When it comes to duct take-offs, spacing and compensation are paramount. If take-offs are placed too close together, less than 24 feet in the center, or too close to an air exchange point (elbow or transition), the air won't have enough time to re-pressurize and create the right turbulent flow. This pressurized air then arrives at the takeoff and drains through a branch, resulting in a loss of pressure. To restore the turbulent flow, a minimum of 24 inches or two feet is required.
The Two-Foot RuleMy colleague John Puryear teaches in his classes the simple “two-foot rule” for takeoffs.
When there is a change in the direction of air in a duct, it takes approximately 24 inches for the air flow to restore its pattern. That's why John recommends keeping takeoffs 24 inches away from any curve, transition, or end cap.
Variants and OptionsWe will cover the steps necessary to perform an accurate and complete detachment of the sheet metal. Now that you've learned to recognize round and rectangular ducts, accessories, and specialties, we'll learn some variants or options you might have if the drawings and specifications don't clarify what type of accessory to use. For instance, if you're dealing with a rectangular duct system, you can use a rectangular takeoff with a flat bottom or a round takeoff with a flat bottom. If you're dealing with a round duct system, you can use a round takeoff with a flat bottom or a rectangular takeoff with a flat bottom.
The choice of which type of takeoff to use depends on the type of duct system you're dealing with. In addition to these options, there are also other types of takeoffs that can be used in certain situations. For example, if you're dealing with an irregularly shaped duct system, you can use an adjustable takeoff. This type of takeoff allows you to adjust the size and shape of the takeoff to fit the irregularly shaped duct system.
HVAC ContractorsI bought new construction in Montana, where the HVAC contractor installed four takeoffs within the two-foot final space in one area and three takeoffs within the two-foot final space in another area. The NCI suggests that the installer use the NCI duct design tables, the NCI Guide to Improved Duct Installation Practices, and your company's duct installation procedure to fix the problem.
John Puryear has experience in duct design, sheet metal fabrication, and installation in commercial and residential heating, ventilation, and air conditioning systems. Most HVAC contractors, depending on their geographical location, have a standard type of duct system that they use when installing equipment, usually during the initial phase of new construction. It's important for HVAC contractors to understand spacing and compensation for duct take-offs so they can ensure that their installations are up to code and efficient. By following John Puryear's two-foot rule and understanding all of the available options for take-offs, HVAC contractors can ensure that their installations are safe and effective.