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Sheet Metal Field Labor Productivity

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Sheet Metal Field Labor Productivity

Chapter #4 – Sheet Metal Field Labor Productivity

After your reports have finished running on the estimating computer software program its time to review the field labor. If you don’t have a computer estimating software program then your all your field labor numbers would be generated using Excel or some other spreadsheet totaling software. Hopefully your estimating software has the capability to breakdown the labor reports for easy analyses.

We will want to look at the labor productivity, meaning how many linear feet of ductwork is being installed per man-day (LF/MD) as one of the methods of labor analysis, or linear feet per hour (LF/HR) if you want to look at it in a micro sense. A man-day is equivalent to an 8-hour day.

Another method of labor analysis is hours/piece (Hrs/Pc) and pounds/hour (Lbs/Hr). When we took the sheet metal off we separated our takeoff into different zones and systems. At this point we would analyze each zone and system separately.

There is no better way to review the productivity of your project, except to go through it line by line as grouped per your System and Zone descriptions. This will help those in the estimating review to build the job in their head, giving everyone involved a good sense of the project. If you apply a factor overall to everything, no one will fully grasp the project as deeply as a thorough review would allow. Some companies do just that—they apply one factor to all the labor based on the type of project.

We will first cover some of the factors that affect productivity. This is often one of the most difficult undertakings of an estimator, the analysis and adjustment of labor for non-typical conditions. If using a computer estimating program with SMACNA labor units, then you would be adjusting from a 1.0 of SMACNA labor.

Measuring Labor Productivity

This is one of the main duties of the sheet metal estimator, as it has the greatest impact on cost. Labor proposes the highest risk to the contractor, as it is one of the greatest variables. The cost for other aspects of the bid, such as material, equipment and subcontracts shouldn’t vary much from their original bid time cost, unless an error was made.

But labor will vary from your assumptions at bid time about how productive the field labor would be able to install the required equipment and material. So it is labor that really needs to be understood in order to provide accurate estimate that will return a reasonable profit for the owners.

SMACNA Labor Benchmark

There are various organizations that publish labor productivity values for the HVAC Sheet Metal industry including SMACNA, the Sheet Metal and Air Conditioning National Association. There are various factors affecting the productivity of labor from some baseline, or benchmark standard. Each project presents various challenges based on differing site conditions. Labor needs to be adjusted from the standard benchmark labor units to match the specifics of the project under consideration.

Most often labor is the riskiest portion of the estimate because of the assumptions made on the productivity that the project is assumed to be able to redeem. SMACNA provides three progressive categories of labor risk, “Normal”, “Difficult”, and “Very Difficult”. As the conditions worsen, the corresponding labor cost will increase. If you are bidding a retrofit project, one in where you are working in an existing building, then SMACNA considers all remodels to be categorized as either “Difficult” or “Very Difficult”.

SMACNA Labor Tables (Database)

If you are using the SMACNA Reference Manual for Labor Units for your field labor productivity then you will apply a factor to align the SMACNA labor with your field labor feedback (Historical Data), if your company maintains such information. SMACNA labor tables were original assembled from a survey of over 300 member firms from the United States and Canada.

The labor units contain 138 tables for the fabrication and installation unit labor values for the installation of ductwork, fittings, hangers, equipment and accessories. Labor units are available in pounds per hour, or hours per piece, in addition to labor for equipment based on the size of the HVAC equipment.

The SMACNA labor units exclude engineering, supervision, field layout (Detailing) or non-standard construction.

You can use the SMACNA labor units that are embedded in your estimating software if you have one, or you could purchase the SMACNA labor units Excel version, that allows you to adjust values using their spreadsheet. The electronic Excel spreadsheet allows you to rate your project in 30 different difficulty categories in an effort to provide you with the difficulty factor of your project compared to a standard project. There are three project difficulty ratings based on the information you enter for your project, indicated as “normal”, “difficult”, or “very difficult.”

Labor Feedback

The best way to determine how much labor is required for a particular task of work, is to review historical data, that is what it took your companies field crew in hours to perform similar work that has already been completed.

If you are fortunate enough to work for a company that tracks their labor, then you will have a labor database to review when assembling an estimate. The database accumulated from labor feedback will give you a reference for how long it takes to install various materials and equipment, depending on the sophistication and accurate tracking and reporting of the labor spent.

Historical data should be used as a guide for your current project, but not without serious consideration for the differences in the respective projects. Just as SMACNA rates the different categories of labor according to their difficulty, you must know under what conditions your historical database reference project labor was installed under. Understanding what effects labor is the first place to begin before any adjustments up or down from a benchmark labor database is performed.

Industry Labor Database vs Historical Data

There are two main ways to bid labor on a project. The first is to use an industry labor database, and the other is to use some form of historical labor feedback from completed projects. The third way is to use a hybrid of the two. Do your takeoff using the industry standard database, such as SMACNA labor units, and then adjust the productivity according to your historical labor.

The most accurate labor productivity is going to be based on the feedback you received from the field on jobs completed that are similar to the one you are currently proposing, making adjustment for any differences. Let’s look at a few ways to record and adjust labor in the field.

Sheet Metal Labor Productivity Units

There are several methods for analyzing labor productivity for any project. You need to find one that you are comfortable with and that matches the specifics of your company. We will cover four methods of measuring labor productivity.

Duct hung in Wood Structure
Duct hung in Wood Structure

Labor can be measured in LF/MD (linear feet per man day), which means that so many feet of duct can be installed in an 8 hour period, the same unit can be defined as LF/HR (linear feet per hour), which means how many feet can be installed in one hour.

