Why older local homes struggle
Much of the St. Louis housing stock predates central air conditioning. Many homes originally used gravity octopus furnaces, and the duct systems were later adapted for forced air, often without resizing returns or sealing joints.
The result is a system that may deliver adequate heat but cannot move enough conditioned air in summer. Second floors overheat, back bedrooms stay stuffy, and the equipment runs constantly.
Signs of duct problems
Rooms that differ by more than three degrees, whistling or booming registers, visible dust streaks around vents, and unusually dusty surfaces all point to duct leakage or imbalance.
Ducts running through an unconditioned attic or a vented crawl space are the biggest offenders. Air leaking from supply ducts in a 130-degree summer attic is pure waste, and return leaks pull hot, dusty air into the system.
How ducts get tested
A duct blaster test pressurizes the duct system and measures leakage as a percentage of system airflow. Under 10 percent leakage is good; many older local systems test between 20 and 35 percent.
Static pressure testing is equally revealing. A technician measures pressure across the air handler; readings well above the equipment rating usually mean undersized returns, an overly restrictive filter, or crushed flex duct.
Repair options and pricing
Manual sealing with mastic at accessible joints and boots typically runs $500 to $1,500 for a small home. Aerosol-based internal duct sealing costs roughly $1,500 to $3,500. Adding a properly sized return in a problem area often runs $600 to $1,500.
Full duct replacement in an older two-story home ranges from $4,000 to $12,000 depending on access. Duct insulation upgrades in an attic generally add $2 to $4 per linear foot.
Getting the work done right
Ask contractors to document static pressure and room-by-room airflow before and after the work. Without measurements, you have no proof the money improved anything.
If you are replacing equipment, address ducts at the same time. A new high-efficiency system installed on a leaky, undersized duct system will never deliver its rated performance.
Frequently asked questions
How much energy do leaky ducts waste?
The Department of Energy estimates 20 to 30 percent of conditioned air is lost in typical duct systems with leaks and poor connections.
Is duct cleaning the same as duct sealing?
No. Cleaning removes dust and debris; sealing closes leaks. Cleaning rarely improves efficiency, while sealing usually does.
Can I seal ducts myself?
You can apply mastic to accessible joints in a basement. Attic and wall-cavity ducts, plus any work requiring airflow testing, should go to a professional.
Sources and further reading
Price ranges in this guide come from quotes and invoices shared with STL Home Pros by St. Louis area homeowners and contractors. Technical and climate references are listed below.
About the author
Nick Cascani is a New York native who has called St. Louis home for the last few years. He believes getting professional home service should be simple and convenient, and he writes these guides to help local homeowners make confident decisions about their HVAC, plumbing, and foundation projects.
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