Duct takeoffs have a big job to do, they transfer air from the main trunk line to new branch duct runs. The better they do this, the more efficiently your duct system performs.
Often, a poorly designed or installed duct takeoff will lead to poor air flow and performance. The causes are numerous:
High-efficiency Takeoff (HETO)
This type of takeoff is often found at the point where ductwork changes direction in a building. They are designed to capture air and direct it into the branch duct. However, they can cause a significant loss of airflow to the branch duct due to turbulence and pressure differences. There are several different types of takeoffs and dampers that can be used to prevent this problem.
The best choice for this type of connection is an HETO (High-efficiency Takeoff). An HETO has a rectangular inlet opening and a flange assembly extending about that opening. The body of the HETO then takes on a cylindrical shape downstream of the rectangular inlet, defining a circular outlet.
These HETOs have a much lower airflow penalty than other duct taps because they do not restrict the flow of air on the upstream side of the flange. They also have a large volume damper that can be easily adjusted by the installer. These are available for both round and rectangular duct work. There has been a recent increase in the use of HETOs on engineer drawings that previously would have been specified with a standard saddle tap.
Regardless of the type of takeoff you choose, it is important to have accurate measurements for all your ductwork. An online estimating software tool can automate the process, streamlining material and labor pricing, and saving you hours of time. Choose a solution that offers SMACNA specifications and customizable specification templates to get the most out of your software investment.
AAB
When it comes to duct takeoffs, there are many styles and types available. Some are more efficient than others. In order to get the best results, you should know how each type works.
For example, a HVAC technician recently told me that his crew used a scoop takeoff to solve an airflow problem. The original takeoff was a straight collar on the end of a 3-piece, 90-degree elbow. When the flexible duct was connected to this collar, the air was shooting past the takeoff opening, not entering it. After installing a scoop takeoff, the technician found that the airflow improved significantly.
Another duct transition that is often employed in commercial applications is the rectangular-to-round takeoff. This fitting has a substantially oversized duct opening that allows for easy entry of air into the duct. These fittings provide a good balance between cost effectiveness and airflow performance. They are also ideal for splicing in branch ductwork lines with a 45-degree angle. They are also known as reducing laterals or wye joints. These ductwork fittings find application in supply air and return air duct systems.
Round
When an engineer’s duct drawing doesn’t have enough room on one page they’ll often show a match line across the page. This is a convenient place to stop your takeoff and begin the next page.
This style of round directional takeoff, also known as a scoop, is designed to grab air from the main duct and direct it into a branch run without needing to reduce the size of the duct itself. This can be a very cost-effective approach, but can result in an airflow penalty at the branch fitting. The metal scoop, or extractor, of this type of directional takeoff will protrude into the air stream and could potentially block or restrict the flow of air in a given direction.
Rectangular-to-round takesoffs are another cost-effective alternative for branch ducts. Typically having a side opening 25-30 percent larger than the duct that’s feeding it, this style of fitting is easier for air to enter and change directions in. They are also less likely to cause an airflow penalty if the duct that’s being fed is properly sized.
A complete HVAC estimating solution for commercial sheet metal contractors is available in our cloud-based application FastDUCT and our HVAC estimating software, FastPIPE. These two products work together to provide a single, integrated software solution for calculating rectangular, round, and oval ductwork, as well as HVAC equipment.
Conical
The duct takeoff is where the ductwork transitions from a main trunk line to a branch. It can be a tricky point in the system due to the fact that if the takeoff is not configured correctly, airflow could suffer.
This can be caused by several things, including poor design or improper fabrication. The best way to avoid this is to check the ductwork drawing and make sure that there are no sheet metal branch connections at this point. If there are, then make sure that the drawings have a section view and an elevation view of the takeoff to see how much the bottom of the duct will rise or fall.
If the takeoff has a slope, then you will need to provide a special type of fitting called a saddle-tap. These fittings can either be used to tap into the side of a round duct or they can be used in place of a full body fitting like a Tee. When tapping into the side of a round duct you will need to cut a hole big enough for the saddle-tap to fit. Then the saddle-tap can be welded to the hole that was cut in the duct.
Another option for a duct takeoff is to use a scoop or extractor fitting. These are a little pricey, but they are a great choice for ducts that will be subjected to vibration or shock. They work well for the first duct they are connected to, but downstream ducts will have reduced airflow. This is similar to a traffic jam when you close off one lane of traffic all at once, it takes time for vehicles to navigate around the restriction.