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Round Duct and Fittings

Sheet Metal Field

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October 2, 2020

Chapter #2 – Round Ductwork and Fittings

In order to do a takeoff, you will need to be able to understand what rectangular duct and fittings look like and how they are represented on the drawings.

Round fitting types and methods of construction can vary by region and between companies.
Engineers use software programs that have standardized duct and fitting symbols, in addition to the ability to create their own representations of what duct and fittings look like on the drawings, but once you know what to look for you can identify the slight differences between designs.

Depending on the situation ducts either start at some cooling or heating source air conditioner, air handler or an HVAC accessory like a VAV terminal, or transition from a rectangular duct as in a square to round fitting.

If the round doesn’t start from a piece of equipment or from a square to round, then it will need some form of starting fitting. Manufactures make various types and connection methods, so you need to become familiar of what is commonly used in your area.

Round Duct Types

Round duct is manufactured with various seam types, the most common being spiral or snap lock.

Spiral Round Ducts

Round duct is used to connect between fittings, accessories and flex duct. Manufactures usually stock standard lengths, but if you have your own spiral machine you can fabricate any length.

Spiral duct is not limited in length, except as required to fit on a truck or in an elevator, or as needed for the insertion of a fitting. Reinforcement rings are generally not required for positive pressure and low negative pressure round ducts. The use of rings helps maintain the roundness of the duct as the sizes get larger.

Longitudinal Seam Round Duct

In addition to the spiral seam there are various other types of seams that run straight from one end to the other (longitudinally), such as a longitudinal welded seam as shown in the video below. The video quality isn’t the best but it shows how you can weld galvanized and stainless steel round ductwork, and how to adjust the machine for various sizes.

Welded Longitudinal Seam

There is also a snap lock version of the longitudinal seam as shown in this next video. This particular type of round duct has an internal gasket that makes the seal.

Snap Lock Longitudinal Seam

Sheet Metal Rollers

When the use of round spiral is not allowed the use of longitudinal welded round duct is required, a piece of fabrication equipment that can roll a piece of flat stock into the required round size.

Specialty exhaust systems that require a secure seam without ridges can be made with a longitudinal seam.

Watch this Sheet Metal Roller video to see a sheet metal roller in action.

Sheet Metal Rollers

Longitudinal vs Spiral Seam

Spiral allows for lighter gages then longitudinal seams when using Unreinforced Round duct as shown in the SMACNA table below. You can see that from 4” to 14” round (Column #1), both Longitudinal Seam (Column #2) and Spiral Seam (Column #3) require the same 28 gage minimum. But, spiral seam can use 26 gage from size 16” to 24” round, while longitudinal seam is limited to a maximum of 18” round when using 26 ga. This indicates that the spiral seam is stronger and more rigid then a longitudinal seam.

You can also see in the table that the larger the round size the greater or thicker the metal gage required.

Special Round Ducts and Fittings

There are various manufactures that make proprietary systems that are meant to save labor such as the Eastern Sheet Metal’s gasket fitting system. The rubber gasket provides a tight seal, making additional sealing unnecessary. Watch the enclosed video to see how easy the ductwork goes together. They also have a double-wall version, in addition to a single and double-wall flanged system, and one for oval.

Single Wall Round Duct with Gaskets

Double-Wall Duct & Fittings

In some cases engineers will specify double-wall duct, often when noise is an issue, but it also provides thermal properties. Double-wall duct comes in round or oval sizes with either a solid inner wall or a perforated inner liner, with insulation sandwiched in between the inner and outer wall.

The outer duct can be purchased with three different seams, spiral lockseam, spiral lockseam with a standing rib, and longitudinal seam.

This next video shows how one shop fabricates double-wall duct with a perforated inner wall and lined with fiberglass insulation. The perforated inner wall is made from a skinny coil of material similar to that of spiral duct.

Making Double Wall Spiral

Engineers will also specify that the duct be made by a particular manufacture so that a certain level of quality can be assured to the owner.

Lined Round Spiral Duct and Fittings

In order to provide acoustical properties, which is the ability to attenuate or reduce sound transmission, lined round spiral duct can be used by adding liner to the interior of the duct. Watch the video below of the John Mansville Spiracoustic Plus liner material being added very easily to spiral duct and fittings.

Fabric Duct

There are special cases where instead of sheet metal you can use fabric ductwork such as made by DuctSox. Where you have long straight runs of exposed ductwork the application of DuctSox may make sense, such as gymnasiums and indoor pools.

You have the option of having the air dispersed through linear vents, fixed orifices, fixed nozzles or adjustable nozzles.

They can come in various color options, and can have zippered sections. Companies can also have their logo imprinted on the fabric or pick from various patterns.

The fabric can be supported by an internal metal ring which provides tension to keep the fabrics shape, or without any internal metal ring in which case when the air is shutoff, the fabric will collapse.

Fabric Duct Hanging Methods

You can use an external cable to suspend the fabric sox or a channel that allows you to pull the fabric like a curtain along the suspended railway.

Applications

Fabric ducts can be used in Gymnasiums, Warehouses, Manufacturing Facilities, Indoor Pools, Auditoriums, Schools, Malls, Restaurants and other exposed locations.

In a lot of these facilities the use of some form of man-lift or scissor lift will be required to reach the heights at which these fabric ducts get installed. Remember to always check the height at which your ductwork needs to be hung.

As shown below the DuctSox (#1) is used in a Gymnasium and is installed at 28’-1” above the floor, which would require some form of man-lift for the installation. The main supply air duct (#4) is run in galvanized rectangular ductwork, including the return air duct (#3).

