Diagnosing and repairing duct leakage is not as simple as it might sound. You look at the duct system, get some tape and mastic, and proceed to seal it up. The obvious points to seal are the S and drive connections, branch takeoffs, and plenum connections to the furnace coil or air handler. The return air additionally, will require sealing of joist headers, and panning. These are the most obvious areas, but in no way are they the only areas that require sealing in order to have an efficient duct system.
The obvious areas are easy to detect, and often are the only areas where duct sealing is applied. However, you should consider many other factors that contribute to duct leakage. Duct leakage can be a result of an infinite number of reasons. Some of the areas that are often missed are:
Stud cavity above the return air wall opening. Air leakage in the wall cavity is caused by plumbing, and electrical wiring that runs through one stud to the other, or up through the top plate into the unconditioned attic. The easy fix for this is to install a block directly above the return air wall opening, and seal with mastic. This will eliminate the negative draw from the entire cavity and allow air to enter only from the wall return grill. Quality HVAC contractors will install this header while performing the original install, but you will find many jobs where this is not done.
Broken or disconnected supply ducts behind finished walls. Finding these air leakage points are quite difficult. Using an infrared camera can be one way to locate the air leakage, as the air in the wall cavity will be much colder than air in other wall areas. The airflow from the register will often be very low. Now this could be for other reasons, but often it will also be from duct leakage.
Supply runs installed through return joists and headers. In many areas, this practice is against code regulations. However, code officials often miss it. The headers are usually cut to allow the passage of a round supply duct, and often the opening is oversized. Often times, the penetration is not sealed properly.
Return duct top air transfer connection to floor joist or panning. This one is the most common that we have found for return air duct leakage. Code officials almost always miss this one. The air seal relies on the duct being tight against the floor duct and panning. What we find is the air leaks under the joist on both sides of the opening, and draws air in from the open joist on either side.
Gap seam between main supply and return duct run. Just as above, the air can leak from where the supply and return ducts run tight next to each other.
Plumbing and electrical penetrations through panned floor joists. Often in new construction of homes other trades such as plumbers and electricians can create leakage points in the return air system when using free draw joists and wall stud cavities.
This list represents the most common findings in our business of correcting duct system problems. We use a combination of tools to locate these leaks. Tools include infrared camera, static pressure gauge, thermometer, and duct blaster. One additional tool that has become the most valuable is a theatrical fogging machine whereby we can see the duct leakage when the fogging escapes the air distribution system. Using these tools, you will be amazed at the places you find air distribution system air leakage.
Duct leakage doesn’t necessarily mean the air is leaking outside the conditioned envelope. That would be the worst-case scenario. However, duct leakage even inside the conditioned envelope is still bad. It is arguably the main reason for inconsistent temperatures in the home. These include the inability of a system to properly cool the second floor level, and excessively colder temperatures in the lower levels.
When we properly seal a duct system the number one response we receive from clients is how much more air is flowing in and out of the return and supply air openings.
This illustrates that sealing duct systems properly and extensively is important to delivered efficiency, cost of operation, comfort, and life of the clients’ comfort system. Use the necessary tools to find the problems that are not necessarily obvious, and seal the system thoroughly. The difference will amaze you and your clients.