DUCT CONSTRUCTIONSMACNA

Sheet Metal & Air Conditioning Contractors National Association

About SMACNASMACNA's mission is to provide products, services, and representation to enhance members' businesses, markets, and profitability.Located in headquarters outside Washington, D.C., the Sheet Metal and Air Conditioning Contractors' National Association (SMACNA), an international association of union contractors, has 1,944 members in 98 chapters throughout the United States, Canada, Australia and Brazil.

Member ProfileSMACNA members perform work in industrial, commercial, institutional and residential markets. They specialize in heating, ventilating and air conditioning; architectural sheet metal; industrial sheet metal; kitchen equipment; specialty stainless steel work; manufacturing; siding and decking; testing and balancing; service; and energy management and maintenance.

Technical Manuals and StandardsThe voluntary technical standards and manuals developed by SMACNA Contractors have found worldwide acceptance by the construction community, as well as foreign government agencies. ANSI, the American National Standards Institute, has accredited SMACNA as a standards-setting organization. SMACNA does not seek to enforce its standards or provide accreditation for compliance.SMACNA standards and manuals address all facets of the sheet metal industry, from duct construction and installation to air pollution control, from energy recovery to roofing. SMACNA's Technical Resources Department fields several thousand technical questions annually from architects, engineers, manufacturers and government personnel.More than 17,000 orders for SMACNA technical manuals are processed and shipped each year from SMACNA national headquarters. This translates into sales of more than 50,000 technical manuals, generating approximately $1 million in income for the association.

Member ServicesThe association offers contractors professional assistance in labor relations, legislative assistance, research and technical standards development, safety, marketing, business management and industry issues.

ASHRAE

American Society of Heating, Refrigerating and Air-Conditioning Engineers

ASHRAE, founded in 1894, is an international organization of 51,000 persons. ASHRAE fulfills its mission of advancing heating, ventilation, air conditioning and refrigeration to serve humanity and promote a sustainable world through research, standards writing, publishing and continuing education. 

ASHRAE   Mission

To advance the arts and sciences of heating, ventilating, air conditioning and refrigerating to serve humanity and promote a sustainable world.

ASHRAE   Vision

ASHRAE will be the global leader, the foremost source of technical and educational information, and the primary provider of opportunity for professional growth in the arts

and sciences of heating, ventilating, air conditioning and refrigerating. 

Construction Materials and Their Normal Usage

A variety of materials have been used in the construction of ducts. Selection of the materials used throughout the duct system, should follow the same careful consideration as the other HVAC system components. The materials used in duct systems can substantially affect the overall performance of the systems. Each material should be selected carefully after considering its advantages and limiting characteristics.

Materials used for ducts include: galvanized steel, black carbon steel, aluminium, stainless steel, copper, fibreglass reinforced plastic (FRP), polyvinyl chloride (PVC), polyvinyl steel (PVS), concrete, fibrous glass (duct board), and gypsum board.

Consideration must also be given to selection of duct construction components, other than those used for the duct walls. Items such as flexible ducts, duct liner, pressure-sensitive tapes, sealants, adhesives, reinforcements, and hangers are described in individual SMACNA manuals, as well as many other publications.

Many of the metals you work with are alloys. An alloy is a metal that has another metal or other substance added to it. Steel is an alloy because it is iron that has carbon or some other substance added to it.

Ferrous metals contain iron. All steel sheets are ferrous. Galvanized sheets are ferrous because they are steel sheets coated with zinc.

Nonferrous metals do not contain iron. For example, copper and aluminum are nonferrous metals.

Some common terms are used to describe the characteristics of metal:

Ductility Temper

Hardness Tensile strength

Ductility is a measure of how much a metal can be worked without breaking. Think of ductility as softness, although this isn’t an exact definition. In sheet metal shops, lead is the most ductile metal used. It can be formed by hand, bent and rebent, and hammered until it is very thin, but it still remains soft. On the other hand, some types of aluminums that have been hardened have so little ductility that they cannot be bent to a 90˚ bend

without breaking. However, the metal you find in the sheet metal shop is usually ductile enough to allow it to be bent for various seams and edges.

