Indoor air quality (IAQ) is a big deal. It doesn't get near as much attention as energy efficiency or green building programs, but it should be one of the highest priorities for anyone who lives in a home. It's actually part of an even bigger deal called indoor environmental quality (IEQ), but I'm just going to focus on IAQ here as I show you four ways your ducts might be hurting your indoor air quality.
1. Leaky ducts pull in bad stuff and distribute it throughout the home
The photo above shows one example of a duct system that pulls in bad stuff and distributes it. I wrote about this case last year when one of the HERS rater companies we work with, E3 Innovate in Nashville, found this disconnected return duct in a nasty, vented crawl space. That's bad enough by itself, but this one also had a dead possum lying close to that bad air vacuum cleaner. Even the best filtration system would be hard pressed to remove the putrefaction from that air.
Sometimes you may think you've got a great filter that's doing its job, but inside the box lies a an IAQ surprise, as shown below. This expensive media air cleaner is supposed to be supported by combs that hold the pleats apart but has suffered from a collapse. The resulting hole at the top serves as an easy bypass for the air, so little was actually getting filtered here.
2. Unbalanced duct leakage increases infiltration
But not all the the bad stuff happens in the return ducts, which are supposed to pull air from the house back to the unit to be conditioned and which run under negative pressure. Duct leakage on the supply side can create IAQ problems as well. Take a look at the diagram below.
This system pulls 10 units of air from the house, sends it back to the HVAC system where it gets heated or cooled, and then puts 10 units back into the supply ducts. In this house, the supply ducts run in an unconditioned crawl space. Of the 10 units air entering the supply ducts, only 7 make it all the way back to the house.