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The Two-Foot Rule

The Two-Foot Rule

When pressurized air comes upon a takeoff for a branch run it loses its turbulent flow. This can cause the duct to overwork.

The solution is to use a larger fitting on the takeoff, such as an HETO (High-Efficiency Takeoff). These are more expensive but worth it for the airflow they provide.

Round Takeoffs

Duct takeoffs play a major role in ensuring the airflow in a duct system is properly balanced and distributed to all rooms in a home or commercial building. The wrong type of takeoff can reduce the overall effectiveness of the ductwork, leading to higher energy costs and/or a shortened lifespan for an HVAC unit.

The main purpose of a duct takeoff is to transfer air from the trunk line into the new branch run that will service an individual room or area. A takeoff that is too small or positioned in the wrong location can negatively affect the airflow in the resulting branch duct, so it’s important to choose the right fit for the job and install it correctly.

A round duct takeoff is the most common type of fitting for a duct branch. This style of takeoff is inexpensive, easy to install and works well on most jobs. The most basic round duct takeoff has a c-collar (or side collar) that taps into the flat duct surface. This type of takeoff captures the full amount of air moving quickly thru the trunk lines upstream of it and disperses it evenly in the direction the duct will flow.

There is a more efficient version of the round takeoff that’s becoming more popular on engineer drawings called an HETO or High Efficiency Takeoff. This type of takeoff has a wider base than a traditional shoetap HETO and allows the installer to direct the airflow into the branch duct. It’s also priced in the more expensive range but offers a better airflow performance than a standard round takeoff.

Directional duct takeoffs may look good on paper, but they can cause more problems than they solve in the field. When two directional takeoffs are installed too close together the air pressure in each one drops dramatically, making it difficult for them to tap off of the trunk line at the same rate.

A duct takeoff that is too large can result in poor distribution of air throughout the duct system. This can cause uneven temperatures, dust and other debris in the room, as well as excessive noise from the airflow. The most effective duct takeoffs are those that match the diameter of the branch duct they’re connecting to. If the duct takeoff is too large it will require a larger-than-normal branch duct that will add cost and weight to the overall ductwork. In most cases, a properly sized takeoff can be tapped into a circular or rectangular duct with an adhesive foam gasket that creates a tight seal and allows the duct to operate at its optimal performance. A specialized duct tape can be used on more complex projects.

Conical Takeoffs

Takeoffs that transfer air from the main trunk line to a branch run should be as efficient as possible. This will allow the installer to deliver a room’s conditioned air as quickly and consistently as possible. The most common takeoffs are round, but there is a significant airflow penalty associated with these types of fittings. There are other takeoffs that improve the airflow performance, but these are often more expensive and can cause a disruption in the airflow pattern downstream of the fitting.

Directional takeoffs, which have some form of metal scoop or extractor, grab the air from the duct and direct it into the branch duct. While these do improve the airflow at the first branched-off duct they are connected to, they can cause problems in downstream branch ducts that can be compared to traffic jams when one lane of a highway is blocked off. These types of directional takeoffs can reduce the flow from a correctly-sized duct by 25-30 percent.

The best commercial takeoffs are rectangular-to-round (often called HETOs) that offer easy air entry via an oversized duct opening. These are available in a wide range of sizes and many include a preinstalled balancing damper for additional control. I see more of these being specified on engineer drawings than the standard saddle tap style that is usually used in residential jobs.