The video below is another brand of fabric ductwork which shows workers installing the duct using a scissor lift.

Fabric Duct

Round Flexible Ducts

Flexible ducts come in various arrangements that include insulated and uninsulated, metallic and nonmetallic. The insulated version would be used to carry conditioned air or for the reduction of noise, while the uninsulated metallic would be used for non-condition exhaust air. The use of flexible duct should be kept to a minimum to avoid excessive pressure drop. Aluminum is the most commonly used material for metallic flexible ducts. Flexible ducts come in various lengths depending on manufacture.

The R-values of the insulated flexible ducts range from R4.2, R6 to R8. Flexible duct comes in lengths of 3, 6 and 25 feet with some manufactures, others may vary.

Flexible Duct Defined

Flexible ducts need to be supported according to the manufactures recommended method but not less than every 5 feet, including the maximum sag of 1/2” per foot between support hangers. This would allow maximum sag of 2-1/2” between two support hangers spaced 5 feet apart. The portion of the hanger that the flexible duct rest upon needs to be at least 1 inch wide to avoid the reduction of the internal diameter. This can be accomplished by a 1 inch wide hanger strap or by providing a saddle for the flexible duct. The size of the saddle would need to be half the circumference of the outside diameter of the flexible duct and sit at the bottom half.

Draw bands must be used to secure nonmetallic flexible ducts to a sheet metal sleeve or collar.

The most commonly used piece of flex duct is for the last 6 feet (2m) where a connection is made to a piece of air distribution. Some projects allow all of the low side ductwork to be run in flex. This is not the best approach due to many factors, one of them is static pressure loss.

Air Distribution Flexible Duct Connection — Air Distribution Flexible Connection Detail

Watch this video for the correct methods of supporting flexible ductwork.

Flexible Duct Support

Insulated Aluminum Flex

Some projects may allow long runs of insulated aluminum flexible duct, but this is not good engineering practice due to the higher resistance created from through inner surface, which creates more static pressure losses and increased energy cost.

Uninsulated Aluminum Flexible Duct

Often used for exhaust system that require a short run of duct.

Start Collar

A round hole is cut into the side of a rectangular duct and the dove tails of the start collar are alternately bent perpendicular, one inside, one outside, and fasten with sheet metal screws and then sealer is applied.

Start Collar with Volume Damper

Often the start collar will have a volume damper included. Keeping the volume damper in the start collar and as far away from the air distribution will help reduce noise transmission through the grille into the room.

start collar with VD — Start Collar with Volume Damper

Flat Saddle Tap

Instead of a spin-in or similar starting fitting, a flat saddle tap can be used to get round duct started from the side of a piece of rectangular duct. Available with a 45 or 90-degree branch.

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Radius Saddle Tap

This fitting is used in lieu of some form of a tee fitting. This will save having to cut the main round duct to install a Tee. This is a round on round fitting. Available with a 45 or 90-degree branch.

Here is a short video on the basics of installing a saddletap. There are electric shears that would work much better than the manual shear that this gentlemen uses in this video. You can stop the video after 2 minutes 38 seconds.

Installing a Saddle Tap

Square to Round

When rectangular duct transitions to round duct on the straight run its called a square-to-round.

90 Degree Elbow

Most turns will either be 90 degrees or 45 degrees. You don’t see a lot of odd angle elbows as they would be more of a custom made fitting and more expensive. Notice the crimped end on one end of the fitting.

Elbows can come in many variations, from adjustable, stamped, pleated, spot welded or fully welded. Here is a video on how an adjustable elbow can be manipulated to create various angles.

90 Degree Adjustable Elbows

Sheet Metal Spiral Elbow Machine

There are machines that lock segments or sections of a fitting together to make a complete fitting. Some of these machines are referred to as Gore Lockers. Two segments are butted together and are spun around as their seams are locked together by the gore locking machine. For a multiple gore elbow this occurs with each piece until you get the quantity of gores required. This is common for adjustable gore elbows. It’s a good time to discuss the different types of elbows.

Spiral Elbow Machine

45 Degree Elbow

There are several different types of elbows based on seam and joint types. As you can see this fitting also has a crimped end allowing it to slip inside the next piece of duct.

Tee-Wye (T-Y)

These are used to make 45-degree branch connections. See the image above where the T-Wye reduces the main branch from 10” x 8” x 8”. Also shown above but not highlighted is an 8” x 8” x 8” Tee-Wye, can you find it. Also note that the acronym used for a this fitting may be spelled many different ways, but they all refer to the same fitting type such as; T-Y, T-Wye, Tee-Wye, etc .

Volume Dampers

Dampers are used to adjust the volume in ducts. These will be used by the air balancing personnel to set the CFM needed for the branch or main that has a manual volume damper.

In the example above the volume damper controls the amount of air that air distribution diffuser CD-6 receives, in this case it indicates 500 CFM (cubic feet per minute)

Reducers

A reducer is used to change size of the duct run. The reduction in size can also be accomplished by using a reducing tee.

Round Joint Types

Most low pressure round fittings will either have a crimped end or be coupling sized.

Crimped Ends

Crimped ends reduce the overall diameter just slightly so that the fitting or duct can slip into the next piece.

Round Couplings

Couplings slip inside the ends of two round ducts to be joined together and then sheet metal screws are fastened to hold it in place. Couplings could be required at a certain size such as connections 18” and larger. The shaded area in the image below is the coupling.

Flanged Connections

There are many manufactures that provide various types of flanged connections like Ductmate™ Spiralmate™, see their website for more information and videos for various connections.