Some contractors like to look at LBS/HR, which means how many pounds of sheet metal that can be installed in an hour. And then there is the HRS/PC (hours per piece), which is defined as how many hours it takes to install a piece of duct or fitting.

MD = 8 hours

  1. LF/MD = Linear feet per man day
  2. LF/HR = Linear feet per hour
  3. LBS/HR = Pounds per hour
  4. HRS/PC = Hours per piece

Linear Feet Per Hour (LF/HR) & Linear Feet Per Man Day (LF/MD)

Ft/MD (Feet per Man Day) is used as a measurement of the labor productivity for a certain ductwork system. If your labor feedback from previous projects indicates that you can install rectangular main ducts at 25 LF/MD (linear feet per man day), then you would apply this productivity factor to the total linear feet of main ductwork you have for the current project if the duct sizes and fitting quantity are similar.

Then you would add any of the labor corrections factors that are applicable for the current project. For example let’s say you have 200 linear feet of main ductwork. You would take the 200 feet and divide it by the historical labor feedback of 25 LF/MD which would equal 8 man days or 64 hours (200 200 LF/ (25LF/MD) = 8 Man Days or 64 Hours.

To this you would adjust for any factors effecting labor. (See course section on “Factors Affecting Field Labor.”) If you receive your ductwork from the fabrication shop KD (knocked down – unassembled), then you will have to add the time it takes to assemble the ductwork on site, which is separate from the time it takes to install the ductwork.

Additionally, if you had 3,000 feet of ductwork and fittings, and your benchmark labor database determined that it would take 800 hours to install this ductwork & fittings, this would equate to 30 LF/MD (30 linear feet per man day)

LF/HR = 3,000 Ft / 800 Hours = 3.75 LF/HR (Linear feet per hour)

To get to linear feet per man day, just multiple by 8 hours in a day.

LF/MD = (3,000 Ft / 800 hours) x 8 Hours/Day = 30 LF/MD

3.75 LF/HR = 30 LF/MD (These two represent the same labor productivity factor)

Pounds per Hour (LBS/HR)

This method is available in the SMACNA labor database as an option for labor analysis. Differing productivity values (Lbs/Hr) are used for different size and gauges of ductwork. As ducts get larger or their gauges get heavier, the duct and fittings increase in weight (pounds).

Example: if you had a 10 feet of 12” x 12” duct at 26 ga, this would equal

STEP-1 Remember that stretch-out is derived by unfolding the duct from a four sided box into a flat piece of metal, so that a 12” x 12” duct unfolded would look like this;

12” + 12” + 12” + 12” = 48” or 4 feet.

STEP-2 Determine how many pounds of sheet metal you have. We have sheet metal ductwork that is 10 feet long by 4 feet wide (stretch-out), which equates to 40 FT2.

10 feet x 4 feet = 40 FT2

STEP-3 Convert the 40 FT2 into pounds by using the conversions value for 26 gauge (0.906 LBS/FT2) galvanized sheet metal, which is.

40 FT2 x 0.906 = 36.24 LBS

STEP-4 Apply your productivity factor (LBS/HR) to the total pounds of duct at this size. Per SMACNA the productivity at this size and gauge is 24.70 LBS/HR. This would calculate out as

36.24 LBS / (24.7 LBS/HR) = 1.46 Hours

The pounds per hour labor productivity factor is the most difficult to visualize. The difficulty comes from the fact that the same five-foot piece of ductwork has different weights at the different gauges, and it’s difficult to tell how much duct weight by looking at hit. It’s easier to count how many pieces have been installed or how many linear feet. What does 300 pounds a day look like, as compared to 25 feet a man-day? It’s easier to visualize the quantity of feet or pieces.

How does the Foreman determine at the end of the day how many pounds the crew has installed? We strongly recommend that you don’t use this method, but choose one of the other labor productivity measuring factors.

Labor Productivity Calculations
Labor Productivity Calculations

Hours per Piece (HRS/PC)

This is the second method available in the SMACNA labor database as an option for field labor analysis. Differing productivity values (HRS/PC) are used for different stretch-out sizes (perimeter). Using the same  10 foot 12” x 12” duct from above, and assuming that we are using 5 foot joint lengths, would mean that we have two (2) 5-foot sections of duct. Using the SMACNA table for low pressure galvanized straight duct with a 4 foot stretch-out and the 26 ga column, we get 0.8611 HRS/PC.

2 pieces x 0.8611 HRS/PC = 1.72 Hours

Using the same 12’ x 12” section of ductwork we can see that depending on which units of labor you use, you derive at different totals. For the LBS/HR unit labor, you get 1.46 hours, while the HRS/PC method gives you 1.72 hours based on the SMACNA database.

Once you decide on which units of labor you will use, then you should stick with it, so that you can develop a sense of familiarity.

Some prefer to use the hours per piece as a means of measuring their labor productivity in the field. This is somewhat similar to the feet per man-day in that if you know the length of each piece of duct

Practice Calculations

Calculate the LF/MD using the following information: (Answers below at end of this course)

Let’s assume your estimating software or manual takeoff has provided you with the following data;

  1. Rectangular Duct = 1,200 feet and 320 hours. (What is the linear feet per man-day?)
  2. Round Duct = 1,800 feet and 240 hours. (What is the linear feet per man-day?)
  3. Adjust the labor productivity to 25 LF/MD for question #1. What are the new hours?
  4. Adjust the labor productivity to 50 LF/MD for question #2. What are the new hours?

