Bad Ductwork - Ductwork is necessary to deliver the air throughout the HVAC system. It is important that the system has integrity with no leaks. Ductwork is made of different materials. Duct systems also have different construction methods which hold the ductwork together.
Pictured above is an old ductwork system (which needs to be replaced) made of duct board, which is a rigid fiberglass board, metal, and flex ductwork. The duct board is cut to size from the ductwork sizing requirements for the tonnage and taped together with special duct tape. For the branches coming off the duct board trunk, metal collars are inserted into holes cut to size in the duct board. Flex ductwork is attached to the metal collars for the branch ductwork.
Bad Ductwork Made into Good Ductwork
The metal collar is sized to the size of the flex duct branch and the flex duct is attached to the metal collar with special duct tape and a large Panduit strap. Some HVAC installation mechanics also use a sealer in the metal collar where it is inserted into the duct board and on the flex duct connection. This makes the connection better and ensures there will be no air leaks at the connection as long as the sealer is covering all connections. Flex duct should have few turns in the runs and be pulled taught. Installed in this manner reduces the friction on the airflow by reducing turbulence in a duct system with minimal bends and turns.
Ductwork systems are also constructed of sheet metal. These sheet metal duct systems are manufactured or pre-fabricated in either round or rectangle ductwork. This ductwork can be connected either using mechanical joints called Slips and Drive, snap seams, or by screwing it together. Either way, all seams in the duct system should be sealed with the sealer to prevent any air leaks. Ductwork also needs to be insulated for a few reasons.
First of all, so that the conditioned tempered air is delivered to the designated spaces without losing all its conditioning through heat loss and heat gain as it travels through the ductwork. Secondly, the duct system needs a vapor barrier to prevent sweating when humid conditions exist outside of the ductwork.
Duct systems that are not properly insulated lose efficiency and the ducts will sweat when operating below dew point temperatures. It is like a glass of ice water sweating on a warm humid day. The water eventually accumulates and causes further problems. If the duct system is made of metal the ducts will rust. The additional insulation on the duct, if any, will become waterlogged and fall off. That reduces the R-Value of the duct system and you lose the efficiency of the system.
Bad Ductwork
A good duct system will deliver the air to the designated space with no air leaks. Additionally, it will do it with little heat loss heat gain while it traveling through the ductwork. Ductwork integrity is important for both the efficiency and longevity of your HVAC system. Without ductwork integrity (a properly sized, insulated, and sealed duct system) your HVAC system loses efficiency and will need to be replaced sooner rather than later.
When you are considering replacing your HVAC system always have the duct system evaluated for proper size of the duct system (for the new system) and for airtight integrity. The HVAC estimator can do a review of your duct system including a “Manual D” sizing calculation to properly fit your new HVAC system. Then, with your new system, you can have a properly sized, insulated, and airtight duct system installed that will give you years of comfort.
Duct Systems and Bad Ductwork
Technical Resource: Refrigeration and Air Conditioning Technology
Hello, I am a consumer interested in replacing my original duct work. After two confusing estimates and limited information I started doing my own research and came across your website. My home was built in 1955, off-grade. My vents or under the floor, metal and insulated. They are in good shape. Some insulation issues. I can feel air coming from under the home when outside. My main concern is air quality. I would assume based on age they are not sealed and dirty.
Is there specific duct material that must be used for a crawlspace? Thanks for any information you can provide. I just want the job done right to have piece of mind. Can you suggest any other questions I should ask?
Hello Lisa,
The best ductwork to give you the best performance would be hard round ductwork where they use duct sealer at all the joints and after insulating the ductwork. It’s very important that the duct itself is sealed with duct sealer (like a paste but dries and seals) and then insulated providing a vapor barrier. At all the insulation joints duct sealer is used again to provide the insulation with a seamless finish.
That does two things: prevents conditioned are from escaping the duct and gives the duct a vapor barrier. Any breaks or gaps in the insulation will allow humid air to condense on the metal duct. That makes the insulation pretty much worthless and causes the metal duct to rust and corrode.
The round hard is usually more expensive but it offers the best performance with less friction in the duct. You can also use metal square or rectangle but you won’t get the same performance for air delivery. Most mid-economical systems use the hard rectangle duct for the main trunk and then flex duct for the branches. The same applies for what I described above for any type of duct system; and that is it must be properly sealed and insulated especially at the boots.
Some contractors have a bad habit of not insulating the boots. The boot is the very end of the duct work were the supply vent is located.
Make sure you get a quality contractor that does good work. If it’s done properly it will last a very, very long time.
The cheapest type would be all flex duct even for the main trunk. I recommend all hard duct (round). Good luck.