Duct leakage is one of the most common problems I find in homes. How bad is the problem? Studies by the Department of Energy have shown that the typical house with a forced air ventilation system loses about 30% of its energy out the duct system! This is absolutely unacceptable because it is completely avoidable.
I've often heard heating contractors argue that ducts leaking inside the house don't matter because the heat ends up in the house anyway. But did you know that a leak in the basement air return system can draw the exhaust fumes out of a water heater or furnace and kill your family?
While this may seem to be a far-fetched example, I studied a house recently where exactly this problem could have occurred. Several large duct leaks in the basement caused the water heater exhaust gas to 'spill' out of the flue any time the furnace ran. This is real. Please, take it seriously.
What are some other problems with leaky ducts?
In this article, we're going to look at common problems and their solutions. Most of these are well within the capabilities of homeowners to fix. And, some of these fixes can save you more energy than replacing your entire heating system with the latest, high technology!
We're looking at the air handler filter port in a high-end home. The air handler is installed in the attic which is essentially open to the outside.
The problem here is pretty obvious, isn't it? The filter is over-sized and there's no cover anyway. So any time the air handler runs, most of the air bypasses the filter and system ductwork and simply sucks in from the attic! In addition, the thermostat was set to "on" to help circulate air around the house, round the clock..
The result? Utility bills probably 100% higher than they need be. Wood floors that warped based on outdoor humidity. Extremely uncomfortable living conditions year round because the indoor humidity simply matched the outdoor humidity. All that because of an over-sized filter and no port cover.
I was called into this job as a troubleshooter because no-one could figure out what was causing the floors to warp.
Most of the time, it won't be so obvious. Maybe just the end of the filter will poke out. But without an air-tight cover, a significant fraction of the system's air will pull in through this gap rather than through the filter. This is the single worst place for an air leak because the pressures are the highest.
Even if you have a filter cover, it is worthwhile taping over it with foil tape if it's not very tight. Rarely are these actually air tight.
Cost to fix: $0.10 - for a two foot section of foil tape.
Inside a Ceiling Air Register
This shows a typical ceiling mounted air supply register. The sheet-metal box that supplies air to the room isn't even attached to the ceiling. Instead, there's a large gap around the perimeter. Again, sloppy construction, replicated thousands of times in this development wastes homeowners' money on a huge scale.
There are two problems with the typical installation:
The thermal image of this area of the ceiling shows just how big a problem these types of duct leaks are.
The bright spot is the hot register, and the whispy areas around it show the air leaking into the attic.
If the gap is small enough, it can be cleaned out then filled with a high quality silicone caulk. If it is larger, I recommend placing rubber weatherstrip in the gap around the entire perimeter. After caulking or weatherstripping, screw a self-tapping sheet metal screw through the ceiling and into the area near each corner of the box. Don't screw it too tightly or you'll damage the sheet rock! Tighten the screws just enough to secure the box and form an air tight seal.
After the metal duct boot is properly attached to the ceiling, foilmastic tape works very well to air seal the connection. This is shown in progress in the following picture:
Duct boot in the process of being sealed
Note: the tape is not actually covering the air opening as it appears in this picture. The round collar where the duct attaches is actually further back than it appears. The tape goes up and around a corner.
While you are working on the duct boot, you may want to secure the duct to the sheet metal. Look inside the box and you'll see a round hole and metal collar where the duct attaches. Wipe down the metal collar and the inside of the duct to remove dust, then use Foilmastic tape to attach the duct to the metal. This allows you to really seal your ducts without crawling around the attic. Ideally though, you want to ensure that the duct is properly attached in the attic. The proper technique is described in the links below.
After sealing the duct, there is almost no heat leakage, as shown by this thermal image. The result is much less lost energy, more comfortable rooms, lower utility bills, and less chance of rotting out the roof in your attic. Well worth spending a few minutes sealing your ducts!
One of the worst, common problems is the detached duct. I've seen this in about 10% of the homes I've evaluated. Just like the leaky filter port, the detached duct can reduce your system efficiency by as much as 50%.
In the example shown here, the duct connects to a very leaky wall return, so the duct is sucking attic air, moisture and dirt into the system.
In addition, the opening to the register in the room below allows attic air to freely flow to and from the house.
During the summer, the air conditioning system will struggle because of all the hot air and moisture being sucked in. During the winter, the air will be dry and cold.
Fortunately, the fix is relatively simple. A flex-duct like this one has two layers. The inner one carries the air and the outer one is a vapor barrier. The key is to attach the inner layer air-tight, mechanically fasten it, then attach the outer layer and fasten it, usually with giant zip ties.
Others have covered this in detail elsewhere. See the following documents for step-by-step instructions.
Toolbase.org article on duct installation and sealing. - PDF file. See page 4
Often, when people are confronted with high utility bills, they're told that they have throw out their perfectly good furnace and buy a new high efficiency heating system. Instead of spending thousands of dollars, wouldn't it make sense to fix the obvious problems and probably save more energy than you would with a new system? If you could do this and only spend an afternoon with some caulk and special sealing tape - maybe $50 worth of materials, wouldn't you do it?
By fixing the three simple, but serious, air distribution problems outlined on this page, you can substantially improve the efficiency of your heating/cooling system. Typical savings are from 10%-20% but if you have more than one of the problems shown here, you could save 50% or more on your utility bills. Not only that, but you would increase your comfort and have a healthier home.
Aeroseal - Duct sealing system
An Introduction to Residential Duct Systems - U.S. Dept. of Energy
Louisiana Dept. of Natural Resourcs - Extensive duct guide
OIKOS Green Building Source - Mastic Gives Ducts the Treatment
Southface.org - Ductwork Q&A
Toolbase.org - Air Distribution Installation and Sealing
EFI.org - A non-profit that sells many weather-sealing supplies. If you're a PECO customer, enter the website here for a 20% discount. Contractors should sign up for an account where they can purchase items not in the consumer catalog.
Duct Mastics - Sealing compound for ducts.
EPDM Weather seal - High quality, D-profile weather strip. Great for placing in gaps to act as an air-tight gasket.
Foilmastic - An excellent tape with a good enough adhesive for all your duct sealing needs.