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Multiple Fittings in a Duct System

All forced air duct systems will have a certain amount of ductwork fittings in the system. Each fitting has a purpose, and each fitting carries with it a certain amount of restriction to the system. The Designer, Installer, or Do-It-Yourselfer's goal should be to minimize the system restriction for best airflow throughout the system.

On previous ductwork pages we discussed the single fitting and it's contribution to the system's overall TEL (total equivalent length). Here we will look at a few ductwork multiple fittings trunklines and estimate their effect on the overall performance of a duct system.

The ductwork pictured in the above diagram is quite common. Many homes have obstacles that the duct system must get around. More often than not though, only one side of the duct system needs to be configured as this illustration. The trunkline going the other direction is usually straight with only a reducer somewhere in the trunk.

The square ell example above is a poor approach to the necessary turns of the trunkline. Each square ell adds approximately 80' to the TEL. Also, remember that air will flow best where there is least restriction.

We can accomplish alot with duct sizing, but we can accomplish even more when proper sizing is combined with proper ductwork multiple fittings design, fabrication, and installation.

The above square ells example can be vastly improved with turning vanes installed inside the fittings.

Seeing these three examples of ductwork multiple fittings next to each other, it becomes apparent which trunklines will perform the best.

The second example with the rounded throats is a much improved approach if the double 90* turns are absolutely necessary. This configuration will add approximately 50' to the overall TEL.

If your home configuration will allow for 45* ells, instead of the 90* ells, addition length to the TEL can be as little as 20'.

Other Ductwork Related Pages

Residential Duct Drawings by Geoffrey R. Stoddard, Detroit, Mich.

Typical trunk and Branch Duct Systems

Plenums and Return Air Drops

Rectangular Ells, Tees, and Reducers

More Ductwork Transitions: Side Take-offs and Vertical Ells

Basic How-To Ductwork Installation

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