Welded Connections

The requirement for a welded duct connection can be found for kitchen grease exhaust systems or some industrial processes and laboratory exhaust systems. Some of the more commonly welded duct materials are galvanized steel, stainless steel and black iron. Joints are often butt welded or flanged.

Often welded kitchen exhaust systems will use stainless steel where exposed to view and black iron for the hidden duct to the grease exhaust fan. Grease exhaust ducts need clean-out doors every so often as required by code for access for cleaning. (Also see chapter on Grease Exhaust Systems).

FRP – Fiberglass Reinforced Duct

Used in industrial processes to exhaust corrosive fumes such as solvent or acid exhaust systems. You can spend your life in the HVAC business and never be involved in a project that requires this material, but watch the enclosed short video so you understand what is involved.

The Proper Use of Round Fittings

There is more than one way to make a reduction or branch connection. As the following example shows you can use two different ways to accomplish the same thing. In the example we have a round main duct that branches into two 12” round branches. You can either use a 14” x 12” x 12” Tee-Wye or you can use a 14” – 12” reducer with a 12” saddle-tap.

Various Fitting Options

In the case above it would be best to use the T-WYE as it is one less connection. Use a T-Wye when the main duct changes sizes, and use saddle-taps when the main remains the same size as shown below. Using the saddle tap and the reducer in the above scenario would require and extra cut of the duct and two fittings, instead one T-Wye fitting.

Round Fitting Test Q2 — 45-Degree Radius Saddle Taps

In lieu of a full body fitting you can use a saddle-tap which allows the main duct to remain the same size without having to cut-in a fitting. A saddle-tap does just as its name implies, it saddles the main round duct. This is accomplished by cutting a hole in the main duct where the branch is to occur and then the saddle-tap is fastened with sheet metal screws and then sealed to provide an airtight assembly.

fitting usage MEP-Academy — Round Fitting Usage

Round Starting Fittings

Whenever you have a piece of duct tapping into the side of another duct you will need a starting fitting if you are not using a whole body fitting of the same shape.

Round on Rectangular Duct

So, if you have a round duct branching off of a rectangular duct you might use one of the following;

Options for branch connections

Spin-in
Spin-in with damper
Dove-tail spin-in
Scoop tap-in
Flat 45 Degree Saddle-tap
Flat Straight Saddle-tap

Each of these starting fittings will get you connected to the side of the rectangular duct and then capable of connecting another round piece to that fitting, such as a length of spiral duct.

Round on Round Duct

Straight Saddle-tap
45 Degree Saddle-tap
Tee-Wye

Single Line Duct Design

Most often you will find that the duct design is done in a single line style, where the actual scale width of the duct is not represented.

In the above example the yellow dot highlights a fitting that may not be feasible to fabricate and what could be a better selection is a Tee-Wye instead of a bullhead tee as shown.

In the above example the Red dot highlights a typical Tee-Wye except that is usually preferable if possible to have the branch that angles off in a 45 degree to be the same size or smaller then the reducing run dimension, in this case the Tee-Wye is a 12” x 8” x 10”. It would be better to use a 12” x 10” x 8” if possible.

In the above example the Green Dot highlights an expensive fitting as opposed to a simple spin-in. What is actually represented by the symbol is a type of tap/Square-to-Round all in one fitting.

There are many different ways to make a branch connection in round duct from a full body fitting like a Tee to a saddle-tap which only requires a hole in the side of the duct big enough for the connection outlet size.

Often there will be several methods to accomplish the same branch takeoff or reduction in size. Whether to use a fitting or some form of saddle-tap and reducer can depend on the specifications or your company preferences.

The above fitting accomplishes the provision of a branch connection and a size reduction in the run. This can also be accomplished by cutting an opening in the side of the main duct for a saddle-tap that provides for a branch connection and then a reducer to provide a reduction in size for the main run. The fitting is comprised of one piece while the other connection method requires two pieces, the saddle-tap and reducer.

Saddle-taps are an effective strategy when you have multiple taps on a main duct that doesn’t change in size on the run. For instance, if you had a main 20” spiral that had four 10” branches it would be best to use saddle-taps to avoid cutting the main for a Tee connection for each branches.

Round fittings will be represented on the drawings either in single or double line format. Either way the specifications will let you know what type of fittings are required.

Round fittings come in various construction seams and joints, which show be defined in the specifications.

We will show you where each of these fittings are used and the different ways they can be represented on engineered drawings.

Engineers or their CAD personnel may not show the fittings as you would actually build the duct. You have to read the specifications and look at the details to determine what is allowed.

Fully Welded Round Fittings

There will be projects that require a fully welded round system. This usually occurs where special an exhaust system contains flammable or noxious gases in the air stream.

Gored Elbows

As shown in the image above these are fully welded 90 degree elbows with flanged joints. The seams are fully welded and the quantity of gores are indicated. There is a 5 and 7 gore elbow identified in the image above. When a greater radius turn is required the engineer will specify more gores for the elbow. This applies to any degree of elbow from the standard 45 and 90 degree elbows to custom angles such as 30 and 60 degrees.

Mitered elbows should comply with SMACNA table 3-1 below which sets the number of mitered pieces based on the velocity (#1) in the ductwork. Unless the specifications call for something different or other limitations are stated, then this SMACNA table can be safely used to figure the quantity of miters (#3) to add to 90, 60 and 45 degree round elbows.

As can be seen from Table 3-1 as the velocity increases (#1) within the elbow, the greater the quantity of miters (#3), and the larger the Centerline Radius (#2) is required. The centerline radius makes the elbow take a wider turn, which allows for better aerodynamics and performance.