Military Project Labor Productivity

This chart is derived from the U.S. Military and shows what they assume it takes to put in rectangular and round sheet metal ducts based on their standard project assumptions. You should establish something similar for your company based on labor feedback from a sample of various project types. This will help you establish your standard benchmark for labor by which all future project labor would be adjusted from, such as SMACNA developed for its membership.

Military Sheet Metal Productivity-Chart
Military Sheet Metal Productivity-Chart

U.S. Military Sheet Metal Labor Productivity Values

Which Labor Unit Method is Best?

We prefer the LF/MD or LF/HR method of labor productivity analysis because it’s visually represented better than the others. For example, if you were to visit a jobsite to review the ductwork that has been installed, and you’re looking at a section of ductwork hanging in the buildings first floor ceiling, could you tell how many pounds of metal it is? This would require you to do some mental gymnastics, or some quick calculation as to what gauge and size the duct was in order to calculate the weight, and then divide by the productivity factor.

Productivity factors using weight in our opinion is not a good labor unit for visually inclined estimators and project managers. There must be a reason why the military uses LF/MD & LF/HR, and that is, it represents an easier way to visualize what has been installed, and for setting a benchmark for field productivity performance.

Proper Takeoff Techniques

Labor productivity factors will vary based on job site conditions as discussed above, but with everything being equal, productivity will also differ based on duct size. There is no way for a crew to install a 64” x 36” section of duct at the same productivity as a section of 12” x 12” duct. So, for this reason your adjustment factors need to be broken down as shown in the military labor unit schedule.

Benchmarking Labor Productivity Performance

When the project is under construction and ductwork begins to get installed, the project management team should begin measuring weekly what was installed and the amount of hours expended for the portion of the project related to the Sheetmetal ductwork. This will let them know if they are hitting their estimated productivity goals.

This is done by taking a fresh new set of drawings out to the jobsite for benchmarking the performance that occurs each week. Each week the ductwork on the drawings that has been installed gets colored on the drawings, documenting the progress of the project. Back at the office, the ductwork that was colored in for that week gets taken off to determine how many linear feet of ductwork was installed compared to how many hours were charged for installing the ductwork that week.

If the sheet metal crew had installed 900 feet of ductwork and they used a six person crew for a week, then the actual productivity would be as follows;

6 Crew Members x 40 Hours/Wk = 240 Hours spent

900 Feet / 240 Hrs = 3.75 LF/HR or 30 LF/MD

Correction/Adjustment Factors

The correction factors shown below are used to adjust a base unit of labor that is considered normal conditions. The correction factors then would adjust this normal condition base unit for any conditions out of the norm. If you review MCA and SMACNA labor units their method is similarly based on a standard unit of measure for which unusual conditions are adjusted. Labor productivity will vary by building type and projects, and may require more or less than the benchmark database labor.

These adjustment factors are just suggestion, and are not hard percentages that you should rely on. All labor adjustments should be something that is derived from experience and historical data that your company tracks. This applies to all the adjustment factors shown in this course.

Height of Installed Ductwork: Anytime your ductwork is hung above 10 to 13 feet you will need to adjust for the increased height. If the field labor can’t work off ladders then you will need some type of scaffold or scissor lift. Add 2% to 10% for ducts hung 11’ to 15’: add 10% to 20% for ducts hung from 16’ to 20’, and so on:

Labor Correction Factor for Ductwork Height
Labor Correction Factor for Ductwork Height

Quantity of Floors: As mentioned above the quantity of floors will have an impact on your labor when the quantity of floors exceeds three. This is the additional time it takes to reach higher and higher floors in the building.

Labor Correction Factor for Quantity of Floors
Labor Correction Factor for Quantity of Floors

Weather Conditions

We all know what it’s like to work in excessively hot or cold weather. Workers just don’t perform at the same productivity levels as they do under normal weather conditions. You can get more done in a nice controlled environment than if you had to work on a cold concrete floor on the 10 floor of a new high-rise that has no walls installed yet, and the cold wind is blowing through the building.

If the project will be built during extreme weather conditions, than you should adjust for a loss of productivity due to the weather. The question is what would the weather conditions be during the duration of the project?

Extremes of either hot or cold weather will result in a loss of labor productivity. Humans don’t function as well when the temperature of the ambient air is outside their comfort zone. This goes for working in areas such as hot boiler rooms or equipment rooms where the temperature is outside the normal range for human comfort.

Temperature Factor on Labor Productivity
Temperature Factor on Labor Productivity

Field Labor Feedback

If you have been in business for any period of time you have established some historical data on the field time required to install ductwork, equipment and accessories. From this historical data, you can project what kind of productivity you should get for the current, similar job. If the jobs are not similar, then you must adjust the productivity up or down depending on the current projects relative ease or difficulty compared to your historical project.

There is no better measure of productivity than your own historical data, but each job will still be unique and should be carefully scrutinized for those areas where productivity could be affected.

Answers to Practice Calculations

  1. 30 LF/MD (1,200 LF/320 hours) x 8 hrs/day)
  2. 60 LF/MD = (1,800 LF/240 hours) x 8 hrs/day
  3. 384 Hours (1,200 LF/ 25LF/MD) x 8
  4. 288 Hours (1,800 LF/ 50LF/MD) x 8
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Sheet Metal Field Installation Course (Free)

Conditions Affecting Field Labor

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Conditions Affecting Field Labor

Chapter #11 – Conditions Affecting Field Labor

There are many factors that affect the cost of installing the sheet metal and HVAC equipment in a building. Here is a list of some of the things to consider when analyzing labor productivity. This list is one of the most important things to think about when deciding on how much labor to include in your project.

