Duct Leakiness Testing

Why Is Duct Leakage Important?

Leaks in forced air duct systems are a major source of energy waste in both new and existing houses. Studies indicate that duct leakage can account for as much as 25% of total house energy loss, and in many cases has a greater impact on energy use than air infiltration through the building shell. Just as important, duct leakage can prevent heating and cooling systems from doing their job properly and resulting in hot or cold rooms.

Here are just some of the problems resulting from duct leakage:

• Leaks in the supply ductwork cause expensive conditioned air to be dumped into the attic, crawlspace or garage instead of into the house.
• Supply ductwork leakage can cause household depressurization which can cause spillage of combustion products (from furnaces, water heaters and fireplaces) into the house.
• Return leaks can pull outside air (hot in summer, cold in winter) into the duct system reducing both efficiency and capacity.
• Leaks in return ductwork draw air into the house from the crawlspace, garage, and attic bringing with it dust, mold spores, insulation fibers and other contaminants.
• Even if ducts are located in conditioned space, duct leaks can still connect to the outdoors. For example, supply system leaks in a ceiling between the first and second floors of a two-story home can pressurize the joist bay, forcing conditioned air outdoors through cracks in the rim joist area.
• Leaky supply ducts simply do not delivery air where it is needed, even if all your ducts are in conditioned space. Thus some rooms get too much air and others not enough, this results in uneven temperatures between the rooms.

The 2012 IECC limits the leakage to 4 cfm per 100 ft² of conditioned floor area when the air handler is installed and 3 cfm when it is not installed.

Measuring Duct Leakage

A duct leakage performance test involves pressurizing the duct system with a calibrated fan to a standard test pressure (25 Pa) and measuring the air flow through the fan. As air is delivered into the ductwork, pressure builds and forces air out of all of the openings in the various ductwork connections or through the seams and joints of the furnace or air-conditioner. The tighter the duct system, the less air you need from the fan to create a change in duct system pressure.

• A Duct Blaster is used to directly pressure test the duct system for air leaks, much the same way a plumber pressure tests water pipes for leaks.
• The Duct Blaster fan is first connected to the duct system at the air handler cabinet, or a return grille. After temporarily sealing all remaining registers and grills, the Duct Blaster fan is turned on to force air through all the ducts, boots, and air handler/furnace.
• Testing procedures can measure total duct leakage (i.e. leaks connected to the outside and inside of the house).
• Duct leakage measurements are used to diagnose and demonstrate leakage problems, estimate efficiency losses from duct leakage, and/or certify the quality of duct system installation.
• A theatrical fog machine can be used along with the Duct Blaster to inject a non-toxic fog into the duct system to visually demonstrate the location and extent of leakage in the ductwork. This is best used in unfinished basements.

Sealing Ducts

First off, before sealing ducts you need to make sure the airhandler can handle the tighter ducts. This starts with a simple static pressure check. If the static pressure is too high, then duct sealing is not an option until the cause of the high static pressure is resolved.HVAC Airflow & Efficiency Testing will show what needs to be changed to get the static pressure within limits.

Assuming the static pressure is within limits or the necessary changes have been made, then sealing the ducts becomes appropriate. How you do it depends on how much access you have to the duct work. For example, if you have an unfinished basement, then most if not all the duct work would be exposed and easily sealed. Mastic and fiber mesh tape are the best materials for the job. This is an easy DIY job, although it will take you time to come up with a technique that works for you. On the other hand, if your basement is fully finished, then a product/service like Aeroseal is one of the only ways to go without making holes in your basement ceiling.

Interesting Links

Dept of Energy - Improving the Efficiency of your Duct System   -or- Cached copy

ACHR News - Duct Leakage Testing   -or- Cached copy

Green Building Advisor - Duct Leakage Testing