SMACNA Table 3-1 Round Mitered Elbow Chart — SMACNA Table 3-1 Mitered Elbows

Machine Made Gored Elbows

Watch the enclosed video to see how a version of the gored elbow can be made quickly in this sheet metal fabrication shop.

Making Sheet Metal Elbows

Adjustable Elbows

The most common 45 and 90 degree elbows are fabricated to have adjustable gores, so that you can spin the gores and make various angles out of the two types.

Spot Welded Elbows

For system and designs requiring a better fitting, you might find the requirement that the gores be SPOT WELDED. Every so often around the circumference of the seam a spot weld will be made to hold the two segments of the fitting together, giving it a stronger bind then that used for an adjustable elbow.

Continuously Welded Elbows

And for systems requiring a tight seal there are fully welded fittings with a continuous weld as opposed to a spot weld every so often around its circumference. This type of fitting will have a continuous weld at the seams and joints.

The cost of the fittings is usually related to the method of fabrication with adjustable type fittings being the least costly, then spot welded, and fully welded being the most expensive fitting type. Having to fully weld the seams of a fitting requires more shop labor and the use of a heavier gauge to allow for a clean weld without destroying the metal, this is why it is the most expensive type of seam and joint for a round fitting. There are other types of elbows used through-out the industry such as;

Stamped Elbows

This elbow is created from two pieces that were stamped out. This requires that the two half sections be welded together along there longitudinal seam with either a continuous weld or spot weld.

Pleated Elbows

Just as the name implies this elbow has a pleated appearance, almost appearing as if it was a piece of flex duct bent in the shape of a 90 degree elbow and then harden. I have personally never used this type of fitting, but just wanted to make you aware of the terminology in case you live in an area where they are used.

Standing Seam Elbows

This fitting is fabricated with segmented pieces with standing seams that lock to an adjoining segment.

Oval Duct and Fittings

Not as commonly specified as round duct, but it has its application. Oval duct is made from a section of round spiral duct, that is ovalized, or stretched into an oval size. See the enclosed video to see this machine in operation.

Flat Oval Spiral Duct

Watch the below video to see the installation of oval duct and fittings. Notice that the reducer being used is undersized on its collar allowing the oval duct to slip over the collar. They are using Unistrut with threaded rods as hangers for the ductwork.

Oval Ductwork

Round Industrial Classifications

This course doesn’t cover Industrial Construction standards, but you should be aware that duct is classified by the medium that is traveling through the ductwork.

Round Industrial Standards will cover round ducts that carry corrosive fumes or particulate matter, like that found in a dust collection system or industrial processes. The non-industrial standards cover ducts from +10” wg (2,500 Pa) to -10” wg, while the industrial standards cover ducts up to a negative -30 wg.

Class 1 – Includes non-abrasive applications: Makeup Air, General Ventilation, Gaseous Emission Control

Class 2 – Includes applications with Moderately Abrasive Material in Low Concentration: i.e., buffing and polishing woodworking, grain dust, etc.

Class 3 – Includes applications with Highly Abrasive Material in Low Concentration: i.e., abrasive cleaning operations, dryers, kilns, boiler breaching and sand handling, etc.

Class 4 – Includes applications with Highly Abrasive particulate in High Concentrations; i.e., materials conveying high concentrations of particulate in all examples listed under class 3 (usually used in heavy industrial plants such as steel mills, foundries, mining and smelting).

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Sheet Metal Field Installation Course

Chapter #1- Rectangular Duct and Fittings
Chapter #2 – Round Ductwork and Fittings
Chapter #3 – Sheet Metal Duct Hangers
Chapter #4 – Sheet Metal Field Labor Productivity
Chapter #5 – Labor Crew Sizes
Chapter #6 – Sheet Metal Duct Sealer
Chapter #7 – Sheet Metal Details and Specialties
Chapter #8 – Grease Exhaust
Chapter #9 – Air Distribution
Chapter #10 – Rental Equipment
Chapter #11 – Conditions Affecting Field Labor
Chapter #12 – HVAC Equipment Labor

Sheet Metal Shop Drawings

MEP Academy Instructor

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September 27, 2020

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Chapter #7 – Sheet Metal Shop Drawings

Shop drawings should be created by someone in the company familiar with detailing. Detailing is the process of taking the engineered set of drawings and converting them into shop drawings that the field will use to install the sheet metal.

Sheet Metal Detailer

The sheet metal detailer’s job is to coordinate the layout of the ductwork with all other trades in order to avoid collisions or conflicts. This position is held by a union member for companies that are signatory to a local Sheet Metal Union. Their job is to ensure that the sheet metal fits within the space and to show all the required dimensions on the drawing so as to make the field installation go smoothly and without wasting ductwork and fittings because they don’t fit into the space shown on the drawings.

The shop drawings will show the ductwork exactly where it needs to be installed in order to avoid building components and other trades.

The sheet metal detailer will review the architectural, structural, electrical, plumbing and other trade drawings in an effort to avoid installing ductwork where these trades have their systems and where structural supports and architectural items are to be installed.

Detailed Shop Drawings

Looking at the sheet metal shop drawing below you can see that additional items are shown on these drawings compared to the engineered set. These drawings are drawn in CAD (Computerized Aided Drawings), which is drawn on a special computer program. If you have a small project then using CAD may not be feasible, and in this cases the detailer will just fill out the sheet metal fabrication shops order forms directly from the engineered set of drawings and from what is discovered during a site visit for existing buildings.

The numbers in the red circles correspond to the following;

#1 (Duct Size & Joint Length) This shows that the rectangular duct that is 70” x 18” in size is 56 1/4” in length. This duct is made from a 5 foot (60 inch) coil width, and the difference is from the joint. 60” coil width – 3 3/4” joint (1-5/8” each joint) = 56 1/4” in length.