Hardness is the opposite of ductility. Hardness is a measure of how brittle a metal is. The harder a piece of metal is, the more brittle it is. In the sheet metal shop, most metals are not extremely hard, because they must be formed in one way or another. However, many metals can be specified in different degrees of hardness for special applications.

Temper is the ability of the metal to retain its shape. Think of temper as the toughness of the metal. Temper is related to hardness. After a tool has been hardened, it is heat-treated again to temper it. A tempered edge is not quite as hard, but it can cut without chipping or losing its sharpness. The cutting edges of snips are tempered. The sheet metals you use in the shop are not tempered.Tensile strength is the strength of metal under a pulling-apart force. It is the number of pounds that a square inch section of the metal can hold on a straight pull before it breaks. It is given in pounds per square inch. Architects and engineers must calculate the tensile strength of structural metal. Tensile strength is not usually important for sheet metal workers. However, it is a term you should know.

An oxide of a metal is the chemical that forms when metal is exposed to the oxygen in the air. Every metal forms a film of oxide on its surface. Each oxide has different characteristics. Iron oxide is rust. Copper oxide has a gray-green colour. Some oxides cannot be seen. The oxide not the metal itself determines the appearance and lasting characteristics of a metal.

Iron Oxide (Rust)Rust is a form of iron oxide. When iron is exposed to the air and to moisture, iron oxide forms. Since iron oxide (rust) is porous and flaky, the oxygen continues to penetrate through the oxide to the metal and continues to form more rust. As more rust forms, the flaky outside drops off and continues to expose more of the iron to oxygen. This is why iron will rust through in a fairly short time when exposed to moisture in air.

Stainless Steel Oxide

Stainless steel oxide is extremely tough and resists the action of most corrosives that dissolve oxides. It forms quickly and is completely transparent. The result is just as if an extremely thin and tough coat of plastic had been formed over the metal.

Compared to iron, stainless steel is practically indestructible. Under normal conditions it will last for many lifetimes. Stainless steel maintains its finish, and most stains do not penetrate into the metal.

Copper OxideCopper oxide is called a patina. It is a tough oxide that resists most chemicals. This is why copper is so long-lasting. The colour of the patina on copper gradually changes over years from brown to green. Applying certain chemicals to copper roofs produces a green patina, which is a desired effect for appearance on some jobs. A brown or green patina gives a soft, warm appearance.

Aluminum OxideAluminum oxide forms almost instantly and is very difficult to dissolve with acids. This is why aluminum is extremely difficult to solder. The aluminum oxides cannot be removed and kept off long enough to complete a soldering job.

Lead Oxide Lead oxide is very tough, so lead is one of the longest lasting metals used in the shop. Because to the tough lead oxide, lead is scraped be fore it is soldered.

Coated sheets have a covering of a different metal or another material (such as polyvinyl). For example, galvanized steel is a coated sheet because it is a steel sheet coated with zinc to give it longer life and prevent rusting. The oxide of zinc is called white rust. Uncoated sheets contain the same material throughout.

The coating on a sheet determines how corrosion resistant it is. The base sheet determines how workable it is.

The advantage of the protective coating is lost if the coating is damaged or destroyed. For example, if you make a sink of galvanized iron and weld the

corners, you have burned off the zinc coating during the welding process and the iron is exposed to rust in those areas.

Gage refers to the thickness of sheet metal. The metric system of sheet metal gages gives the sheet thickness in millimetres. The U.S. Standard Gage is used in the United States and Canada to identify iron sheet and stainless steel.

The system of identifying sheet metal gages has developed gradually over a period of many years, based more on tradition than logic. Frankly, there isn’t a great deal of logic in sheet metal gages. In fact, even the terms gage and gauge mean the same though they are spelled differently.

You can remember the approximate thickness of a gage in fractions of an inch. For example, 11 gauge is approximately 1/8” and 16 gage is approximately 1/16”. From 16 gauge on up, every sixth gage is approximately half the thickness. These gages and their sizes can help you remember the system:

11 ga. – 1/8”

16 ga. – 1/16”

22 ga. – 1/32”

28 ga. – 1/64”

A. Galvanized Steel

Galvanized sheet metal is soft steel sheets coated with zinc. There are two methods of applying the zinc. In the most common one, the steel is dipped in an acid bath for cleaning and then is dipped in molten zinc, in the other; an electroplating process does the coating.