Working Height

The working height has a great impact not only on labor productivity, but also on the cost of rental equipment. There is a big difference in labor between hanging duct at 12 feet versus hanging duct in a large convention center with a 30 foot hanging height. The latter will require that you work off of some form of elevated platform, whether by scissor lifts or built up scaffolding, the addition of this extra step will require additional planning and installation time to perform the work.

Hanging Duct
Hanging Duct using a Scissor Lift

The height at where your hangers attach to the structure has an impact on field labor, because the hangers are installed ahead of the ductwork and the upper attachment is at the highest point.

Watch the following video and look for the conditions that affect the labor of these workers which include at least the following:

  • Working Height
  • Safety Requirements
  • Material Lift

The question becomes what is your benchmark standard or normal working height from which adjustments will be made. At what height do you bring in a scissor lift or other elevated working platform?

Job Site Conditions

Job site conditions will affect the productivity of the field crew along these three differing scenarios; New construction, retrofit or remodel, and working in an Occupied space. The question is, what type of construction is it?

Ducts above Ceiling
Ducts above Ceiling

New Construction usually allows for less obstruction to performing your labor because walls ceilings and other construction obstructive building components aren’t installed as of yet. This allows you more room to maneuver around with labor, equipment, tools and materials.

Remodels or retrofit projects are just the opposite because you will need to work through the existing construction. All the walls, ceilings and other trades are already installed and possibly obstructing easy access to the work you will need to perform.

Occupied spaces pose another labor intensive scenario because you will need to work around business hours or people while they conduct their business, and existing furniture in addition to all the considerations for a remodel.

Tight Spaces: Attics, crawl spaces, equipment rooms and tight shafts are but a few of the difficult areas due to restricted movement of field labor. You will need to consider these spaces separately and adjust accordingly.

Working in Existing Facilities

There are a lot of factors that come into play when working in an existing facility. First of all will you be working around existing walls, or will they be tore down before you begin your work? This is an important factor, because if you have to move your ladder from one room to another to install your ductwork, that will greatly reduce your labor productivity.

How congested is the ceiling space where you have to install your ductwork? Do you have to work through the existing ceiling grid? Are you responsible to take down the ceiling tiles and reinstall them when you’re finished with your work? Are you responsible for any damaged ceiling tiles? Your labor productivity will be greatly impacted if you have to work through the existing ceiling grid as this will also limit the length and size of the duct sections you can fit through the grid.

Who is responsible cutting and patching of the walls if you need access to a shaft where the duct tie-in will occur, or for the framing of a fire damper? Who is responsible for coring or saw-cutting of floors or concrete walls for the passage of sheet metal ducts?

As can be seen, there are many factors you have to deal with when contemplating the installation of ductwork in an existing facility. That’s why it’s important to attend any pre-bid conferences or job walks. Remember SMACNA considers remodels or retrofit projects to be categorized as either “Difficult or Very Difficult”.

Working Shift

The time at which a worker has to perform a labor task has been documented to affect their productivity. A crew working the swing shift or graveyard shift will produce at lower productivity rate than their counterparts working during normal business hours. You basically have three windows of 8 hours, starting with normal business hours, followed by the 2nd shift (swing shift), and then the 3rd shift (graveyard shift).

Do you have to work around employees during business hours or while the store remains open? Do you need additional time to protect the furniture, flooring or walls?

The question is, when will the work be performed, and does anything need protecting?

Some union workers are paid for 8 hours for only 7-1/2 hours of work when working swing or graveyard shift. Check your union agreement for this requirement or something similar.

Crew Density

You would normally think the larger the crew size the greater the amount of work that would get done, but actually the opposite can be true. There is a point where too many workers on a project site will reduce your productivity levels due to inefficiencies’ caused by too many workers. SMACNA list three crew densities, “Normal”, “Moderate”, and “Extreme”.

Building Square Feet

The size of the building will cause a loss of productivity as the building square feet becomes larger and larger. It will take a lot longer to walk to the work location or to track down tools or material. How do you keep track of all your workers and their productivity in a building the size of a large mall?

It’s unfortunate but there are some workers that will slack off when supervision is out of sight. This is part of how crew sizes can get too big as to cause a lack of overall productivity as stated above. The question is how many square feet is the work area?

Quantity of Building Floors

This is similar to building square footage, except that you are dealing with multiple floors. The higher the building is, the more floors to deal with and workers to keep track of. Up to three floors maybe considered normal, but as more floors are added, several labor factors come into play that will cost additional hours.

In addition to loss of productivity from the additional floors, you will need to take into consideration the additional time it takes to get to the upper floors, such as waiting on manlifts in new construction or waiting on elevators in existing construction.

This is an increase in hours for the labor to get to the upper floors, and for material handling, that is getting the equipment and materials to those same upper floors. Adding 1% to 3% of labor hours per floor for every floor above the 3rd floor is a good starting point for adjusting for additional floors. The question to ask is how many floors is the building?

Building Site Size

Also, similar to the above is the size of the site. Large square feet buildings with many floors and a large construction site is a triple whammy. How far away must your workers park from the building? How long does it take for them to get from where they park to where they will begin their work? Site that are very large may also necessitate some form of small vehicle, like golf carts or gators to get around from one area to another. The question is, how big is the site, and how far away are the workers from their work area when they arrive on site?

