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How Your Duct System Affects Indoor Air Quality

Indoor Air Quality
Reading Time: 3 minutes

Indoor Air QualityArt Plumbing and Air Conditioning’s House Whisperer points out an extreme example of duct deterioration while one of our installation technicians, Tony, looks on.

In my last post on the Art Plumbing and Air Conditioning Blog, we started talking about the very serious problem of indoor air pollution. We addressed the first component in achieving the best air quality possible by keeping the coil of your air handler clean using UV lights. Again as a review, the duct system, the attic, the air pressure in your home, the quality of your windows and the coil inside your air handler are just some of the things that impact the air quality of your home. Today I want to focus on the duct system and how it can impact your indoor air quality.

First off, when was the last time you thought about your ductwork?  I swear I just heard a collective scream of “never”! Here, once again proving the “out of sight – out of mind” theory. The ductwork has five basic parts:

  • supply & return plenums
  • main trunk line
  • branch lines
  • cans
  • supply & return grills or registers

The supply and return plenums are what are actually connected to the unit itself in the typical split system installation. Often times these are made with duct board or fiberboard, which is 1.5“ thick and sealed to prevent air loss and mold growth. Starting at the supply plenum, we then tap in a main trunk line which allows static pressure to build up for proper airflow. This is not the case in every home as duct systems can be designed in many different ways, what I am describing here is a typical application. We will get into the fun world of static pressure down the road.

The branch lines either come off a mixing box or the trunk line itself and these are connected to the cans in the wall or ceiling. The most important part of the duct system are the grills or registers, as they determine what direction the air is going to be pushed in and at what volume. If you count the supplies grills in the home, and then multiply by two or three, that is the number of connections that you will have in the attic to get the air to each of the rooms.

Fun fact for the day:  air travels through a duct system at anywhere from 350-450 feet per minute. Why is this important you ask? The simple answer lies in the aforementioned duct connections.  Most duct systems have 25-35 connections in them and if I go further right than Rush Limbaugh, and say that in a ten year old duct system each connection is only leaking 1%, you are losing 25-35% of the conditioned air to your attic.

Wait! Weren’t we talking about indoor air pollution? Yes, yes we were.  Back to those leaking duct connections, and a phenomenon called the Venturi effect (Bernoulli’s Principle). The fast moving air through the duct system creates a little miniature tornado at each of those leaking connections in the duct system pulling dust, mold, pollen and other allergens into the home. Take a look at the grills in your home, if you see streaking along the ceiling or dust on the fins themselves, you likely need to have you duct system cleaned and sealed.  A well-sealed duct system is another step on the path to achieving the ultimate in indoor air quality.  As always, you can schedule a complimentary duct evaluation with one of our Comfort Consultants by calling 1-888-TELL-ART or visit to make your appointment today.

P.S. Cleaning your ducts without sealing them is similar to having your truck completely detailed, and then going four-wheeling in the Everglades… the dirt’s gonna come back! House Whisperer out!

Gregory Frazier, The House Whisperer, Art Plumbing & Air Conditioning


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