SMACNA’S Duct construction Standards Metal and Flexible states: “unless other wise specified, steel sheet and strip used for duct and connectors shall be G-60 and or G-90 coated galvanized steel of lock-forming grade.

Gauged: using U.S. Standard Gauge

Items normally constructed of galvanized iron are as follows:

1. Air ducts for standard ventilation and air conditioning systems not subjected to extreme acid fume or humidity.

2. Casings and housings for coils, air washers, fans and filters.3. Roof ventilators and cowls.4. Volume control dampers.5. Intake and exhaust louvers.6. Hangers for ducts.7. Spray booths.8. Hoods of all types.9. Fire dampers.

Advantages

1. Economical2. May be used in direct contact with concrete3. Easy to work, easy to join in shop and field.4. Durable long term performance5. Light weight6. Low expansion.7. Stain resistant.

Precautions

1. Do not use in conjunction with copper.2. Do not use in severely corrosive atmospheres, particularly

industrial and chemical environments.3. Insulate with bituminous coating when in contact with copper,

redwood or red cedar. Also verify compatibility with chemically treated wood.

4. Use appropriate flux and solder, neutralize flux after soldering.

B. Black Iron, Mild Steel Sheets

Mild steel or black iron is a strong steel with very low carbon content. 0.05% to 0.25%

Rust is a form of iron oxide. When iron is exposed to the air and to moisture, iron oxide forms. Since iron oxide (rust) is porous and flaky, the oxygen continues to penetrate through the oxide to the metal and continues to form more rust. As more rust forms, the flaky outside drops off and continue to expose more of the iron to oxygen. This is why iron will rust through in a fairly short time when exposed to moisture in air.

Gauged: using U.S. Standard Gauge

Items normally constructed of black iron

1. Boiler breechings (smoke pipes), Gas fired units if acceptable.2. Hoods.3. Belt guards4. Dampers and hoods conveying high temperature air of gasses. 5. Ducts requiring paint or special coating.6. Cabinets7. School lockers

Advantages

1. High strength, rigidity, durability.2. Paintable.3. Easily welded.

Precautions

1. Very low corrosion resistance, must be painted or coated to suit its environment

C. Sheet Copper or Lead Coated Copper

Lead coated copper is copper that is coated with lead on both sides. It has a characteristic gray colour and is used primarily to limit staining of concrete, stone, etc., and where the gray colour is desired

Copper oxide is called patina. It is a tough oxide that resists most chemicals. This is why copper is so long-lasting. The colour of the patina on copper gradually changes over years from brown to green. Applying certain chemicals to copper roofs produces a green patina, which is a desired

effect for appearance on some jobs. A brown or green patina gives a soft, warm appearance to copper.

Lead oxide is very tough, so lead is one of the longest lasting metals used in the shop. Because of the tough lead oxide, lead is scraped before it is soldered.

Gauged: using Ounces per square foot

Items normally made of copper or lead coated copper

1. Exposed ducts where permanency is important and painting is either impossible or expensive.

2. Shower and swimming pool exhaust ducts where extreme humidity conditions occur.

3. Exhaust doors and ducts where the fumes are harmful to other materials

4. Roof ventilators or cowls for permanent installation where painting is not practical.

5. Intake and exhaust louvers for appearance or where servicing or replacement is difficult.

Advantages

1. Resistant to corrosion in air and moisture2. Easy to work, easy to join in shop and field3. Not corroded by masonry, concrete or stucco when flashed or

embedded therein.4. Accepts solder readily.

Precautions

1. Use copper or copper alloy fasteners.2. Use appropriate flux and solder, neutralize flux after soldering3. Avoid direct contact uncoated aluminum, steel, galvanized steel

and other non-compatible metals

D. Lead

Lead is a dull gray metal used for roofing, flashings, water proofing sound isolation, and as a radiation barrier. On exposed roof surfaces it develops a soft gray patina over the years.

Lead oxide is very tough, so lead is one of the longest lasting metals used in the shop. Because of the tough lead oxide, lead is scraped be fore it is soldered.