Quantity of Hours in a Work Week

Research has shown that after 40 hours of work in a week your labor productivity begins to drop off, and becomes worse over extended periods of time if no reprieve is given. Some may thrive on additional work, but the average worker will lose the ability to remain focused if kept consistently week after week working more than 40 hours. Worker fatigue sets in, and productivity drops. This will have an effect on labor productivity. The question is, how many hours a week are the workers required to work to complete this project on schedule?

Construction Cleanliness Requirements

Depending on the requirements of the project, there are various levels of cleanliness specified by the engineer or dictated by the type of construction. Does the project require a dust free environment, or is it a remodel in a hospital where you might need to work in containment carts? Is it a cleanroom, where you will be required to gown up to prevent contamination of the product? The question is, what are the cleanliness working requirements?

Security Access

Does the project require that the worker and their tools go through a security protocol before entering or leaving the facility? If the project is in a prison, jail, courthouse, amusement park, defense contractor or similar type facility, then it’s possible the workers will lose part of their 8 hour day getting cleared going in and out of the facility. The question is, do the workers or their tools need to go through security each time they arrive or when leaving?

Repetitive Work

If the project consist of work that is repetitive, where each space is exactly the same layout as the previous one, then there should be some labor savings. It’s like doing the same puzzle over and over again, the brain becomes conditioned as to what is expected next, as opposed to having to do a new puzzle every time. The question is, is the work repetitive?

Availability of Labor

How much skilled labor is available for you to call upon to execute this project if you were successful at winning the bid? There are times that various union halls have been drained of the available workers to be called out for a project. If you are a non-union company, where will you find trained workers to complete this project in accordance with the project schedule?

If you maintain a steady crew of workers, and this project doesn’t conflict with any other project commitments you have, then maybe you can get by with the existing crew, but as you grow and expand, the need for additional workers will arise. The question is, do you have the availability of labor required to complete this project on time according to the construction schedule?

Compression of Project Schedule

This factor impacts the crew size as previously discussed, but also involves additional losses caused by having to complete a project in a faster time frame than normal. A compressed schedule means more trade people and their tools, materials and equipment fighting for limited space, all in an effort to complete the project in an accelerated schedule.

The idea is that anytime you are required to accomplish something beyond a reasonable time, it will impact labor, and possibly quality, requiring additional reworking of previous rushed work. The question is, is there enough time allocated in the schedule to complete the project in a reasonable time?

Condition of Project Documents & Responsiveness of Engineering Team

If the plans and specifications are lacking in information and completeness, then time could be lost waiting around for decisions to be made and RFI’s (request for information) to be answered. The question is, are the documents well done and ready for construction with minimal RFI’s required?

Material Handling

When installing a lot of sheet metal ductwork and air distribution devices, then time must be considered for how you are going to identify what goes where and how you get it to the location where it gets installed. Material handling gets the duct from where it is delivered at the site to the location where it is going to be installed.

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Sheet Metal Field Installation Course

Sheet Metal Duct Sealer and Leakage

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Sheet Metal Duct Sealer and Leakage

Chapter #6 – Sheet Metal Duct Sealer and Leakage

Duct Leakage

Duct leakage is a factor of the static pressure in the ductwork that allows for leakage to occur through holes, seams, joints and penetrations. Leakage is a loss of energy, which means a loss of money. This is why it’s so important to seal the ducts and to prevent leakage. Leakage can also occur at equipment and accessories that are installed into the system including dampers, smoke/fire dampers, VAV Boxes and access doors.

SMACNA Duct Seal Classes

Ducts are required to be sealed according to SMACNA Table 5-1 based on the duct pressure class (#1), which is broken down into three categories of pressure. Depending on the pressure class of the duct, there will be a corresponding Seal Class (#2) from C to A.

As the pressure increases in the ductwork the applicable sealing requirements (#3) will apply to various aspects of the ductwork, starting with Transverse Joints Only (Seal Class “C”), then Transverse Joints & Longitudinal Seams (Seal Class “B”) and finally All Transverse Joints, Seams and Wall Penetrations (Seal Class “A”).

Recommended Leakage Class
Recommended Leakage Class

Duct Sealing

There are various sealing requirements.

This video shows how duct sealer is applied with a brush. It appears to be a messy job, but the idea is to make sure that all the joints and seams are air tight so as to avoid duct leakage, including brushing sealer over all screws used in fabrication. Ducts that leak are wasting energy and preventing the conditioned air of doing its job of providing a comfortable environment for the occupants.

Duct Sealing

There are various types of sealer including some that are required to be of low or no VOC (Volatile Organic Compounds) when a building is seeking to get LEED (Leadership in Energy and Environmental Design) certified or when requested by an environmentally conscious owner. The IEQ Credit 4.1: Low Emitting Materials is worth 1 point in the LEED certification program. This LEED credit applies to various sealants and adhesives used on the interior of construction projects in addition to duct sealer, items such as carpet and floor adhesives.

The Adhesives, Sealants and Sealant Primers must comply with South Coast Air Quality Management District (SCAQMD) Rule #1168.

Joint Sealing

Joints are sealed with Butyl Gasket to prevent leaks at the connection points between ducts, fittings and accessories.

Applying Gasketing

Duct Leakage

SMACNA doesn’t recommend pressure testing on duct pressure classes of 3” wg and less, as its not economically feasible. Be sure to check the specifications and your local or state energy code for this requirement. Leakage testing could be shown in the Testing & Balancing or the Ductwork section of the specifications.

