Spiral Flat Oval

Spiral Pipe of Texas


Spiral Flat Oval Duct

It’s Not Rectangular Duct With the Corners Rounded Off

Each year we look at thousands of commercial HVAC duct systems with design concepts that are stuck in the 1980’s. These designs have been costing building owners — and installing contractors — millions of dollars in operating and installation costs. All because of one simple graphic that appeared on page 2.4 of the 1990 SMACNA HVAC Systems Duct Design manual that appeared to support the “common knowledge” of the time.

In comparing a simple duct system — 40’ of straight duct, a 90-degree elbow, a transition and three installed taps — it suggested that the installed rectangular duct section ($775 to $1375) was less expensive than the installed flat oval duct section ($975 to $2100). After 25 years and thousands of real-world cases where that suggestion was proven to just not be true, we are still battling the 80’s mentality that “flat oval is too expensive”.

But a lot more than just the “cost” has changed in the last 25 years. If just one of the following statements is true about your upcoming project, you need to immediately change your design to one using round and spiral flat oval duct.

  • I have an AHU external static pressure of 2” WG or more
  • My system is relatively free of obstructions and follows the most efficient layout
  • My local energy code follows ASHRAE Standard 90.1-2010 or later
  • I have concerns about sound
  • Except where needed for noise control, I prefer ducts to be externally insulated to prevent future environmental concerns and make duct cleaning easier
  • I would like the ducts to be as small as practical
  • I’d like to follow the ASHRAE Advanced Energy Design Guides for my application
  • I don’t want excessive duct leakage
  • I want the least expensive duct system
  • I care about what a duct system costs — to install and to operate

If yours is a commercial HVAC project, you probably answered yes to most, if not all, of the above statements. We are going to give you the reasons why spiral flat oval duct is your best choice to satisfy each of those concerns:

Why Flat Oval Duct?

Pressure and Duct Construction

Most designers require that duct construction follow the SMACNA HVAC Duct Construction Standards. That manual is an excellent structural guide for building ducts and follows a simple premise — what is the minimum metal thickness and reinforcing to limit wall deflection to a specific amount?

Round spiral duct has no wall deflection in positive pressure, but both rectangular and flat oval ducts have flat surfaces that will deflect. Flat oval will have less than a comparative rectangular because you can subtract out the curved sides. You also have those 4-ply spiral lockseams every 4 ½” to help break up the flat surface. But the reason we bring up “2 in. WG or more” is that static pressure determines how much deflection a duct surface will have.

The SMACNA manual has gauge-deflection charts for seven different pressure classifications for rectangular duct — ½”, 1”, 2”, 3”, 4”, 6” and 10” WG. But only one for spiral flat oval duct — 0” to 10” WG.

So unlike rectangular duct, if you follow the SMACNA standards you will need to use the same metal gauges for ½” WG pressure as you do for 10” WG. Material costs greatly affect the installed cost of a duct system, and a one-even-gauge difference amounts to around a 20% weight difference. The differences between rectangular weight and spiral flat oval weight can be substantial in the “low pressure” range, but things start to even out in flat oval’s favor as the pressure class increases.

Straightforward, Efficient Duct Layouts

A flat oval duct system is not all flat oval. A lot of it is actually round spiral duct. And no one contests that round spiral duct is less expensive and more efficient than rectangular duct. Most flat oval duct systems are only flat oval in the major trunks. And those branch fittings are usually round fittings — efficient round taps manifolded to the flat oval trunks. So the “flat oval” in your duct system is mostly straight spiral pipe, elbows and transitions. This is where you can really start to save money.

Spiral flat oval duct is usually provided in 8’ to 10’ lengths, and can be made up to 20’ long. That compares to about 56” for a standard coilline rectangular duct, so you have a lot less duct joints and duct sealing (we’ll get to that later). Obviously, the less elbows and transitions you have, the more straight duct savings you will pick up with flat oval. But don’t assume the cost of flat oval fittings are exorbitant compared to rectangular fittings, because it’s not the case.

Unlike the 1980’s, both flat oval and rectangular fittings are now made on those same plasma torch tables with automated programming. And some of the new seam welding and lockforming equipment make the assembly of flat oval fittings just as easy as banging together the Pittsburgh locks on rectangular fittings. An efficient duct layout will be mostly straight ducts, and spiral flat oval duct will be the less expensive choice.

ASHRAE Standard 90.1

Many of the jurisdictions we work in are just now getting building code updates that bring them up to the 2010 or 2013 ASHRAE Standard 90.1 energy code. So it’s not surprising that the new energy codes have not yet started affecting how engineers design duct systems. Here’s a spoiler alert — if you still design duct systems around rectangular ductwork, you are not going to meet the new energy codes without taking some extraordinary measures.

