How much does duct leakage matter?

How much does duct leakage matter? Short answer: Perhaps more than anything else in energy efficiency.

According to the Department of Energy, the typical U.S. home has 20 to 30 percent duct leakage. On a 4-ton system in the Arizona desert, that’s about one ton of air *per minute* of air that is lost to the attic and/or pulled from the attic, unfiltered.

Here’s a quick primer on duct leakage in a typical Phoenix-area home with ducts running through a vented attic during the summer:

Supply duct leakage means that cold, conditioned air is blowing into the attic rather than being delivered back into the home. So, the homeowner just paid for the air to be cooled, only to be lost to the attic.

It gets worse. Any air that is removed from the house and blown into the attic has to be replaced -- nature abhors a vacuum. With a supply duct leak, the house becomes a partial vacuum. For the house to reach equilibrium, hot, dirty and sometimes contaminated attic air replaces the filtered air being removed from the house.

The end result? You’re heating your house while trying to cool it – two steps forward, one step back. It’s not just the air that gets hot; it’s the home’s interior and exterior walls; the attic air heats the inside of the walls as it makes its way in through your light switches and outlets. The minute your A/C shuts off, the walls radiate that heat into the home. The A/C, then, cycles more than it needs to.

Sometimes, the supply leakage is so bad that it pulls volatile chemicals and sewer gases into the home and can make combustion appliances like gas water heaters and clothes dryers backdraft, resulting in carbon monoxide infiltration.

Whew. That's a lot to consider. But what about return leaks?

Return leaks create similar problems, but from a different angle. With return leaks, hot, dusty air is pulled into the A/C system and sticks to the coil, blower and other components. What doesn't stick is blown unfiltered into the home, resulting in dust and poor A/C temperature splits. The house pressurizes and blows conditioned air to the outside & attic.

Determining how much your ducts leak is a vital first step to making your home healthier, safer and more efficient. This is done with an independent energy audit.

Of course, I am an energy auditor, and I'd like for you to contact me to have an audit performed on your house.