Gauged: using pounds per square foot

Advantages

1. Extremely workable, conforms to surfaces on which it is applied

2. Very resistant to atmospheric corrosion.3. Limited staining of adjoining surfaces.

Precautions

1. Reacts with uncured concrete and mortar: use bituminous coatings for protection

2. Eliminate rough projections on underlying surfaces.3. Allowances must be made for high thermal expansion rate.4. Use appropriate flux and solder, neutralize flux after soldering

E. Aluminum

Aluminum is used extensively as a substitute for galvanized steel. But more care must be used, gauges of metal must be heavier and more reinforcing installed.

In warm air systems, where the air in the ducts varies considerably in temperature, aluminum will expand and contract more than steel. Unless this movement is compensated for, it will cause the ducts to be noisy

Aluminum has a much lower melting point than steel and therefore should not be used where high temperatures are liable to occur. For exterior ducts ventilators, louvers, etc., it resists corrosion without paint.

Aluminum oxide forms almost instantly and is very difficult to dissolve with acids. This is why aluminum is extremely difficult to solder. The aluminum oxides cannot be removed and kept off long enough to complete a soldering job.

Gauged: using decimals of an inch

Items normally constructed of aluminum:

1. Roofing, flashing and numerous other architectural applications.2. Duct systems for moisture-laden air.3. Ornamental duct systems.4. Sometimes aluminum is substituted for galvanized steel in HVAC

duct systems.

Normal uses are the same as steel with the following exclusions.

1. Kitchen exhaust ducts where grease accumulation may cause a fire2. Ducts carrying air or fumes at temperatures over 600 deg. F.3. Ducts from showers or pools where the water is chlorinated.4. Fire dampers or collars enclosing same.5. Duct in or attached to masonry walls where moisture is continually

present, which would cause the lime in the walls to corrode the metal.

Advantages

1. Lightweight corrosion-resistant material.2. Will not stain adjacent surfaces.3. Ductile, malleable, and easily worked.

Precautions

1. Use aluminum or stainless steel fasteners.2. Cannot be soldered. Use rivets and sealer or weld joints.3. Avoid direct contact with dissimilar metal, and with concrete or

mortar. Coat with bituminous paint when in contact with these materials

4. It is not recommended for through-wall flashing. If used, it must be coated

F. Stainless Steel

Stainless steel is a durable, maintenance free, corrosion resistance material with a silvery appearance. The 300 series typically used for roofing and flashing applications are alloys of steel incorporating chromium, nickel and manganese. Type 316 also contains molybdenum. Series 400 does not contain nickel, is less corrosion resistant, and is used primarily for interior applications.

Stainless steel oxide is extremely tough and resists the action of most corrosives that dissolve oxides. It forms quickly and is completely transparent. The result is just as if an extremely thin and tough coat of plastic had been formed over the metal.

Compared to iron, stainless steel is practically indestructible. Under normal conditions it will last for many lifetimes. Stainless steel maintains its finish, and most stains do not penetrate into the metal

Gauged: using U.S. standard gauge

Items normally constructed of stainless steel:

1. Exposed ducts that are not to be painted and a bright finish is desired.

2. Shower and pool exhaust ducts.3. Intake and exhaust louvers.4. Kitchen range hoods complete or as trim on galvanized or black

iron hoods.5. Fume exhaust hoods when other metals are not satisfactory

Advantages

1. Excellent corrosion resistance requiring no artificially applied surface protection coatings.

2. Self cleaning, requires little or no maintenance

3. Not affected by mortar or concrete.4. Does not stain adjacent surfaces.5. Superior resistance to metal fatigue

Precautions

1. Clean surfaces after fabrication to remove contaminants that can lead to surface corrosion

2. More expensive than other materials3. Use special stainless steel-type flux, appropriate solder, and

neutralize flux after soldering.Stainless steel is available in 44 different alloys with various finishes and colours.

G. Flexible Pipe

1. Metal industrial.2. Wire reinforced, fabric, plastic.3. Band reinforced fabric.4. Insulated and Acoustical

H. Double metal pipes

1. Galvanized iron and aluminum.2. Aluminum and aluminum.3. Aluminum and stainless steel.4. Galvanized iron and stainless steel.5. Stainless steel and stainless steel.

I. Double Filled Pipes

Uses- High temperature and high humidity chimneys:

Class A – Masonry filledClass B – Double pipeClass C – Single pipe

J. Sonair Duct

Spiral wrapped paper, vapour barrier, paper and aluminum foil.