Some states, like California require pressure testing on small systems (spaces 5,00 Ft2 or less) when 25% or more of the ducting is located in a non-conditioned space, one not intended to be occupied by people. This applies to both new and renovated projects.

Also some of the utility companies that offer energy saving incentives or rebates require pressure testing to confirm a maximum level of leakage in order to qualify for their program.

Sealing Ductwork
Duct Leakage Test

Duct Leakage Allowed

SMACNA publishes Table A-1 “Leakage as Percentage of Flow in System” that if specified in your local code or the design specifications, shows the percentage of leakage (#5) at various Static Pressures (#1) and Leakage Classes (#2). This is also effected by a calculation (#4) of the Fan CFM divided by the Ducts Surface Area (#3), shown as; CFM/Duct Surface Area. The product of the calculation is available at 2, 2.5, 3, 4 & 5 (#3).

Duct Leakage Percentage Table
Duct Leakage Percentage Table

Note that as the pressure increases (#6) so does the percentage of duct leakage. As an example, if you had a duct rated for 2” static pressure and the specifications called for a leakage class of 16, you would get a rate of 13% leakage, assuming your calculation (#3) for Fan CFM divided by Duct Surface Area equaled 2 or less.

Item (#7) above shows what the Table 5-1 “Recommended Leakage Classes” above would look like on this leakage rate chart. Item #7 shows for rectangular duct (highlighted in blue), while item #8 on the chart above is reflective of round duct (highlighted in green).

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Sheet Metal Duct Hangers

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Sheet Metal Duct Hangers

Chapter #3 – Sheet Metal Duct Hangers

Air conditioning ducts and pipes need to be supported according to code approved methods. You need to confirm that the hangers you intend to use are approved in the local area where the building is located. Mechanical engineers will specify the approved hanger methods in the specifications if provided.

Hanger requirements vary based on locale, but will often consist of two to three parts, the upper attachment (highlighted in blue) which attaches to the structure, the vertical hanger piece (highlighted in green) and the lower attachment (highlighted in red) which attaches to the duct.

There are many different upper and lower attachment types. A few are shown here for concrete.

Hanger Upper Attachments

The upper attachment portion of the hanger attaches to the building structure. The type of upper attachment to use is conditional upon the type of structural element it will be attach to. Common materials are wood, concrete and steel beams. The hanger’s upper attachment will then be connected to the vertical support piece which most commonly is a hanger strap, threaded rod or wire. Hanger strap are made from strips of galvanized metal ranging in gauges from 16ga to 26ga

Concrete Inserts

In commercial construction, concrete is often used to construct the floors, which can be constructed in place with forms or poured onto some form of metal decking.

The concrete insert will need to be installed before the concrete is poured when using metal decking. This will require that the hanger insert (the upper attachment) be placed on the floor above where the hanger is required. For example, if you are hanging duct or piping on the first floor, you would need to put the concrete insert into the floor of the second floor. Watch the video below to get a better understanding of what we mean.

Blue Banger Hanger

(See image below) You could use a simple bent flat bar (#1) that penetrates the floor deck (#3) and which gets covered in concrete to fix it in place. Field labor would spot the location for the hanger and punch a hole in the metal deck, then drop the sheet metal hanger through the punched hole, making the hanger available for the floor below.

With this type of hanger, the layout occurs on the floor above. This is best accomplished using some form of GPS laser layout tool, that automates the process based on CAD drawing hanger locations.

Hanger Strap
Galvanized Hanger Strap

 

Concrete Deck Installation Procedure

This upper attachment is inserted through a hole that is punched into a metal deck. The metal deck gets concrete poured onto it creating the solid floor. The metal deck remains in place as part of the floor assembly.

Blue Banger Concrete Insert
Blue Banger Concrete Insert
Concrete Inserts
Concrete Inserts Installation Procedure
Concrete Inserts Installation Procedure
Concrete Inserts Installation Procedure

Formed in-place Concrete Floor Inserts


This hanger upper attachment is hammered into the wood forms before the concrete is poured. After the concrete has hardened and the wood forms are removed leaving the upper attachment embedded within the finished concrete floor. The upper attachment allows a threaded rod (vertical hanger) to be screwed into its exposed female threaded bottom portion.

Powder-Actuated Concrete Shot Pins

When working in existing structures with concrete flooring, you might use a concrete shot pin if approved by the specifications and building ownership. This allows you to use a special powder actuated tool that shoots the upper attachment into the concrete. The tool uses a small shell filled with gun powder to force a pin into the concrete. The power-actuated shot pins should not be used in light weight concrete or concrete that is less than 4” thick.

Powdered Actuated Shots
Powdered Actuated Shots

Using a powder actuated shot that shoots your hanger into the concrete is a fast and convenient way to attach your hanger. Below is the Gripple system that uses a wire instead of a strip of galvanized hanger strap, but the process is exactly the same.

Powder Actuated Shot Fire Solutions

Watch the below video from Hilti a major manufacture of construction tools to see a version of this tool in action.

Hilti Powder-Actuated

Concrete Anchors

When powder actuated shot pins aren’t allowed you could use drilled anchors for concrete attachments. The benefit of power-actuated or drilled concrete anchors is your ability to locate them precisely where you want them from the same floor that the vertical hanger will be attached.

Expansion anchors will require the drilling of a hole for the insertion of the anchor. This process is a little more labor intensive then power-actuated shots and concrete inserts.

Beam Clamps

Just as the word implies this upper attachment clamps onto a structural steel beam for support of the ductwork. There are numerous types of beam clamps based on the type of structural steel support that it will attach to and the differing beam clamp designs by the various manufactures.

