What is a duct leakage test?
A duct leakage test is a pressurization test that puts a number on how much air escapes your duct system. Because ducts are hidden in attics, crawl spaces, and wall cavities, you cannot just look at them and know how leaky they are, so the test forces air into the sealed system and measures the loss directly. The tool is a Duct Blaster, a small calibrated fan that mounts to a return grille or to the air handler cabinet. As the fan pushes air in, gauges hold the ducts at exactly 25 Pascals and report the airflow the fan is delivering. Since the ducts are sealed at every register, whatever the fan has to supply to keep them at 25 Pa is exactly what is escaping through the cracks.
There are two versions of the result. Total leakage measures everything that escapes the ducts, including leaks into a wall or floor cavity that is still inside the house. Leakage to outside measures only the air that escapes to somewhere unconditioned, like a vented attic or crawl space, which is the leakage that actually costs you money. Total leakage is the number building codes usually check because it is quick to run; leakage to outside takes a second tool and gives the truer picture of wasted energy.
How is a duct leakage test performed?
The technician turns the system off, then temporarily seals every supply and return register with tape or a cover so no air can escape the normal way. The filter comes out and the filter slot is covered. Next they connect the calibrated fan to the duct system, usually by sealing it over a large return grille or bolting it to the open air-handler cabinet, and run a hose from the pressure gauge into a supply duct. They ramp the fan up until the ducts read 25 Pascals, and the gauge converts the fan's airflow into the leakage number in CFM25. The whole thing is a diagnostic test, not a repair, and it involves no gas, refrigerant, or mains-voltage work.
To get leakage to outside, the tester adds a blower door, a second calibrated fan mounted in an exterior doorway, and pressurizes the whole house to the same 25 Pascals as the ducts. With the house and the ducts at equal pressure, air no longer moves through leaks that open into the living space, so the Duct Blaster now reads only the air escaping to the attic, crawl space, or outdoors. That is why a full energy audit often runs both fans at once. Once the test flags where the system is losing air, the fix is sealing the joints with the right material; see duct mastic sealant for why brushable mastic beats tape.
What is the difference between a duct blaster and a blower door test?
A Duct Blaster tests the ducts; a blower door tests the whole house. They look similar (both are calibrated fans with pressure gauges) but they answer different questions. The Duct Blaster seals to your duct system and measures leakage out of the ducts. The blower door mounts in an open exterior door and measures how leaky the entire building envelope is, reported as air changes per hour at 50 Pascals (ACH50). One tells you your ducts are loose; the other tells you your walls, windows, and rim joists are loose.
The two get run together for a reason. A blower door alone cannot separate duct leaks from envelope leaks, and a Duct Blaster's total-leakage reading cannot tell how much of that loss goes outside versus into the house. Pair them (house and ducts held at the same pressure) and you isolate leakage to outside, the number that actually drives your heating and cooling bill. A whole-home energy audit is where most homeowners meet both machines.
How much duct leakage is acceptable?
For new and replacement duct systems, the International Energy Conservation Code (IECC) sets the limit at about 4 CFM25 of total leakage per 100 square feet of conditioned floor area. So a 2,000 square foot house is allowed roughly 80 CFM25 total. Older code editions were looser, allowing 6 to 8 CFM25 per 100 square feet, and the threshold has tightened with each cycle. Codes distinguish a rough-in test (run before the drywall goes up, when leaks are easy to reach and fix) from a postconstruction test on the finished system, and both have to pass. Many codes also waive the test entirely if all the ducts and the air handler sit inside the conditioned space, because leaks that stay inside the thermal envelope do not waste energy the same way.
For an existing home nobody makes you hit a number, but the codes still give you a useful yardstick. A tight modern system runs in the low single digits of CFM25 per 100 square feet; the leaky older systems that lose that 20 to 30 percent ENERGY STAR warns about often test two to four times over the code limit. If your test comes back high, the money is in sealing the accessible runs first. Sealing is also a far better spend than duct cleaning, which the EPA does not recommend as routine maintenance.
How much does a duct leakage test cost?
A standalone duct leakage test typically runs $100 to $400, depending on your area and whether the tester has to set up for leakage-to-outside with a blower door. On new construction the test is usually bundled into the HERS rating the builder already pays a certified rater to perform, so it does not show up as a separate line item. As part of a whole-home energy audit (blower door plus Duct Blaster plus a walk-through), expect the audit to run $200 to $600, and many utilities discount or rebate it.
The test is required by code for most new and replacement duct work, and a passing result is often what unlocks an efficiency rebate or a real-estate energy disclosure. Even when it is not required, paying for a test before you seal tells you where the leaks are and gives you a before-and-after number to prove the work paid off. If leaks turn out to be torn or crushed sections rather than loose joints, see ductwork repair and replacement cost for when a run is past sealing.
How can you tell if your ductwork is leaking without a test?
Several everyday symptoms point to leaky ducts before any fan gets set up. Rooms far from the air handler that never get comfortable, big temperature swings between rooms, higher-than-expected bills, and rooms that get dusty fast are all classic signs, because a leaky return pulls dirty attic or crawl-space air into the system while leaky supplies dump conditioned air where nobody lives. Ducts running through a hot attic or a damp crawl space are the most likely to leak and the most expensive when they do.
You can hunt for leaks yourself without a Duct Blaster. Turn the blower on, then run a wet hand along the accessible joints and feel for escaping air, or look for streaks of dust blown onto the duct near a seam, which mark where pressurized air has leaked for years. The connections closest to the air handler and the plenum leak first because the pressure is highest there. A hidden benefit of sealing: a system that was fighting a leaky duct network can finally keep up, which is why you should seal and test before you assume you need a bigger unit. If you are replacing equipment, size the new system to the real load with the BTU calculator rather than matching whatever tonnage was there before.
Frequently asked questions
How do you perform a duct leakage test?
Turn the system off, seal every supply and return register, and remove and cover the filter. Connect a calibrated fan (a Duct Blaster) to a return grille or the air-handler cabinet, run a pressure hose into a supply duct, and ramp the fan until the ducts read 25 Pascals. The fan's airflow at that pressure is your leakage rate in CFM25. Adding a blower door to pressurize the house to the same level isolates leakage to outside.
How much duct leakage is acceptable?
For new and replacement ducts, the IECC limits total leakage to about 4 CFM25 per 100 square feet of conditioned floor area, so roughly 80 CFM25 for a 2,000 square foot house. Older code versions allowed 6 to 8. The test is often waived if all ducts and the air handler are inside the conditioned space. Existing homes have no legal number, but leaky older systems commonly test two to four times over the current code limit.
How much does a duct leakage test cost?
A standalone duct leakage test usually costs $100 to $400. On new construction it is normally bundled into the HERS rating the builder already pays for. As part of a whole-home energy audit with a blower door, expect $200 to $600 total, and many utilities discount or rebate that audit.
What is the difference between a duct blaster test and a blower door test?
A Duct Blaster tests the duct system, measuring how much air leaks out of the ducts in CFM25. A blower door tests the whole building envelope, measuring how leaky the walls, windows, and floors are in air changes per hour at 50 Pascals. They are often run together so the tester can separate duct leaks from envelope leaks and measure the leakage that escapes to outside.
How can I tell if my ductwork is leaking?
Watch for rooms far from the air handler that never get comfortable, large temperature differences between rooms, higher bills, and rooms that get dusty quickly. To find leaks by hand, run the blower and feel along accessible joints with a wet hand, or look for streaks of dust blown onto the duct near a seam. Leaks are worst at the connections nearest the air handler and plenum, where pressure is highest.