Central AC System GuideA central air conditioner can be perfectly sized and maintained and still waste a large share of its output in the ductwork. Cooled air that escapes through leaks and poor connections never reaches the rooms it was meant for, so the system runs longer to make up the loss. Sealing ducts is one of the most cost-effective ways to recover that lost capacity.
How much air a duct system loses
Duct leakage is larger than most homeowners expect. ENERGY STAR reports that about 20 to 30 percent of the air moving through a typical duct system is lost due to leaks, holes, and poorly connected ducts.
The Department of Energy describes the same problem, noting that a home can lose about 20 percent of its heated and cooled air through the ductwork. Either way, roughly a fifth to nearly a third of what the air conditioner produces can vanish before it does any good.
Where the leaks hide
The worst leaks are usually out of sight. Ducts running through attics, crawl spaces, and garages leak into unconditioned areas, so the escaped air is a total loss rather than a stray draft in a room.
Joints and connections are the common failure points. Seams between duct sections, connections at registers, and the boots where ducts meet the ceiling all loosen over time, which is why a careful sealing job follows the runs rather than treating one visible gap.
Sealing restores airflow
Leaks and airflow are two sides of the same problem. Air lost to the attic is air the blower cannot deliver to the rooms, and ENERGY STAR reports that airflow problems can reduce a system's efficiency by up to 15 percent.
Sealing recovers that airflow. Once the ducts hold pressure, the same equipment moves more conditioned air to the living space, so rooms cool faster and the system cycles less.
The payoff of an efficient system
Duct sealing supports the broader case for efficient cooling. ENERGY STAR reports that nearly half of a typical home's roughly 1,900 dollar annual energy bill goes to heating and cooling, so losses in that category carry weight.
Tightening the system where it leaks captures part of that spending. Sealed ducts, clean filters, and correct charge all work toward delivering the cooling the household already pays for.