Do you know that ear-popping feeling of flying on an airplane? The cabin’s air pressure is reduced as the plane climbs – on purpose. Now, imagine you are in your home, and your ears start to pop. Wait a second…
Maybe (almost positively) your home’s air pressure will never change as dramatically as an airplane soaring 10,000 feet above sea level. However, the results of changing pressure caused by leaking air ducts can be devastating to energy bills and cause discomfort for occupants. In fact, duct leakage is one of the top three energy suckers in most homes, sharing the unfortunate spotlight with only cable TV set-top boxes and air leakage.
There are a few ways to test if a home has unbalanced pressure: building scientist Joe Lstiburek’s “look, lick, and squirt test” or with a manometer, a pressure measuring device. Manometers measure the indoor pressure as compared with that of the outdoors. The “look, lick, and squirt test” involves just that: looking if things like spider webs are moving, licking a finger to feel air movement near a cracked door, and squirting smoke near a cracked door to determine if it is pulled through to the other side.
If you found that your home is leaking, your next question is most likely from where? Duct leakage can appear in two forms: supply duct leakage, resulting in negative pressure, and return duct leakage, resulting in positive pressure.
Supply duct leakage
The air handling unit (AHU) is running at 900 cubic feet per minute (cfm). Without a leak on the return side, the system successfully intakes all 900 cfm from the interior of the home. However, with a leak on the supply side, 100 cfm are lost into the attic, leaving only 800 cfm to be pushed back into the conditioned space. This means a loss of 100 cfm of air in the conditioned space of the house, or a negative pressure inside.
Return duct leakage
The system is again running at 900 cfm. A properly functioning AHU will pull 900 cfm from inside the conditioned space through the AHU and back to the conditioned space through the supply duct. However, this system has a leak on the return side, and the duct is pulling in 50 cfm from the attic and is therefore only taking 850 cfm from the conditioned space. With only 850 cfm leaving the space, the AHU will still push 900 cfm back into the home. The extra 50 cfm now inside the conditioned space create an inbalance, specifically a positive interior pressure.
