Duct Blaster Test

I often hear people say they don’t like forced air. This is sort of like saying you don’t like plumbing. Whether or not we use an air-distribution system for heating, we can and should be using one for cooling, ventilation, whole-house air filtration, humidification, dehumidification, and to circulate air that would otherwise be stratified or stagnant.

Many energy efficiency and green building programs have performance standards for duct system tightness, and now require a duct blaster test as well as blower door testing. Some HVAC contractors also conduct their own duct testing upon a job’s completion. The test should be done after the rough-in of your duct systems and before insulation and drywall, so you can locate and seal any leaks in the system before they become hidden.

Here’s how the duct blaster test works: The technician starts by sealing off all the return and supply boots, typically with special duct masking tape, but anything that seals the opening will work. Cardboard works, and I also have seen a test done with friction-fit rigid foam pads. Once the boots are sealed, your rater will attach a fan to the blower compartment of your furnace or air handler, and will remove any filters in the return plenum. At this point, the only intentional opening in your duct system should be through the duct blaster. The rater will attach the same pressure gauge used for the blower door test to measure the pressure difference in the ducts. The more airflow it takes to create a certain pressure difference in the duct, the more holes there are in your duct system.

Ask your rater to correlate the duct leakage at 25 pascals as a percentage of the total design flow of the system (percent of flow at 25 pascal). The designed flow rate is typically 400 CFM per ton of cooling, i.e. a 3-ton A/C unit will need to move about 1,200 CFM across the A-coil. That said, you’ll want to work with your mechanical contractor to establish the actual designed flow rate. Divide the measured duct blaster flow at 25 Pa by the air handler’s designed flow rate and multiply by 100. So, if the test shows a leakage rate of 60 CFM at 25 pascal on a 3-ton air handler, this leakage is 5% of the rated flow. (60 CFM /1,200 CFM) x 100 = 5% of air handler flow at 25 pascal.) The lower this percentage, the tighter your duct system. Typical energy program total duct leakage targets range from 3% to 10%.

Ducts that leak into unconditioned space, like attics and crawlspaces, have more severe energy and durability penalties (think supply trunks leaking warm, moist air into a cold attic). So you also want to know what percentage of your duct leakage occurs outside of conditioned space. This is called a “duct leakage to outside” test, and it differs slightly from the total duct leakage test in that your rater measures only that leakage into or out of the ducts from outside the conditioned space. Ask your rater or program representative about your program’s requirements. You will want this number to be as small as possible; aim for less than 3% of air handler flow.