Badger Beam Clamps

The Gripple beam clamp shown in the video below is used with their proprietary hanger system that uses wires with a quick fastening, locking mechanism.

Gripple Beam Clamp
Hanger Upper and Lower Attachments
Beam Clamps

Wood Rafters

Various commercial construction projects and lots of residential projects are build out of wood. There are different ways of hanging duct from the wood structure, the easiest being a hanger strap nailed to the wood beam, but if your duct is large or if the local code requires something more stringent, there are other ways of hanging duct. Here is a detail that is at the more expensive end of the methods by which to hang from a wood structure, but it gives you an idea of hanging from wood beams.

Duct Hanger Wood Stud
Duct Hanger Wood Support
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Metal Decking

There are many different manufactures that make hanger components. Here are various upper attachments for metal decks from Badger.

Badger No-Drill Metal Deck Hangers

Wood Form Concrete Deck

This upper attachment is used where wood forms are used for the laying of concrete. The upper attachment is nailed into the wood forms so that its held in place. The concrete deck will be poured encasing the upper attachment, which will be buried in the concrete providing a very rigid support. Then from the floor below the lower attachment will be attached to the embedded upper attachment. Watch this video to see how this is accomplished.

Vertical Support Members

The most common vertical supports are either sheet metal hanger strap (strips of galvanized sheet metal) or threaded rod. Hanger strap can be made in the shop from stocks of flat sheets or it can be purchased from vendors that sell rolls or bundles of pre-cut hanger straps.

The vertical support member is based on construction standards which take into consideration the size and weight of the duct.

All-Thread
Vertical Hanger Supports


Using all-thread is usually accompanied by a piece of angle iron or Unistrut horizontal support member for under rectangular ducts and some form of half or full angle ring for round duct.

1) Single Strap      2) Two Piece with Full Ring     3) Split Ring Hanger

1) Here a galvanized hanger strap is looped around the duct and fasten onto itself. This vertical support and lower attachment are one piece.

2) Here is a galvanized hanger strap attached to a lower attachment that is comprised of a full circumference sheet metal band.

3) This shows you the option of using a hanger strap or rod for the vertical hanger, and a split ring lower attachment support.

You can buy sheet metal hanger strap in rolls and cut them to length or order them from a fabrication shop pre-cut to the desired length.

Lower Attachment

The lower attachment is the portion of the hanger assembly that attaches to the duct. As shown above for round ductwork, this can either be a single strap of sheet metal, a full circumference ring or a split ring.

The lower attachment for rectangular duct can also use a continuous galvanized strap hanger wrapped under the bottom of the duct and fasten with sheet metal screws or rivets to the duct. For larger ductwork a vertical rod or hanger strap would be attached to a piece of angle iron, Unistrut or galvanized angle run horizontally under the duct.

Hanging Ductwork

Hanger Spacing

The distance between hangers is specified in the local code. The often cited reference for hanger requirements is SMACNA HVAC Duct Construction Standards. Hanger spacing in SMACNA is either every 4’, 5’, 8’ or 10’. Its best to use hanger spacing of 8’ or 10’ to maximize the span between hangers and reduce the amount of time for installing hangers.

As ducts get larger or spacing between hangers increases they will require a heavier gauge hanger or a thicker hanger rod.

For example, the following two ducts require very different vertical support materials at a 5 foot spacing interval.

Duct #1 – 60” x 16” requires a 1” wide 20 gauge strap or 3/8” rod

Duct #2 – 18” x 12” requires a 1” wide 22 gauge strap or 12 gauge wire

Things to Remember; The larger the duct or the spacing between hangers, the greater the strength of the hanger supports.

According to SMACNA, hangers are required on horizontal ducts within two 2 feet (0.61 m) of each elbow and within four 4 feet (1.2 m) of each intersection. Hangers are normally manufactured using galvanized steel strips or threaded steel rod, but in areas where there is a corrosive environment the use of electro-galvanized hangers provides additional protection.

The type and strength of the hanger is based on two important aspects;

  • The spacing between hangers
  • The size of the duct

The chart below shows the required vertical hanger support thickness required based on the half perimeter of the rectangular duct for the 8’ and 10’ spacing. See SMACNA table 4-1 for the 4’ and 5’ spacing requirements.

Rectangular Duct Hanger Chart-MEP
Rectangular Duct Hangers

Minimum Size Hanger Example;

Duct Size = 48” x 12”. Perimeter equals P= 48 + 12 + 48 + 12 = 120”

P/2 = 120”/2 = 60”

The chart below is for round duct minimum hanger sizes.

Round Hanger Chart
Round Hanger Chart

Hanger Length

If you’re using a software program for estimating than there will be a default length set in the database, usually 3 to 5 feet in length.

Hanger Capacity

The table below shows the maximum load (Lbs) a single vertical support member can support. (data from SMACNA Table 4-1) Starting from the bottom left we have a 1” x 22 gauge hanger strap that has a maximum load of 260 pounds, while all the way to the right of the chart we have a 1-1/2” 16 gauge hanger strap that has a maximum load of 1,100 pounds.

Single Hanger Maximum Load
Single Hanger Maximum Load

Trapeze Hangers for Multiple Ducts

The use of trapeze hangers will require a load calculation to determine what materials will be required to support the proposed load on the hanger and its upper and lower attachments. Trapeze hangers allow for the support of multiple ducts or a combination of ducts and pipes. A structural engineer maybe required to ensure that the structure can support the weight of the items supported on the trapeze hanger assembly.

