existing ductwork quite readily. If they are used to replace the branches off the main trunk – whether rectangular or round – the same size spiral duct can simply be used in place of whatever was used before. If the main trunk (usually rectangular) is being replaced, there will be a flat oval shape of the right size spiral duct that can be used in its place.

Pros

• There will be fewer joints and they will be sealed more tightly than they formerly were with other types of ductwork.
• Quieter and cleaner.

Cons

• Not easily found in some parts of the country.
• May be more expensive than standard ductwork because it is less available and because it is a beefier, stronger type of material.

B. Ducts in Plain Sight

In the modern home in America, one of the newer and growing trends in HVAC design is that of exposed spiral ductwork. By exposed, we mean just that – the ductwork that is usually hidden above the ceiling or in the walls is actually installed inside the living space of your home where it can be seen.

The fact that spiral can be found in galvanized steel, aluminum or copper means it can accent many different interior design schemes. It can also be painted any color to either match or accent other colors in the home – even a different color in each room.

Pros

• If existing ductwork is being replaced, walls and ceilings will not have to be torn apart in the process. This means much less labor and mess.
• When installed in a home with relatively high ceilings, spiral ducts produce a modern look that is attractive to the eye.
• Spiral ducts can be insulated on the inside so that condensation doesn’t build up on the outside when in air conditioning mode.

Cons

• It Needs high ceilings. Exposed spiral ductwork will be below the ceiling. Unless your ceilings are at least 9 feet high – and preferably higher – the finished look may not be very appealing to the eye and could give the rooms a “cramped” feeling.
• The appearance Isn’t for every interior design style. No matter how nice it may look to some people, others will always think it has that “not finished” look. For many people, HVAC ducts are something that should be kept out of sight – not hanging over their heads!

FAQs

Q: Why does my current ductwork contain both rectangular and round ducts?

A: Many HVAC duct schemes have a large rectangular “trunk” that connects to the plenum of the air handler where the fan is located for moving the air. Coming off from this trunk are several round “branches” which split the airflow and distribute it to the various rooms in your house.

Q: Should spiral ductwork be insulated?

A: It depends on location. Ductwork in conditioned space doesn’t need insulation. Ducts in an uninsulated attic or crawlspace should be, and you should notice an improvement in climate control/comfort and in lower energy costs.

Q: Is spiral ductwork installation a DIY job?

A: Probably not, for these reasons. Putting the ductwork together is doable for DIY homeowners with good skills. But making sure you have the right size ductwork to achieve the proper air pressure and airflow in your system takes pro-level calculations. Plus, cutting into a sheet metal plenum to form a “perfectly” round hole of “exactly” the right size for the connection is not an easy task.

Using Spiral Ducts

So when should you choose spiral ductwork to replace your existing ductwork? When –

• You want to upgrade to the most energy efficient and quietest type of ductwork available.
• You want to improve the air quality and keep mold and mildew out of your HVAC system.
• Your old ductwork has failed and you cannot access the ceilings or walls to replace it.