Effects of Duct Leakage on Indoor Air Quality

Timothy M. Himes


Effects of Duct Leakage on Indoor Air Quality

This past summer, my family noticed an unusual odor in our house. At first, my wife thought that we may be dealing with a dead rodent. The odor, and the search for the culprit, intensified over the course of the next few days. The strongest odor concentration was found in the garage, located in the basement. However, the odor was also noticed inside the house on the first floor. How was the odor traveling upstairs, through a locked door?

The house is equipped with a split HVAC system, comprised of an air handler located inside, and an air conditioning condenser located outside. The air handler has a gas-fired burner for heating, an evaporator coil for cooling, and a fan for moving air. A return duct, as its name implies, returns air from the house. The supply duct, likewise, supplies conditioned air to the house. The fan pulls air from the house, like a vacuum cleaner, and pushes conditioned air back through the supply vents. The ducts are intended to be sealed to avoid leakage.

The source of the odor was determined to be decomposing grass that had accumulated under a push mower, which happened to be stored next to the air handler. Using a smoke pencil, I located a leak in the joint where the return duct connects to the air handler. The smoke was drawn into the leak, pinpointing the defect location. The odor dissipated in the house a short time after the leak was sealed. 

Example of Smoke Pencil
Example of Smoke Pencil

This simple situation reminded me of the importance of minimizing leaks in ductwork. First, duct leakage is one of the leading sources of energy waste. My HVAC system was not only ingesting garage odors and ruining the indoor air quality, it was also pulling unconditioned air into the system. This extra unconditioned air created an additional cooling load, an unwanted burden, especially where I live in the Birmingham, Alabama area. Second, had the leak been more significant, the ingested air could have increased the pressure inside the house. Much like a balloon with a leak, additional air pressure causes increased leakage of conditioned air out through doors and windows. Further, increased indoor air pressure suppresses airflow from the supply vents throughout the house.

Unlike residential construction, commercial and institutional building HVAC systems are designed to bring in a defined amount of fresh air. In order to accommodate the additional fresh air, the HVAC systems are equipped with a means to exhaust the same amount of air to maintain a reasonable pressure balance in the building.

However, most residential HVAC systems are not engineered to introduce fresh air. Rather, fresh air is brought in when doors are opened and closed. If you notice unusual odors in your house, like I did from the garage, or perhaps from a stuffy attic, recall this lesson in duct leakage. Time spent locating a duct leak can preserve indoor air quality and reduce the expenses associated with heating and cooling your home.