A few months ago, I wrote a Duct Dynasty article on the ideal duct system. The most frequent comments and questions I received about this article were regarding branch takeoffs. So, I thought it would be fun to take a deeper look at branch duct takeoffs and how to overcome some of the most common problems associated with them.
AN AIRFLOW PRINCIPLE TO REMEMBER
As air moves from a supply plenum or main duct into a takeoff it changes direction. Airflow has a hard time making turns at high rates of speed, just as we would driving down the interstate. When we leave the interstate, the exit ramp is hopefully a gradual and smooth transition, since we’re moving really fast.
With this in mind, let’s think of the supply plenum or main trunk of a duct system as a fast-moving interstate with branch duct takeoffs serving as off-ramps. Do your duct system installations resemble interstates with exit ramps or windy country roads with small driveways?
Today, many duct systems would cause major traffic accidents or missed turns if they were roads. Just as we can’t make 90-degree turns in our vehicles going 70 mph, we can’t expect air to make 90-degree turns in similar conditions. As you choose branch takeoffs, remember this concept.
DIFFERENT TYPES OF TAKEOFFS
Branch takeoffs should transfer air from the main trunk of a duct system to individual branch runs as efficiently as possible. The easier it is for them to accomplish this, the greater your chances for delivering the proper amount of airflow to a room.
There are various styles and shapes of takeoffs commonly used in duct system installations. As we preview each type of takeoff, consider the pros and cons of each with regard to their impact on airflow.
This is the most common branch takeoff many of us encounter. Round takeoffs range from those using scrap sheet metal pipe with tabs cut on one end to those using a traditional tap-in side collar or sheet metal elbow. The one thing they all have in common is the opening mounted to the duct is also the same size as the branch duct it feeds.
Round takeoffs are the most cost-effective of all branch takeoffs. While they do work, there may be an airflow penalty to deal with when using them. In some scenarios, these fittings can cause a properly sized branch duct to function like one much smaller in diameter due to the way air enters it. Remember, air doesn’t like to make 90-degree turns.
The next progressions from round takeoffs are conical and square-to-round takeoffs. These provide a mix of both cost-effectiveness and increased airflow performance.
The airflow penalty with these fittings is greatly reduced since the duct side opening is often 25-30 percent larger than the branch duct it is feeding. It’s easier for air to change direction and enter this style of takeoff. If you prefer round, these are a good choice to consider.
Directional takeoffs have some form of a metal scoop or extractor fitting (turning vanes that protrude into the airstream). The idea is to grab air from the duct and direct it into the branch fitting.
While the concept looks great on paper, these fittings can cause more problems than they solve due to their field application. When placed into the airstream, directional takeoffs work great for the first branch duct they are connected to. However, branch ducts downstream suffer.
Think back to the traffic example, if you suddenly block off one lane of traffic all at once, you’ll have a backup. Airflow responds the same way inside a duct system when scoops or extractor fittings are used. It takes some time for traffic to get going again once vehicles move around the lane restriction.
When these takeoffs are staggered in a row, there is no chance for air to straighten back out after the fitting. This results in reduced airflow to the next branch duct in line. These takeoffs do have their place, but you need to understand their limitations.
Daniel Vaughan, an HVAC technician, told me he solved an airflow issue using a scoop takeoff. The original takeoff was a straight collar about 2.5 feet downstream from a ductboard, three-piece, 90-degree elbow with no airflow going into the flexible branch duct.
When he took the flexible duct off the collar, he felt air shooting past the duct opening, not coming through it. With limited space and the right conditions, a scoop takeoff solved the airflow complaint. This small correction gained a customer for life.
Whenever possible, the rectangular takeoff is my choice for airflow performance. For round branch ducts, these rectangular-to-round fittings allowed easy air entry due to a substantially oversized duct opening.
When working with rectangular branch ducts, such as those found in commercial applications, a 45-degree rectangular transition fitting that has a 30-40 percent larger opening creates a smooth pathway for air to move into the fitting.
HIGH-EFFICIENCY TAKEOFFS (HETO)
The HETO, or High-efficiency Takeoff, is one I hear wonderful reports on. These takeoffs feature a rectangular-to-round design that allows for easy airflow entry into the fitting with an oversized duct opening.
Additional HETO options include a gasketed flange for duct mounting with a sealed connection, which often contains a preinstalled balancing damper. HETOs are priced on the high-end range for takeoffs but are well worth it for the airflow performance they provide. You get what you pay for, and these fittings are perfect examples of that.
TAKEOFF PLACEMENT MATTERS
You can go through the work of getting the best takeoffs our industry has to offer and still have issues if you don’t plan their placement on the duct system. Poor placement is one of the top reasons some registers get very little airflow, even though the duct may be sized and installed correctly.
My colleague John Puryear teaches the simple “2 Foot Rule” for takeoff placement in his classes. When there is a change in air direction in a duct, it takes roughly 24 inches for airflow to restore its pattern. Because of this, John recommends keeping takeoffs 24 inches away from any turns, transitions, or the end cap.
MORE THAN DUCT FITTINGS
I hope a look at this important duct accessory changes your point of view regarding them. One of the motivating factors among many professionals when selecting takeoffs is cost.
Instead of selling sheet metal duct fittings to your customers, start selling what they achieve. You’ll be surprised how many customers upgrade in the name of comfort.
Publication date: 4/24/2017