So, what changed? The new ASHRAE energy codes are the culmination of years of study and debate to address something we’ve known for years — duct systems leak too much and it’s been costing billions of dollars. One 2005 study conservatively estimated duct leakage cost as $2.9B per year. The new codes have eliminated some faulty practices that have existed for decades. First of all, there is no more “Class B” and “Class C” duct sealing. All ducts — even low pressure ducts — must be sealed to Class A.

That means all joints and all seams except spiral lockseams must be sealed. And even though all ducts are not required to be leak tested, they must still meet the same leakage requirements of the duct that must be tested. And the maximum permitted duct leakage is “Class 4” — 4 CFM per 100 ft2 of duct surface at 1” WG. You need look no further than the SMACNA HVAC Air Duct Leakage Test Manual to see it clearly shown that the expected leakage of a properly sealed rectangular Class A sealed duct system is Class 6. For round and flat oval duct systems it is Class 3. Can rectangular duct be sealed enough to meet this new standard? Probably, but not without buckets of sealant and practices far beyond what most engineers and contractors are used to. This applies to the low pressure duct as well.

When you get into the economics of this change it makes a lot of sense. Choosing the right duct options to meet these new standards can have little or no net impact on cost. But trying to “fix” the wrong products to meet the new standard will cost a lot. In the end, an estimated cost to the owner of $1.75/CFM/year for duct system leakage — regardless of where in the system it was happening — makes the payback for better systems worthwhile.

ASHRAE Advanced Energy Design Guides

“Round duct is preferred over rectangular duct. However, space (height) restrictions may require flat oval ductwork to achieve the low-turbulence qualities of round ductwork.” That is the wording in your ASHRAE Advanced Energy Design Guides. It is a practical and common sense approach to sustainable construction and to meet the net energy usage goals of ASHRAE and the industry.

Duct Noise

Do you have a version of the venerable “Trane Ductulator”? It’s okay, we know you use them, even though we are all supposed to be using static regain duct design programs by now. Look on the right hand side in the red-orange area of the wheel, where “Air Volume – CFM” overlaps “Velocity – FPM”. Did you ever notice how there is a darker shading on velocities greater than 2000 FPM and a note to verify that your velocities are correct?

That’s because rectangular ducts are not recommended to be used over 2000 FPM velocity. Those square corners in rectangular duct cause far more turbulence than the rounded profiles of round and flat oval duct, and turbulence increases dynamic pressure losses and noise. For ducts suspended above an acoustic ceiling to meet a design RC of 35, the recommended maximum velocity for rectangular duct is 1750 FPM versus 3000 FPM for round and flat oval (2015 ASHRAE Handbook – HVAC Applications Chapter 48).

If you move the same mass of air but can do so at a higher velocity by changing from rectangular to round and flat oval, what does that mean? It means you could be using smaller ducts. Yes, higher velocities result in higher pressure drops, even with round and flat oval ducts. But overall, the fittings are more efficient already. Reducing leakage allows you to reduce your needed volume. Lastly, using a static regain design program lets you really start getting a smaller, more balanced system without needing those other noisy things you put in rectangular duct systems — balancing dampers.

Externally Insulated Ducts

If you compare internally insulated rectangular ducts versus internally insulated flat oval ducts, you will usually see a big price advantage for the rectangular duct. That’s because you insulate rectangular ducts — at least you did 25 years ago — with glued and pinned duct liner. Round and flat oval ducts had to be doublewall construction.

We’ve had a huge change in the industry since 1990 because we now see the industry move away from duct liner and towards external insulation wrap. Even where ducts are internally insulated for acoustics we are now seeing rectangular ducts required to be double-wall construction.

Rectangular double-wall duct is more expensive than round or flat oval double-wall duct. And for external wrapping, that old advantage for insulating rectangular versus flat oval not only goes away, it actually favors flat oval. Because of those rounded corners, an “equivalent” flat oval duct has less surface area — and less required insulation — than it’s equivalent rectangular duct.

Smaller Ducts

See above. More efficient, higher velocity ducts can be smaller.

Insulated Flat Oval Ducts

Insulated Flat Oval Ducts

There are a lot of misconceptions about ductwork, but the one that is probably most egregiously wrong is the one about “double-wall flat oval duct is just too expensive to use”. While based loosely on a few facts, and maybe kind of true for the way ducts were fabricated and installed 25 years ago, the notion is outdated and needs to be reevaluated for modern duct systems. You really need to be using more flat oval duct and less rectangular duct to remain competitive and meet modern performance standards. Let’s take a look at a few of the facts about these modern duct systems.

