Comparing Good and Poor Duct Designs

Below, we show you two different drawings of two different duct designs that are intended to carry airflow into the house. Both designs are intended to be used in the same house to help you understand and recognize the differences between an energy efficient duct design and poor duct design.


There is a proper and appropriate way to use the design shown above such as in certain situations where space is limited. However, the improper way is often employed and abused by contractors wanting to cut costs of time, labor and materials. Here, we are explaining how this design is used improperly:

Instead of a trunk, a square box called a plenum is used. The plenum is attached to the air handler and from that, all the supply ducts are attached and strung at different lengths and directions to distant parts of the house which makes the duct work look like octopus tentacles. The problem with this design is that the shortest runs get the most air — therefore, the rooms farthest from the air handler are starved. Also, the supply ducts are competing for air flow where they are attached at the plenum — the ducts in the direct air flow stream get the most flow.

We typically find the return air ducts (not show in the picture) to be undersized in these types of duct systems because the layout does not allow for flexibility — the contractor basically has to use whatever space for the return duct has been created rather than what is actually required. We also find a lot of undersized or oversized units associated with this duct design — the diameter of the supply ducts is random and does not necessarily match the CFMs the unit is designed to release.


In the example above, you can clearly see that each supply duct is spaced nearly equal distance from the main source of air supply known as the supply trunk. This ensures even distribution of heating and cooling — meaning that each room in the house will be at approximately the same temperature. The return air supply (not shown in the picture) is equally important and should be properly sized — if enough air is not being sucked into the unit, it will create a situation that causes the compressor to work harder and can lead to ice building up on the coil during hot summers (which means no cooling). When the return air supply is undersized, what you will feel coming out of the supply vents is a weak stream of air, and it will take longer to bring the house to the proper temperature. Contractors have devices which they are able to use to detect the proper air flow and should be able to measure and report this to you after the work is completed.

Each supply duct carries a determined amount of cubic feet per minute (CFMs) of air flow, and the size of the unit determines the amount of air flow that will be conducted through the entire duct system. Therefore, the diameter of the supply ducts and number of supply ducts should equal the amount of CFMs the system is designed to push. Too many supply ducts of improper size, and there is weak air flow. Too few, and the supply ducts will blow like a hurricane wind. Where the supply ducts are placed on the supply trunk is also important so that one duct does not deplete air flow from another. That will result, for instance, in a bathroom having a strong air flow, and a bedroom barely getting any air flow, depending in which room the ducts are attached.

In the picture, an example of a transition is indicated. The purpose of a transition is to reduce the size of the trunk to keep consistent air pressure the entire length of the trunk so that farther rooms have the same air flow as those nearer to the air handler. A properly designed duct system will evenly distribute air throughout all parts of the house.

UPDATE 2018: Many counties in the state of Georgia now require HVAC contractors to submit computerized heat load calculations which will accurately determine the size unit and ducting you will need to properly heat and cool your house.


The photo gallery below shows some of the poorly designed plenums and duct systems that we have removed from houses over the past 21 years. Caution: These are examples of what NOT to do!

The rough sketches shown above are intended to be examples for instructional purposes and do not represent the actual duct system design proposed for use in your house.

Copyright 2018. Williams Heat, Air & Refrigeration, Inc.