#2 (Plenum Size & Length) This is the plenum off of the VAV box, which is a duct that is 20” x 17-1/2” in size by 36” in length.

#3 (Duct Elevation) This shows the bottom and top elevation of the ductwork. This shows that the bottom of the duct is 9’-11” off of the floor, and that the top of the duct is at 11’-5” off of the floor.

#4 (Service Access) – According to the manufacture or local code authority, each piece of equipment, valve or accessory that needs adjustment or service is required to have an unobstructed access area. As shown in item #4 the hatched area next to the VAV boxes must be kept clear so that a service technician can access the controller.

#5 (Ceiling Height ) – The height of the ceiling is indicated to be 8’-2”

#6 (Benchmark Distance) – The location of the edge of the ductwork is indicated as being 10’-0” from the column line.

#7 (Wall Opening) – Top and Bottom elevation of Return Air Boot opening in Fire-Rated Wall, as can be seen by the Fire/Smoke Damper.

#8 (Direction) – BF = Bottom Flat. This is a transition fitting that is changing the size of its depth. In order to inform the fabrication shop and the field installer which side remains flat these acronyms are used. It’s also possible to install a concentric transition, where both sides converge evenly.

#9 (Duct Offset) – This indicates that the duct needs to drop by 16” in order to get under the large main supply air duct. It’s important to remember this when you are working with a set of engineered plans that haven’t been detailed into shop drawings. Remember to add extra fittings when crossing ducts.

#10 (External Insulation Wrap) – The dotted line on the outside of the duct indicates that this ductwork will get wrapped with insulation, most likely by your insulation subcontractor.

#11 (Internal Liner) – These dotted lines on the inside of the duct indicate that the ductwork is lined with acoustical liner.

From Shop Drawing to Fabrication Equipment

From the shop drawings the detailer will either draw the required duct and fittings onto an order form or the CAD software will download the information to the shop fabrication equipment with or without the shop superintendent’s modifications.

For those that have an integrated software system where the CAD drawings can be directly sent to the fabrication equipment, this will save a lot of time by not having to draw out each piece required to be fabricated or enter data into the coil line or plasma cutter.

Detailing for Retrofit Projects

Detailing for existing buildings can be done in a similar manner as that of new construction, except that you will need to do a site survey to document what is in the space where new sheet metal or HVAC equipment will need to be installed. Again, based on the size of the project and the requirements of the RFP (Request for Proposal) the use of CAD may or may not be used.

For small retrofit projects the detailer will visit the project site and make the measurements required to get the needed ductwork and fittings fabricated. There is no need to go through the expense of drawing everything in CAD.

Control / Reference Point

The construction project will have one or more control points from which everything can be measured from in order to ensure the accurate location of walls and other trades. The control point is determined by a field engineer and has an X, Y, Z reference point.

Navisworks by Autodesk (Collision Detection)

Most large new construction projects are built within some form of modeling software like Autodesk Revit, Bentley or many others.

In the enclosed video at about the three minute mark you will see that the Navisworks software will pick up a collision between a sheet metal duct and a structural beam. Navisworks combines the different designs from the various trades (Architectural, Structural, HVAC, Electrical & Plumbing) and combines them together, and then searches for clashes that occur between them. Clashes occur when two different designers are trying to occupy the same space in the building, like the duct that is placed where a structural beam is located.

Using Autodesk Navisworks helps eliminate change orders and costly field errors by finding them before they get installed.

AutoDesk Navisworks

Shop drawings are done in AutoCAD, Autodesk Revit MEP or some other modeling software that allows for the coordination with other trades using Navisworks to ensure that the sheet metal will fit within the building without hitting anything.

Revit

BIM (Building Information Modeling)

The use of BIM is widely used throughout the industry for commercial contractors doing new construction projects. Often the requirement to use BIM is specified in the design criteria for many Federal, State and local Municipalities in addition to the private sector. BIM make coordination easier and reduces the amount of change orders due to the collision checking before construction begins.

BIM – Building Information Modeling

The sheet metal estimator will need to provide hours in the estimate to cover the detailing requirements per the RFP (Request for Proposal) or ITB (Invitation to Bid). If there are no requirements then your companies minimum level of detailing required to accomplish the ordering of ductwork and fittings.

Companies that maintain historical data from completed project will be able to look at metrics from those projects which could help determine the hours required for the current project. A metric like, percentage of detailing hours to total field hours would be helpful.

10% Detailing (historical data feedback)

Example: Current Project has 2,000 field labor hours.

Calculate detailing: 2,000 Field Hrs x 10% = 200 detailing hours

Below is a screen shot of a small section of the Sheet Metal Material & Labor Summary tab of the MEP Academy Estimating spreadsheet that allows you to enter a percentage of your total field labor for detailing.

Detailing on Estimating Spreadsheet — Detailing Option on MEP Academy Estimating Spreadsheet

Summary

The detailer will create shop drawings by reviewing the location of all the architectural, structural, electrical and plumbing elements in addition to the various other trades that require space within the building for their component.

The detailer will coordinate with the other trades to ensure that everything fits within the space allotted. It’s possible that software like Navisworks will be used to detect collisions automatically within the software program, thereby minimizes field change orders.

Detailing can be done on a computer using some form of CAD, or on smaller project detailing can be done by hand and drawn out on an order form.

Detailed drawings (shop drawings) shows the size, length and duct joint information for each piece of ductwork and fittings, along with their elevation height.