Uses – Concrete forms, slab concrete, attic installations, may be used with or without collars.

K. Transite or Asbestos Board

The transite, that is produced today, is a completely fireproof composite material and a non-asbestos product. Transite HT, and Transite 1000, are currently available fiber cement boards that contain no asbestos. Instead it contains crystalline silica which has been classified by The International Agency for Research on Cancer (IARC) as being carcinogenic to humans (Class 1). Crystalline silica is also known to cause Silicosis, a non-cancerous lung disease.

The use of asbestos to manufacture Transite was phased out in the 1980s. However prior Transite was made of 12-50% asbestos and cement, leading to its frequent use for such purposes as furnace flues, shingles, siding, and wallboard for areas where fire retardancy is particularly important. It was also used in walk-in coolers made in large supermarkets in the 1960s, 1970s and even the 1980s. Other uses included roof drain piping, sanitary sewer drain piping, and HVAC ducts. Because cutting, breaking, and machining transite releases carcinogenic asbestos fibers into the air, its use has fallen out of favor.

1. Fume exhaust systems.2. Extremely high temperature ducts.3. Stacks for gas heater vents.

L. Plastic Duct

Polyvinyl chloride, commonly abbreviated PVC, is a widely used thermoplastic polymer. Around the world, over 50% of PVC manufactured is used in construction. As a building material, PVC is cheap, durable, and easy to assemble. In recent years, PVC has been replacing traditional building materials such as wood, concrete and clay in many areas.

Polyvinyl chloride is used in a variety of applications. As a hard plastic, it is used as vinyl siding, window profiles, pipe, plumbing and conduit fixtures.

PVC pipe plumbing is typically white, as opposed to ABS, which is commonly available in grey and black, as well as white.

Corrosive fume exhaust systems. (P.V.C. should not come in contact with Petro Chemical Gasses). As there are many types of plastics available, use and construction should be recommended by the manufacturer.

M. Glass Fibre Ducts

Is use in interior, low pressure (2” in water gage max.) heating, ventilating, and air-conditioning ducts where either thermal or acoustical insulation is required. Round or square forms are produced.

Note- Construction recommended by supplier.

N. Spun Rock Wool or Fibreglass Aluminum Backed Board

Used with fittings where there is a change of direction.

Forming- Special cutting and notching tools are used to form mitred corners on the duct. Aluminum backed tape is used as a seal on the seams.

O. Polyvinyl Steel (PVS)

Polyvinyl steel is a polyvinyl-chloride plastic coating heat fused to galvanized steel. Two-mil and four-mil coating thicknesses usually are standard, with steel gages available from 26 ga through, and including 14 ga. This product is most popular in spiral formed pipe and is available in flat sheets and coil stock of lock-forming quality.

P. Concrete

Used in underground ducts, air shafts.

Advantages

Compressive strength, corrosion resistance.

Precautions

Cost, weight, rough surface (high friction) porous, fabrication (requires forming processes).

Q. Turneplate

Turneplate is sheet iron or steel coated with an alloy of about 4 parts lead to 1 part tin.

Used for roofing, gutters and downspouts, and casket linings and in the manufacture of gasoline tanks for automobiles, oil cans, and containers for paints, solvents, resins, and so on, it has largely been replaced by other, more durable steel products that are easier to manufacture.

Advantages

Has a higher resistance to acids and other corrosives.

Precautions

Softer coating than galvanized, more easily scratched.

R. Tin plate

Tin plate is black iron coated with tin. It has a clean shinny appearance.

Advantages

It is used for food containers

Precautions

Not as long lasting as stainless steel. It is not used much anymore.

AIR SYSTEMS

Air flows in ducts due to a pressure difference created by a fan. The air at the outlet side of the fan creates a positive pressure and the air at the inlet side of the fan is in a negative pressure. The speed at which the air moves or its velocity is measured in FPM (feet per minute) and the volume of air that moves threw the duct is measured in CFM (cubic feet per minute). The speeds at which the air moves and the quantity of air moving threw the duct create pressure on the duct walls called static pressure measured in WG (water gauge). As the static pressure increases so does the need to increase the gauge of the material that the duct is made of, or to reinforce the duct. SMACNA’S manual “HVAC DUCT CONSTRUCTION STANDARDS” shows us the proper gauging and reinforcements.