Riser Supports

When a duct rises up through multiple floors or up within a shaft it will need to be supported at each floor or every other floor, with a piece of angle or other structural element that is capable of handling the weight of the riser. The riser support can be attached to the duct with sheet metal screws, rivets, bolts or welds. The riser support will often be attached to a concrete floor with anchors, a wood floor with screws, to structural steel with welds or bolts, or embedded into the concrete floor.

Make sure to include riser supports every floor or other floor, with at least a riser support every 12 to 24 feet based on the size and weight of the duct. Angles or channel can be attached to the side of the riser duct as shown in the images below.

Riser Supports
Duct Riser Supports

 

Riser Supports
Riser Supports

Wall Mounted Duct Supports

Duct running up an exterior wall will need some form of support to hold it in place. Depending on the height of the attachment some form of mechanical lift maybe required to allow a worker to safely make this connection to the structure.

Ductwork Riser Wall Supports
Ductwork Riser Wall Supports

Roof Duct Supports

Ducts are commonly run on roofs because of the lack of attic space or for convenience and ease of installation.

All roof ducts will need some form of support and attachment to the structure if required, unless Dura-blok’s or similar supports are allowed. Look at page 4 of this PDF for Dura-Blok supports that are commonly specified.

Roof Duct Support
Roof Duct Supports

Stack Supports

Exhaust stacks or boiler flues that extend above the roof could require some form of support. Here guy wire (aircraft cable) is used to keep the exhaust stack steady in windy weather.

Stack Supports
Stack Supports

Duct Stack Support – Guy Wires

Hangers in Commercial Construction

In new commercial construction projects the type of hanger you use will be based on the materials and methods of construction within the parameters of the local code. Are the floors poured concrete on metal Decking, if so you will have a system similar to the one shown in this video.

Construction Decking

The type of hanger you use will depend on the materials that you need your hangers to be supported from. Is the deck or floor above constructed of concrete, metal wood or some combination of materials?

Seismic Restraints

For those living in areas prone to earthquakes, equipment, pipes and ductwork might require seismic restraints. Based on the code in your area and the seismic zone that the property is located in; there are various seismic restraint requirements. Standard hangers are inadequate for seismic restraint, and will require additional reinforcement and support.

Duct Seismic
Duct Seismic

Duct Seismic

Hanging Duct
Hanging Duct using a Scissor Lift

Technology for Installing Hangers

On large commercial projects it’s possible that the contractor is using the latest in technology to install hangers. Trimble is a company that has specialized in GPS systems and has created the “Robotic Total Station” for laying out hangers in less time than traditional methods.

By importing the BIM (3-D Model) drawings into the Trimble program the system will identify the exact location on the jobsite for each hanger using laser technology. Watch the video below to see this technology in action.

The use of building information modeling (BIM) and its ability to export data for use in jobsite technology tools has increased field productivity rates far above existing manual methods. At the jobsite the instrument is positioned so it can be aligned with two building site control points. This allows the instrument to precisely locate the hanger location points using distances and angles matching the design drawings.

If installing hangers without the latest technology, you would need to breakout the old tape measure and find a control point to start from and measure to each hanger location. A control point is a reference point that allows you to start from a reliable spot in the building to measure from for accuracy. This would require that you have a set of printed detailed drawings that indicate the location of each hanger and its distance from some reference point in the building.

After you locate the location for each hanger, you’ll need to install the Upper Attachment, then the lower attachment hanger assembly. If you are using concrete inserts on metal decking, then you would mark the location on the deck using the laser guided tool for precise placement of the hanger. At that point you would punch a hole in the deck for the upper attachment using a special tool.

There are various upper and lower attachments based on the type of construction (new construction or retrofit), structural element to hang from and the code required hanger materials and methods. It’s important to know what your ductwork will be supported from, such as the concrete deck above, structural beams, wood rafters or joists.

Various Manufactures Hanger Systems

Below are some additional methods of hanging ductwork that are proprietary to the specific manufactures, such as Gripple and Ductmate.

Gripple “Fast Trak”

https://youtu.be/x_PBQDknKPU
Gripple

Ductmate Clutcher

Below is Ductmates® proprietary “Clutcher” hanger system. The Clutcher hanger system meets all of SMACNA’s upper and lower attachment requirements if installed per the manufactures installation guidelines.

The Clutcher by Ductmate


Gripple’s Duct Trapeze

Hanging Fiberglass Duct

Less commonly used is fiberglass ductwork. Here is a hanger system by Gripple that makes the hanging of fiberglass duct much easier than traditional methods. In this video the Gripple system competes side by side with the traditional method.

Gripple

Flexible Duct Hangers

Flexible ducts require a shorter distance between hangers because flexible duct lacks the rigidity to avoid excessive sagging. Flexible duct requires a maximum distance of 5 feet between hangers, but check your local code for more stringent requirements. According to SMACNA the maximum sag is 1/2” for every foot between hangers, this means that a 5 foot maximum span between hangers would allow a maximum sag of 2-1/2 inches.

Flexible Duct Hangers
Flexible Duct Hangers

Summary

The following is what we have learned in this lesson;

  1. Duct is hung with up to three separate hanger pieces; the Upper & Lower Attachments, and the vertical member.
  2. The upper attachment used depends on the structural component that it will be attached to.
  3. Powered actuated tools should not be used on concrete decks less than 4” thick or made of light-weight concrete.
  4. The choice of hangers is driven by the local codes and specified by the engineer
  5. Hangers are influenced by the size and weight of the duct.
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