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Chapter #1 – General Layout of Drawings
Chapter #2 – Architectural Drawings
Chapter #3 – Plan, Elevation, Section Views and Details
Chapter #4 – HVAC Mechanical Drawings
Chapter #5 – Understanding HVAC Symbols
Chapter #6 – Drawing Scales (How to Read Scales including Metric Scales)
Chapter #7 – Sheet Metal Shop Drawings

How to Read Drawing Scales

MEP Academy Instructor

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September 27, 2020

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Chapter #6 – Drawing Scales (How to Read Scales including Metric Scales)

It’s important that you understand how to read the various scales of architectural and engineered drawings. You will learn the following in this section.

How to read an Architectural scale (mostly used for buildings in the U.S.). Architect scales, such as 1/4˝ = 1´-0˝ (1/48 size) or 1/8˝ = 1´-0˝ (1/96 size)
How to read an Engineers scale (mostly used for roads and topographical measurements) Engineer scales, such as 1˝ = 10´ or 1˝ = 50´
How to read a Metric scale (mostly used for buildings in other parts of the world)
How to determine the scale of a drawing where the scale isn’t indicated

It’s not practical to draw a building to full scale, so various ratios are used to represent the actual size of the building. The scale provides a quick method for measuring drawn objects, such as the length of ducts, pipes, and electrical conduits.

The scale is usually shown in the lower right hand corner of the drawing or under the title of the page. There are often many different scales used in the same set of drawings, as they can be on floor plans, elevation pans, section views and details.

Often times you will find all three bits of information located together as in the above drawing and as shown in the insert below. The Title of the Drawing, the scale and the North arrow indicator.

Bar Scale

The bar scale here is accurate even when reduced, so if your drawings have one of these all you have to do is put your ruler up to the scale bar and see which of the scales match exactly the numbers on the bar as shown below. If none match, then your drawings aren’t to scale and weren’t printed correctly.

Set your ruler so that the zero (0) on your scale aligns with the zero (0) on the bar scale, then check to see if the rest of the numbers line up exactly.

Bar Scale Chart 1/4" = 1'-0" — Bar Scale Chart 1/4″ = 1′-0″

As seen above the zero’s (0) and the four’s (4) line up exactly between the ruler and the drawings bar scale, so you know you got the right scale. If they didn’t match, then you would try another scale on your ruler until you found one that did.

If there isn’t a bar scale, then there are other methods to confirm the drawings scale. It’s important to confirm you have the correct scale, otherwise all of your material lengths will be incorrect.

Determining the Scale when No Scale is Shown

Sometimes the scale is not shown on the drawings are is indicated incorrectly. By using some know distance like a doorway or the distance between columns, you can determine the true scale.

Everything on the drawing has been drawn to some scale so it’s a matter of finding something that you know the dimension of and laying your scale down next to it to find the right scale. For instance, most doors in our local area and in the U.S. are about 3 feet wide, so if you measure a doorway it will let you know the proper scale to use.

Checking the scale using the doorway and the 1/4″ = 1′-0″ scale.

Checking the scale using a doorway (This indicates that 1/4″ is incorrect)

Looking at the above we can see that the doorway measures 1-1/2 feet on a 1/4′ scale. Unless this is a home for the 7-Dwarfs in the tale of Sleeping Beauty than the scale isn’t a 1/4″. Next we’ll try the 1/8″ = 1′-0″ scale as shown below.

Using a 1/8″ = 1′-0″ scale gives a measurement of a 3 foot doorway, which is the correct dimension. This verifies that the correct scale to use is the 1/8″ scale.

You can also use a dimension that is already given on the drawings such as the width of a duct that has been drawn using double lines as shown below;

Using a section of duct to determine the scale

Using the dimension of the ductwork it is determined that the scale is 1/8″ = 1′-0″ as shown in the image above. The 50″ x 30″ duct measure 4′-2″ on our scale which is equivalent to 50 inches.

How to Use the Metric Scale (SI Units)

The Metric system is widely used around the world and on some Government projects, so it’s imperative that you understand both methods. The following scale is derived by measuring the drawings and multiplying every centimeter (cm) on the drawing by the denominator of the scale ratio such as 1:50 which means that for every 1 cm measured on the drawing it is equivalent to 50 cm in real life.

Basically the scale is represented by two numbers in a ratio, with the first number being how many centimeters are shown on your drawing followed by the second number which is separated by a colon (“:”), then the matching length of the real item to be built.

If we blow up the image of the scale, we see that there are two scales referenced at one end as opposed to the imperial version that showed 1/4” on the left side and 1/8” on the right side of the scale.

Metric Scale 1:50

The two scales shown are 1:50 and 1:500. What they represent is the following;

1:50 means that when you measure 1 cm on the drawing it is equivalent to 50 cm of the real item to be built. 1:50 is also equivalent to 1/2 of meter for every cm on the drawings, because 100 cm is equal to a 1 meter.

Metric Scale 1:50. 1 cm = 1/2 meter — Metric Scale 1:50. Measuring 1 cm on the drawing = 1/2 meter

Metric Scale 1:500

Using the yellow highlighted 1:500 scale means that when you measure 1 cm on the drawing it is equivalent to 500 cm of the real item to be built. 1:500 is also equivalent to 5 meters for every 1 cm on the drawings, because 100 cm is equal to 1 meter; 500 cm is equal to 5 meters.