Duct System Reinforcement: Whose Responsibility?By Todd TalbottAirflow Group Engineering and Marketing Manager, United McGill Corp., Groveport, Ohio

Duct reinforcement is an essential element of proper duct system design that is often overlooked, especially for negative pressure systems. Even when reinforcement specifications are addressed, they are often so vague that the designer, fabricator, and installer each assume that the others have taken reinforcement into consideration. On most projects, reinforcement becomes a major consideration only if there is a problem in the field. The consequences of inadequate reinforced ductwork are rarely noticed in commercial building applications. Positive pressure supply systems rarely exceed six in. WG, and few negative pressure return air systems exceed –3 in. WG. Duct systems generally meet approval if the design volume of air gets from the fan to the diffuser with no structural failure and within the budget.

However, problems that avoid detection initially can result in costly retrofits in the future. Over time, inadequately supported positive pressure duct can experience serious leakage and noise problems. Duct walls that continually pressurize and depressurize in variable air volume (VAV) systems can eventually increase leakage at duct joints, thereby requiring the fan to push more air through the systems to meet the original design criteria. How much more air? That depends on the quality of workmanship in fabricating, installing, and sealing the ductwork. The “oil-canning” effect can also cause excessive noise problems that could require installation of expensive noise abatement equipment.

Reinforcement SpecificationsReinforcement specifications are intended to minimize duct wall deflection,

thus preventing potential leakage and noise problems in the commercial/institutional arena involve negative pressure systems exceeding the common return air system pressures of –2 to –3 in. WG. These systems, when constructed of common commercial gauges, will experience structural failure if not properly reinforced. Herein lies the problem. The construction standards referenced by most commercial specifications do not properly address all reinforcement issues. Potential disaster awaits duct systems when reinforcement issues are not addressed during the design stage.

The most prominent construction standards found in specifications today are published by SMACNA. The premiere commercial standard is SMACNA’s 1985 HVAC Duct Construction Standards. The following paragraph comes from Section 1, “Basic Duct Construction,” under the subsection titled, “Reinforcement Arrangements”.

“Fabricators and installers are obligated to select feature from among the joint, seam, reinforcement, and support options that will result in a composite assembly that will be serviceable within the express and implied performance criteria identified herein. Experience in construction is valuable; no representation is made that all detail and knowledge necessary to select, fabricate and install a workable assembly is implied. Indiscriminate selection and poor workmanship compromise construction integrity. Conversely, the obligation to make suitable selections does not constitute and obligation to compensate for a designer’s negligence in specification application. A construction standard must be applied by a designer to the requirements of the individual project within the range of its limits.”

Who’s responsible?So, who is responsible? Designers, fabricators, and installers! The above

paragraph implies that the fabricator and installer are responsible for selecting reinforcement and other construction details from among SMACNA’s options to meet specific performance criteria. Gauge/reinforcement options allow the contractor and fabricator to select reinforcement combinations that offer the best

price advantages providing they are within their manufacturing capabilities. Together, the contractor and fabricator can evaluate reinforcement, joint, connector and support options to reduce costs further. Selection and workmanship are the combined responsibility of the fabricator and installer.

However, designers cannot totally dismiss themselves from duct construction responsibilities. Construction standards are available for various applications; one manual does not cover them all. Therefore, the designer is responsible for choosing the construction standards according to the specific application.

Defining ResponsibilitiesThe following example typifies how vague wording in commercial

specification can cause severe problems for all parties concerned when responsibilities are not clearly stated.

A specification states: “Put 80ft of 14 by 41 flat oval exhaust air ducts for the animal cage room ventilation system. Ductwork is to be of galvanized steel able to withstand an operating pressure of –6 in. WG. The duct must be conformance with SMACNA’s HVAC Duct Construction Standards. Four-bolt flat oval duct connectors are required”.

According to SMACNA’s 1985 HVAC Duct Construction Standards (pp. 3-11, Tables 3-4), Flat oval Duct with a major axis dimension of 41 in. should be constructed of 22 galvanized sheet metal. The proposal submitted by the fabricator to the installing for the sheet metal duct and fittings. The fabricator offers two four-bolt oval connectors price options: shipped loose and shop installed.