  • Double-wall flat oval duct used to be compared to lined rectangular duct without an internal metal shell. For modern VAV systems, we have seen a trend over the last 20 years to use internal insulation for a certain distance from the fan, usually 25-30 feet, for sound attenuation, then use external wrap insulation on the remaining ducts. Those ducts, even the rectangular ones, now typically have an inner metal liner. The difference of installed cost between double-wall flat oval and double-wall rectangular is usually negligible in $/ft2.
  • Ahh, the “cost-per-square-foot” comparisons! Did you know that a flat oval duct has less “square-feet-per-lineal-feet” than an equivalent rectangular duct? On average, about 8% less. So, you really are less expensive for the system using double-wall flat oval versus double-wall rectangular.
  • Modern duct systems are expected to be sealed (Class A sealing for all duct systems per ASHRAE Standard 90.1-2013). Old estimating algorithms required an 8% add for sealing rectangular ducts. There is virtually no add for sealing flat oval duct. The spiral lock-seams are exempt from the sealing requirement, there are no corners to be sealed, there are usually less than half as many transverse joints (10-12 foot lengths versus 4-5 foot lengths of rectangular), and those transverse joints are typically flanged and require only butyl tape gasket to seal.
  • Let’s not forget the cost of the duct leakage. A rectangular duct system sealed to Class A standards is expected to leak 6 CFM/100 ft.2 of duct surface at 1” WG. A flat oval duct system sealed to Class A is expected to leak 2 CFM/100 ft.2 of duct surface area. For a 2.0” WG system, that is 9.4 CFM/100 ft.2 and 3.1 CFM/100 ft.2 respectively. At a system leakage cost estimate of $1.75/CFM/year the payback versus lined rectangular duct is less than seven years‡‡. It’s immediate when compared to double-wall rectangular.
  • Flat oval duct systems are not all flat oval. The duct is usually ovalized because of height restrictions. As soon as those restrictions are satisfied — and generally for all branch runouts — the duct should be designed as round. As much as half of a medium pressure duct system designed as rectangular duct should be round — and installed round duct is less expensive than installed rectangular duct (even double-wall round versus lined rectangular duct).‡‡‡
  • Sound. It’s probably the main reason you were using duct with internal insulation in the first place. Turbulence causes sound and the corners of rectangular ducts cause turbulence. The problem worsens in the constricted spaces immediately downstream of air handling units where you seldom have the opportunity to get laminar flow. The wording from some of the ASHRAE/AIA/IES Advanced Energy Design Guides sums it up pretty well:
    “Unwanted noise in the ductwork is a direct result of air turbulence. Round duct is preferred over rectangular duct. However, space (height) restrictions may require flat oval ductwork to achieve the low turbulence qualities of round ductwork.”‡‡‡‡

‡ The $1.75/CFM/year is a consensus figure used by several industry committees to determine payback for system improvements and includes both energy and cost savings from reduced equipment and ductwork for lower net CFM’s.
‡‡ Installed duct costs are based on the 1992 National Mechanical Estimator (Ottaviano) for installed duct systems with mark-up factors for labor and material.
‡‡‡ From 1992 NME, installed cost of insulated medium pressure rectangular duct at $6.88/ft2 versus $6.52 comparative cost for installed double-wall round spiral duct ($3.35/ft2 installed single-wall round spiral X 2.3 double-wall mark-up X 0.92 ft2 adjustment X 0.92 duct sealing adjustment).
‡‡‡‡ Example from 30% Advanced Energy Design Guide for K-12 School Buildings

Insulated Flat Oval Ducts

Insulated flat oval ducts from Spiral Pipe of Texas are provided as “double-wall” construction. The outer metal shell is the basis of construction, designed to contain the air at the specified pressure. The inner metal shell holds the insulation material in place and assures a smooth airstream. The use of metal spacers to maintain liner concentricity is not recommended. We recommend that for flat oval ducts with sufficient flat span to exhibit “sagging”, that flanged transverse connectors be used. They greatly increase field installation productivity while reducing deflection in both the inner and outer shells.

Outer Metal Shell

The outer metal shell of straight duct may be spiral flat oval lock-seam duct or longitudinal-seam welded duct. We make 516 spiral flat oval duct sizes that can be used for double-wall construction, excluding only the 3” and 4” minor axis duct sizes. We make double-wall flat oval duct with outer shell dimensions ranging from 13 x 5 to 124 x 36. Standard construction and gauges are in accordance with SMACNA standards for +10” WG. Fittings can be made with either fully-welded construction or “tack-welded-and-sealed” construction.