Metric Scale 500cm (1 cm on the drawing equals 5 meters) — Metric Scale 500cm (1 cm on the drawing = 5 meters)

Metric Scale Measuring 2 cm on the drawings equals 10 Meters — Metric Scale 1:500 (2 cm on Drawings = 10 Meters)

To verify that you have the correct metric scale you can use a Bar Scale or find a known dimension on the drawings and put your scale next to it until you get the correct scale.

doorway metric measurement — Using a Doorway to Determine Metric Scale

Making a measurement with a known length

Metric Scale Chart 1:50, 1:100 and 1:500) — Metric Scale Chart (1:50, 1:100 and 1:500)

Architectural Scale

The most common scale in the USA is 1/8″ = 1′-0″ & 1/4′ = 1′-0″.

We will also show you how to verify that the scale indicated on the drawings is correct. Often times the engineer incorrectly marks the scale or no scale is indicated, so having a way to verify the scale is important to an accurate takeoff. The scale is indicated in various locations on the drawing. Each engineer has their own preferred location. The scale is often found under the floor plan or section name description as such;

Reading a manual scale is easy if you understand the basics of each scale size. As shown in the image below there are two scales on the same side of the same scale. The 1/8″ and 1/2″ scales are show on the same side.

Reading a Scale — How to read a scale. A lot of scales offer options for scale lengths.

Software Programs

If you own modern estimating software then the scale is set in the computer according to the scale indicated on the drawings. Occasionally the scale indicated on the drawings is incorrectly marked, so knowing how to confirm the scale is important.

There are also other programs like Blue Beam or Adobe that have various scale reading features of their software packages.

Next is Chapter #7 “Sheet Metal Shop Drawings” to see how drawings are prepared for use in the field.

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Chapter #1 – General Layout of Drawings
Chapter #2 – Architectural Drawings
Chapter #3 – Plan, Elevation, Section Views and Details
Chapter #4 – HVAC Mechanical Drawings
Chapter #5 – Understanding HVAC Symbols
Chapter #6 – Drawing Scales (How to Read Scales including Metric Scales)
Chapter #7 – Sheet Metal Shop Drawings

Understanding HVAC Symbols

MEP Academy Instructor

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September 27, 2020

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Chapter #5 – Understanding HVAC Symbols

In order to understand how to read HVAC drawings, you have to understand the road signs (HVAC Symbols). These vary from one engineer to the next, but there are some similarities that will help you figure the differences out. HVAC Symbols work like the road signs you are familiar with; they allow you to discern their meaning by a visual icon or image with very little use of words.

Without HVAC symbols the drawings would be crowded with words and sentences in an attempt to explain what is easily explained with a symbol.

There are symbols for all kinds of equipment, duct specialties, piping components and for all other trades. Symbols are the unspoken language in the construction trade. They convey meaning without words or in conjunction with abbreviations.

There should be a Legend of symbols on the mechanical drawings.

You will find that there is no industry standard and that symbols can vary from one engineer to the next. Often if they are using the same CAD program and the symbols that are standard with the program then you will see similarities.

Don’t be confused by the variations of the same symbol from drawing to drawing. Focus on what the symbol is trying to convey.

Supply, Return & Exhaust Symbols

The following are used often throughout the mechanical drawings to indicate which type of air is in the ductwork, or which type of air distribution is being referenced.

The three most common being supply, return and exhaust. An “X” in a square or round shape is indicative of Supply Air, while a single Diagonal line indicates return air.

The following symbols indicate the type of air but also that of a riser duct. A riser duct is one that penetrates a floor. When the line is solid, it indicates that the duct is going up, and if the line is dashed, it indicates that the duct is going down.

Duct Risers in Shafts

In multi-story buildings it’s probable that you will have a shaft, which is usually a drywall enclosure that is fire rated to allow utilities, including HVAC ductwork to travel from one floor to another. When the ductwork leaves or enters the shaft it requires a fire damper or a combination fire/smoke damper, in order to prevent fires or smoke from traveling from one floor to another.

When looking at the shaft, you can determine which direction the ductwork is going by whether the lines are solid or dotted. Solid lines indicate that the ductwork is going up, and dashed lines (Hidden Lines) indicate the ductwork is going down.

Again, to distinguish which direction the above ducts are going, you only need to remember the rules governing hidden lines. The shafts on the left side show hidden lines (dashed lines) in the riser indicating that the shaft is headed downward, while the shafts on the right show solid lines indicating that these are duct risers going upward.

The above shows what a rectangular duct looks like in each case, while the below represents a round portion of duct that rises up and down. The round duct riser goes down on the left side as indicated by its dashed lines, and the other side goes up as indicated by the solid lines.

Air Distribution

Air distribution devices are used at the end of branch ducts or tapped onto the side of ducts to allow air out of or into the ductwork. Air distribution grilles, registers, diffusers and linears are chosen to allow for the proper dispersion of the air into the room based on acceptable noise criteria, throw distance and throw pattern. The arrows show the direction of the airflow. Supply shows the arrow exiting the air diffuser, while return and exhaust show the arrow pointing into the grille.

Supply Return Exhaust Grilles

he arrows show the direction of the airflow. Supply shows the arrow exiting the air diffuser, while return and exhaust show the arrow pointing into the grille.

Below is an example of what it might look like on the drawings. Note that different engineers will use different symbols. The below image shows the symbols indicating supply, return and exhaust air distribution devices.

Air distribution grilles, registers and diffusers will often have an acronym that matches an air distribution schedule on the drawing where the equipment is listed, which will define the make and model number for each piece & indicate the CFM.

Ductwork Symbols

Most ductwork is either shown in single line or double line. There are also various types of ductwork that have their own symbols, such as flexible duct which is often shown as a squiggly line.

Ductwork can be internally lined for acoustical or thermal reason, that is for the reduction of sound transmission or for the reduction of heat loss or heat gain. This is usually indicated by a dashed line within a solid pair of lines.