The installer gives the fabricator a contract for the ductwork with four-bolt flat oval duct connector installed in the shop. The job is fast track, so after receiving approved submittals from the designer, the installer releases the fabricator to send material to the job site. Eight months later, the installer informs the fabricator that the duct collapsed and wants to know what the fabricator is going to do about it. The fabricator claims no responsibility because the exclusions page in the submittal package plainly notes that the reinforcement would not be provided.

The installer claims no responsibility, having assumed that the four-bolt oval duct connectors selected by the designer were the reinforcement. The designer informs the installer that four-bolt flat oval duct connectors were selected for their attractiveness as connectors, not for their reinforcement value. The designer will not take responsibility for the misunderstanding because the specification plainly states that the installer is responsible for providing ductwork that meets AMACNA standards, including reinforcement. The designer also points out that the installer is responsible for the reinforcement because the

fabricator stated it would not be provided. The installer approaches the fabricator and demands help solving the problem, claiming that it was the fabricator’s responsibility to detail the reinforcement required even though it was not supplied. Who is responsible for the reinforcement?

After researching SMACNA’s 1985 HVAC Duct Construction Standards to determine what reinforcement is required, the fabricator the fabricator finds no guidance for reinforcing round, flat oval, or rectangular systems exceeding –3 in. WG. The installer requests an add from the designer for the unspecified reinforcement. The designer finds that SMACNA’s Rectangular Industrial Duct Construction Standards and Accepted Industry Practice for Industrial Duct Construction both address reinforcement for systems exceeding –3in. WG and demands that the installer pay for fixing the problem.

The fabricator informs the installer that following the reinforcement guidelines in SMACNA’s Rectangular Industrial Duct Construction Standards is impossible because 16-gauge sheet metal is the minimum allowed. The 14 by 41 spiral duct is 22-gauge. Accepted Industry Practice for Industrial Duct Construction (p. 8, Table 2-A) requires that 22-gauge duct be reinforced every foot, which is very expensive. The fabricator and installer both inform the designer that these other publications are not referenced in the specification and are for industrial applications outside the construction standards used for this application.

Fortunately, there is no other flat oval duct on the job. Ultimately, the fabricator agrees to supply reinforcement, provided that the installer assumes installation expenses. But there is still the problem of paying delay back charges. The installer and fabricator join forces, demanding that the designer assume these expenses due to the weak specification and the fact that the designer approved all submittals. If this project had been a larger project involving more money, there would probably have been litigation.

ConclusionThe only win=win scenario is for the owner, designer, installer and

fabricator to out a solution together. This rarely happens, especially on the larger projects, and usually all parties think they are in the right and the lawyers are brought in. Who is responsible? There is no guaranteed after-the-fact solution because someone stands to lose profits and respect for admitting fault. The best solution is prevention! Consider the following suggested responsibilities:

The designer is responsible for choosing construction standards for the job, specifically detailing expectations or modifications thereof.

The installer is responsible working with the fabricator to determine which gauge/reinforcement option offers the best price and still meets the designer’s performance criteria. Usually, the installer can save money by purchasing and installing the reinforcement and by properly coordinating the hanger/support layout with the reinforcement spacing.

When the installer and fabricator provide price options for the engineered ductwork systems, the designer should keep those prices in confidence and not to shop around for a better price before or after the bid. The fabricator should write a letter to the installer, copying the designer, calling attention to the need for reinforcement and the fact that it is not included. However, the fabricator should provide the installer with reinforcement requirements for the duct provided. The fabricator may provide an option price for reinforcement at the installer’s request.

The designer should review the submittals and verify that the duct construction conforms with the performance criteria selected.

The designer should not allow the installer to release the fabricator to supply material to the job site until all concerns about the submittals have been resolved and approval has been given.

The new SMACNA’s HVAC Duct Construction Standards was published in October 2006 and covers gauge/reinforcement guidelines for round, flat oval, and rectangular duct for +10 to –10 in. WG in standards commercial gauges. The designer, fabricator, and installer should all share the responsibility for duct construction. Accountability, cooperation, and coordination among all parties are essential.

