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The Importance of a Good Ductwork Takeoff

A ductwork takeoff is a critical part of any HVAC estimating project. A great piece of software can streamline this process and save a contractor a fortune in time, labor and material costs.

When pressurized air comes upon a restrictive duct fitting like a take-off, it loses its turbulent flow. Spaced properly according to the Two-Foot Rule, these take-offs will relieve unnecessary pressure on the duct system.

Duct & Fittings

The ductwork in your building keeps air moving efficiently and prevents buildup of dust and mold. But that’s not the only thing it does – it’s also an essential part of the ventilation system to keep your building a comfortable temperature and safe to inhabit. To make this possible it’s important that you use quality duct, pipe, connectors and fittings to keep your building comfortable.

Typical sheet metal ducts are made of galvanized mild steel. The thickness can vary depending on the needs of the project. This can include acoustic or fire rated needs. The most common size is rectangular although round duct can also be used and is cheaper than the equivalent rectangular duct.

Clamp-together ducts reduce the amount of installation time by using rolled edges that snap together with a quick-clamp. They are popular for industrial applications such as wood processing, food and grain handling and dust collection. They are available in a wide variety of materials and thicknesses to accommodate the different ventilation processes required.

Starting fittings connect to the end of a piece of duct to create an entry point or exit for the conditioned air. They can be square or rectangular and can be beaded if the duct will have an insulation covering or acoustical liner. They can also be welded or solvent weld. Welded fittings are typically welded with Pittsburgh seams and will require additional field labor to install.

The corners and joints of a duct must be sealed to ensure the conditioned air does not leak. This can be done with a variety of sealers including duct tape but if you want long term durability and performance we recommend using an aluminum tape that is designed for this type of project.

If your ductwork will be transporting conditioned air then it’s usually required by code to have it wrapped or lined. This is to avoid energy loss and reduce noise transmission. It’s also often required for acoustical reasons as it absorbs or eliminates the vibration of the conditioned air.

90 degree turns in forced air ducting are to be avoided whenever possible. If the design does call for them, the impact to the ductwork can be reduced by adding turning vanes. These can be fabricated at any sheet metal shop and will help to improve air flow and fluid dynamics in the ductwork.

The Takeoff

When a ductwork system fails to deliver the airflow it’s designed for it often can be traced back to a bad takeoff. Ducts that have sharp turns, impede airflow, or are too long can all cause issues such as drafts, hot or cold spots, high energy costs and even shortening the lifespan of your HVAC equipment. Fortunately, a little planning can help you avoid these problems and ensure your building gets the airflow it needs to function properly.

A duct takeoff is the main point where branch ducts connect to it from various locations in your facility. These branch ducts transfer conditioned air from the main trunk line to specific rooms or areas of the building. It’s important to keep in mind that each duct run will be impacted by turbulence and pressure differences which will make it difficult for the branch ducts to receive adequate airflow. There are a variety of different takeoff styles and shapes used in ductwork that can impact this flow.

For example, a rectangular duct takeoff has a flat facing on one end to mount to the trunk duct and a tapered opening on the other. This shape captures more volume on a smaller space than a round duct tap which can result in greater airflow downstream. Using the right type of takeoff for each duct in your system can dramatically improve its performance and overall efficiency.

Other styles of duct takeoffs include the shoetap style (which is flat on both sides), and a rectangular hex tap with a rounded base. The latter is referred to as a High Efficiency duct takeoff (HETO). The HETO design provides a wider base where it taps into the flat trunk line and narrows down to the pipe or flex duct size it will be connecting to. This is a more expensive option but can be worth it in a commercial job.

When working on a construction drawing it can be helpful to deselect any layers you’re not using for the sheet metal takeoff. This can save you a lot of time as well as improve your accuracy. When using a specialized estimating software tool that offers this feature you can also customize the appearance of your drawings, highlight different parts in various colors and even pause the takeoff to review your work.

The Fittings

The main ductwork that delivers the air for HVAC systems has two pressures; supply air and return air. Keeping these separate is crucial for good air delivery. The pressures are rated or classified by their velocity and static (static pressure). Each time the air encounters a change in direction through a transition, square-throated elbow, or takeoff, it loses some of its turbulent flow. It takes about 18 to 24 inches of distance from that point for the air to re-pressurize and resume its turbulent flow. This loss of pressure is called friction loss.

To avoid friction loss, all of your ducts should have smooth surfaces and should be properly sized to the size of their openings. To do this, you need a proper estimating and detailing process that includes the use of accurate drawings with elevation views.

The drawing details will help you determine the best type of duct to use for each run. For example, for long stretches of straight runs, a round duct will provide the most cost effective solution. But for shorter stubs, or directional changes in the ductwork, you may need a directional tee or extractor fitting.

Directional tees and extractors have some sort of metal scoop or turning vanes that protrude into the airstream and grab air from the duct. These are a good choice when it comes to changing the direction of a duct because they don’t have that airflow penalty that a traditional takeoff with a tap-in side collar has.

But be careful if you’re using these in a long row because they have the same effect on the airflow performance of branch ducts downstream. Think of it like blocking off one lane of traffic. It can take awhile for vehicles to get around the lane restriction and start moving again.

Another option is to use a rectangular takeoff with an opening that’s 25-30 percent larger than the duct it feeds. This is a very cost effective and airflow efficient alternative to the directional tees and extractors. US Duct offers a wide variety of duct branch connectors to meet your needs. Whether you’re adapting from a duct board, to a duct with a round opening or to an existing duct that has metric openings, we can supply what you need.

The Estimate

A ductwork takeoff involves more than simply seeing where the ducts can fit. The physics of air flow and the specifications of heating and cooling units must be matched to ensure the equipment is placed in locations that will optimize its performance. Poorly matched equipment and ductwork can result in several costly problems, from shortened lifespans to unbalanced air pressure that exposes building occupants to high levels of dust, pollutants and odors.

Using the best ductwork design will minimize these issues and maximize the efficiency of the system. One important consideration is minimizing turbulence, which occurs when the duct changes direction. The simplest way to reduce turbulence is by installing a turning vane, which will smooth out the change in direction. Another method is to choose rectangular ductwork, which allows for greater air volume downstream.

Another issue is how the ducts are connected. Ideally, ducts should be joined by a sheet metal fitting. This type of connection is most efficient because it provides a tight seal and minimizes the amount of air that escapes between the joints. This type of joint is also easier to install, which can save time and labor costs.

It’s also important to note how far the duct rises on an elevation view of the project. This information can help you estimate the amount of insulation required to cover the ducts. Additionally, if you are estimating for a system with an air conditioner or exhaust fan above, the engineer will often show a stub of ductwork that must be installed to connect to the duct.

Lastly, you’ll want to pay attention to the arrows on the drawing. Ducts that are pointing up (UP) will increase their bottom elevation, while those pointing down (DN) will decrease their bottom elevation. This can have a significant impact on the amount of insulation you’ll need for the ductwork, as well as the overall thickness of the duct insulation.

Using an automated online estimating software tool will streamline the ductwork takeoff process and provide accurate measurements that can be exported directly to your spreadsheet for material pricing and calculation of labor rates. Ensure you’re using the most up-to-date software by choosing a solution that offers SMACNA specifications and customizable specification templates. In addition, the right estimating software will enable you to import all quantities automatically and instantly, saving you hours of work on manual calculations.