In retrofit projects where the existing ductwork needs to be removed, the symbol that indicates this is usually some form of diagonal hash marks as shown below. Labor must be included in your estimate to remove this ductwork.

When existing duct is removed as shown above, but not all of the existing ductwork gets removed, there is a symbol that indicates a point at which the existing duct on one side of the line remains, while the duct on the other side of the line gets removed (Demoed)

When new ductwork is attached to existing ductwork, engineers will indicate this by using a point of connection symbol. The difference between the new ductwork and the old can also be determined by the type of line used. Lines that are dotted, dashed or thinly drawn are indicative of existing ductwork, while solid lines indicate new.

Routing ducts through buildings often require that they rise or drop to get around obstacles. This is indicated with an arrow and with a letter; R (Rise), D (Drop). As a sheet metal estimator you should add a couple of angles (45-degree elbows) or an offset fitting.

A special type of duct is used to prevent vibration and noise from transmitting down the ductwork from the source. The flexible duct section is used for equipment, but there are also special flex section used for seismic breaks.

Dampers

Fire Dampers

Fire dampers are shown as a dotted or solid line through the duct at the point where it penetrates the fire rated wall, accompanied by a solid diamond, square or other shape, often with the abbreviation FD (Fire Damper) as shown below.

Smoke/Fire Damper

This is a combination of two types of protective dampers, a fire damper, which protects against fires, and a smoke damper, which closes off the air by motorizing a damper closed.

Volume Dampers

The air in an HVAC system requires balancing in order that the air gets to the room in the quantity that the engineer intended. Volume dampers are shown as various straight lines perpendicular to the duct with a small handle.

Automatic Control Dampers

In order to control dampers automatically they need to have some form of actuator that will modulate, open or close to perform some function of the HVAC system.

Auto Control Damper — Automatic Control Damper

Miscellaneous Equipment

There will be miscellaneous pieces of HVAC equipment that will be inserted into the ductwork. Make sure to review what each symbol represents, as piece of equipment installed in the ductwork often requires transitions or special connections. As shown below this in-line coil which is larger than the ductwork will require two transitions.

Duct Mounted Coil

This next symbol represents a piece of equipment, in this case a heating coil as shown by the red outlined box in the image below, but it could be any type of coil.

One of the ways to tell this is a coil is that there are pipes attached to it. Usually you will see two pipes entering the coil, but the engineer has chosen to show only one of the two pipes.

Symbols for a Typical VAV Terminal & Ductwork

The image below is of a typical VAV (Variable Air Volume) terminal, this is a box connected to the ductwork that varies the volume of air based on the thermostats setting.

This symbol represents a piece of equipment, in this case a Variable Air Volume terminal box with a lined plenum. The VAV provides a variable amount of air to the diffusers based on the thermostat setting and current room conditions. This could also represent a CAV (Constant Air Volume) terminal box.

Point of Connection (POC). This indicates where the new ductwork connects to the existing ductwork. Everything from this point forward is new, and everything before it is existing. Often, the new ductwork is shown in dark lines, while the existing is drawn lightly. Also, sometimes the (N) new ductwork includes an (N) before its size, to indicate its new.

As we learned above the “X” within a box refers to Supply Air, so this here is referencing a Supply Air Diffusers. This also shows that the SA (Supply Air) diffuser blows air in four (4) directions, as indicated by the arrows.

Volume Dampers are used to balance the system so that the correct amount of air (CFM) is provided through the diffuser to the space.

VAV (Variable Air Volume) Terminal. This is a piece of equipment purchased from a supplier, and manufactured by Titus, Metal Aire, Krueger, or similar. This VAV terminal will adjust the volume of air delivered to the space based on the demand of the thermostat.

Return Air Grille with a sound boot. This grille provides for air to return from the space back into the attic where it will make its way back to the system by the negative pressure created by the HVAC system. On the Return Air Grilles there is a sound boot, or a piece of lined sheet metal with an elbow to prevent noise from being carried to other spaces.

Return Air Grille Symbol — Return Air Grille

Thermostat

The thermostat controls the temperature setting for the room and controls the HVAC equipment. The symbol for a thermostat is often just a circle with a “T” inside as shown below. The thermostat shown below is controlling a VAV box.

Indication of Demolition

Items that are hashed out as shown in this picture indicate items of a retrofit project that need to be removed. This show two fans and duct that needs to be demoed and removed.

Equipment Tags

These are special symbols that identify the equipment by an acronym and a number. This helps tie together the equipment schedule that list all the equipment with tags that can be located somewhere on the floor plans to show where that piece of equipment resides in or on the outside of the building. This equipment tag (AC-1) represent Air Conditioner #1.

Drawing Notes

Drawings will use numbered tags that refer to a column of numbered notes that might be off to one side of the drawings. Putting the description or notes directly on the drawing next to the item could make the drawing messy and hard to read, it’s easier to put a small number next to the item that the note references.

Detail and Section View Symbols

As mentioned previously, these are drawing road signs that point you in the direction of another drawing where a detail or section view is available. These special symbols identify what other drawing that you need to review to understand in more detail the area referenced.

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Next is Chapter #6 “Drawing Scales” to see how scales effect your takeoff and how to read scales including metric scales.

Chapter #1 – General Layout of Drawings
Chapter #2 – Architectural Drawings
Chapter #3 – Plan, Elevation, Section Views and Details
Chapter #4 – HVAC Mechanical Drawings
Chapter #5 – Understanding HVAC Symbols
Chapter #6 – Drawing Scales (How to Read Scales including Metric Scales)
Chapter #7 – Sheet Metal Shop Drawings