INFORMATION REQUIRED FOR DUCT CONSTRUCTION

Various types of information are required in project plans and specifications in order for the fabricating and installing contractor to provide the duct system performance intended by the system designer. Among those are:

1. A comprehensive duct layout indicating sizes, design airflows, pressure class, and routing of the duct system.

2. The types of fittings to be used based on the designer’s calculations of fittings losses (i.e. square versus 45 degrees entry taps, conical versus straight taps, etc.)

3. Use of turning vanes or splitter vanes.4. Location of access doors5. Location and type of control and balancing

dampers.

6. Location and type of diffusers.7. Requirements for duct insulation.8. Location and types of any fire protection device

including fire dampers, smoke dampers, combination fire/smoke dampers, and ceiling dampers. Building codes require this information to be shown on the design documents submitted for building permit.

9. Details of offsets required to route ductwork around obstructions (columns, beams, etc.)

Pressure Classifications

Old system New system1) Low Pressure 0” to 2” Water Gauge

1) 0” to ½” Water Gauge

2) ½” to 1” Water Gauge3) 1” to 2” Water Gauge

2) Medium Pressure 2” to 6” Water Gauge

4) 2” to 3” Water Gauge

5) 3” to 4” Water Gauge6) 4” to 6” Water Gauge

3) High Pressure 6” to 10” Water Gauge

7) 6” to 10” Water Gauge

Each duct system shall be constructed for the specific duct pressure classifications shown on the contract drawings. Where no pressure classes are specified by the designer, the 1in. WG (250 Pa) pressure class is the basis of compliance with these standards, regardless of velocity in the duct, except when the duct is variable volume: All variable volume ducts upstream of VAV boxes has a 2 in. WG (500 Pa) basis of compliance when the designer does not give a pressure class.

Ductwork and supports shall conform to HVAC Duct Construction Standards Metal and Flexible, Third Edition, 2005. Where fittings of configurations not shown in the HVAC-DCS are shown on the contract drawings, they shall be constructed at though they were therein.

Duct dimensions shown in the contract drawings are for airflow area. When ducts are acoustically lined, their dimensions shall be increased as necessary.

Duct pressure classes are to be identified on the contract drawings.

Duct shall be sealed as specified in the HVAC-DCS.

Metal nosing shall be used on leading edges of each piece of lined duct when the velocity exceeds 4000 fpm (20.3 m/s) otherwise, it shall be used on the leading edge of any lined duct section that is preceded by unlined duct.

Installation Standards for Rectangular Ducts Using Flexible Liner

Flexible duct liner of the specified material, thickness, and density shall be furnished and installed where shown on the contract drawings.

Unless otherwise indicated, the net free area of the duct dimensions given on the contract drawings shall be maintained. The duct dimensions shall be increased as necessary to compensate for liner thickness.

Each layer of duct liner shall be attached with 90 percent coverage of adhesive at the liner contact surface area.

All transversely oriented edges of liner not receiving metal nosing shall be coated with adhesive. Liner shall be neatly butted without gaps at transverse joints and shall be coated with adhesive at such joints before butting.

Liner shall be folded and compressed in the corners of rectangular duct sections of shall be cut and fir to ensure butted edge overlapping. Longitudinal joints in the duct liner shall not occur except at the corners of

ducts, unless the size of the duct and standard liner product dimensions make them necessary.

Fasteners shall be located with respect to interior dimensions and regardless of airflow direction as in the accompanying table.

Velocity Transversely Around Perimeter

Longitudinally

2500 fpm (12.7 mps) and less

At 4 in. (102mm) from longitudinal liner edges, at 6 in. (152 mm) from folded corners and at intervals not exceeding 12in. (305 mm)

At 3 in. (76mm) from transverse joints and at intervals not exceeding 18in. (406 mm)

2500fpm (12.7 mps) to 6000 fpm (30.5 mps)

At 4 in. (102 mm) from longitudinal liner edges, at 6 in. (152 mm) from folded corners and at intervals not exceeding 6 in. (152 mm)

At 3in (76 mm) from transverse joints and at intervals not exceeding 16 in. (406 mm)

Where dampers, turning vane assemblies, or other devices are placed inside lined ducts or fittings, the installation must not damage the liner or cause erosion of the liner. The use of metal hat sections or other build out means is optional; when used, build outs shall be secured to the duct wall with bolts screws, rivets, or welds.