Spiral Flat oval duct can be used for negative pressure, especially long runs of return air at low pressures, but that is an area not currently covered in the SMACNA standards. An ongoing project through SPIDA (Spiral Duct Manufacturers Association) has produced gauge/construction/pressure testing on some size ranges, but they have not been published as of this time. Please consult the factory for guidance from this testing. Longitudinal seam flat oval outer shells can be made in any size, and in gauges ranging from 22 ga. to 3/8” thick.

Spiral lock-seam outer shells are normally provided in corrugated construction for increased rigidity. They can be ordered without corrugations by request. Insulated flat oval duct is increasingly used as an architectural feature in buildings, and we offer the same appearance and finish options as we do for our round ducts. Double-wall insulated flat oval ducts can be made in lengths up to 20’-0”, but we recommend that ducts in exposed applications be limited to 8’-0” in length so they can be shipped “standing up” to minimize damage to the appearance. We also recommend that even non-exposed double-wall flat oval ducts be limited to 10’-0” lengths because they have twice the weight of single-wall ducts and most construction sites lack the equipment to safely offload large sections.

Outer metal shells are available in all materials used for our round products — G90 and G60 galvanized steel, Paint Grip mill phosphatized steel, A60 galvanealed steel, 3003 aluminum, 304-L and 316-L stainless steel, aluminized type-1 steel, Agion anti-microbial steel and PVS/PVC coated steel.

Inner Metal Shell

The inner metal shell may be spiral flat oval lock-seam duct or longitudinal-seam welded duct. They are available as solid metal, perforated metal (23% open area), and perforated metal with Mylar erosion barrier. Inner metal shells are available in the same materials as the outer shells. The function of the inner shell is to contain the insulation liner material and maintain the cross-sectional profile of the airstream. It is not the “pressure shell”. Unless specifically ordered, standard construction is to tack-weld all seams except the spiral lock-seams, and omit sealing. Spiral flat oval lock-seam duct is provided corrugated to increase rigidity.

Insulation Layer

Our standard insulation material is 0.75# density Knauf AtmosphereTM Duct Wrap with ECOSE® Technology. This glass mineral wool product is purchased in thicknesses 50% greater than the intended annular space, the compressed to approximately 1.0 PCF density. It is available in 1” and 2” thicknesses. As an alternative “fiber-free” insulation, we also offer Armacell AP/Coilflex® elastomeric foam insulation in 1” and 2” thicknesses.

Less Leakage

Flat oval ducts are expected to leak half of what rectangular ducts do. The reasons are pretty obvious. First of all, they have no longitudinal seams to seal. Spiral lockseams have negligible leakage (various tests carried out in accordance with AMCA 511) which have earned them their sealing exemption in ASHRAE Standard 90.1. But more significantly, they usually have less than half as many duct joints as rectangular duct (10’ or greater lengths versus 56” lengths). And most duct leakage occurs at the transverse joints. Half the joints – half the leakage.

Less Expensive

There have been presentations at the ASHRAE/AHR show most of the last 10 years from SPIDA (Spiral Duct Manufacturers Association) where example systems have been laid out using round, flat oval, and rectangular product and the relative installed costs were compared. Even using “equal friction” comparisons and not static regain design comparisons, the flat oval systems always come in less expensive — installed — than the comparative rectangular systems meeting the same leakage and performance standards.

We’ll be happy to show you our own comparisons as well as published comparisons from SPIDA and some of our competitors. There’s actually at least one engineering firm we know of that will teach engineering and contracting firms how to redesign rectangular duct systems with round and spiral flat oval duct, using the Trane static regain duct design program, and guarantee a cost savings greater than his consulting fee on their first job. It is less expensive.

Less Expensive to Install and Operate

When you use flat oval and round spiral duct systems instead of rectangular duct you are going to save money. Not just to buy the system, but also to operate it. Round and flat oval ducts have higher dynamic efficiency, lower noise, less leakage, and lower heat gain (less surface area, higher velocities for the same air mass).

So the choice should be clear. Round spiral duct is the best choice for duct construction, but if you don’t have the room, flat oval is still a better choice than rectangular duct. So if you still look at spiral flat oval duct as an expensive rectangular duct with the corners rounded off, you need to move out of the 1980’s and see the changes that have taken place in the last 25 years.

Proudly Made in Ft. Worth, Texas

Available heights of 3” to 36” minor axis. Available widths of 15” to 129”.

Available in single-wall and double-wall.

Contact our sales department or your local Spiral Pipe of Texas rep for additional information or quotations. We want to be your source for flat oval ducts and fittings.

Submittal Downloads

1995 SMACNA Standards – Spiral Pipe and Fittings

2005 SMACNA Standards – Spiral Pipe and Fittings

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