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On this page I will introduce you to the various materials and techniques used in the installation process.

 You may be suprised to learn that nearly all problems and break downs result from installation mistakes. I hear some customers say that "oh I have lived in a house with this brand of equipment or that brand, and I won't buy that brand because it was noisy or certain rooms were uncomfortable. I would explain to that customer that it is not the fault of the specific brand of equipment that was installed, rather it was the installation. Every piece of equipment needs a specific amount airflow as well as each room in a building will also need a specific amount of air. The duct system is responsible for this. A furnace or air handler as a limited capacity to move air. If a duct system is to restrictive the furnace fan will not deliver the correct amount of air through the heat exchanger and coils. As a result the equipment will overheat. The furnace and duct system may also be noisy if this is the case. Restrictive duct work is a very common problem.

Take a moment to meditate before diving in.

 "On the surface talk is cheap but the bottom has a rocky reputation."

Duct design and installation is crucial to the performance of your heating and cooling system in many ways. It can affect your comfort, health and wallet. Ducts that are in unconditioned areas will gain and loose heat to those areas. Unconditioned areas are places like your attic, crawlspace and garage. Duct leaks in these areas can really cause dramatic problems. As you might guess it is most efficient to locate the duct work in conditioned space. The best case scenario is a basement where metal duct work is the most economical material to use. Ducts can also be hidden in a bulk head. Metal duct work is has the advantage of having the least resistance to air flow and is an economical choice as long as it doesn't need to be insulated. It can be cleaned with out worry of damage to the material. Metal duct work has the longest life span of any of the materials. It can be insulated for use in unconditioned spaces but increases the cost of the job. Metal Duct has the poorest sound deading qualitys. When using Metal duct work it is advisable to use metal for the trunk and use either a combination of metal pipe and flex or just flex for the individual branch runs.

For clarification there are several different scenarios for duct work. There is a supply side which is the duct system which moves air from the furnace to the individual "supply boots and registers" from which air blows into each room. With in this system the air will move from the furnace through the "plenum" and or the cooling coil in the case of a gas furnace. From the supply plenum the air will flow into and through the "supply trunk" and out the "suplly runs". Where the branch run attaches to the trunk the connection is known as a "take off" or "collar". The transition from the branch run through the floor, wall, or ceiling is known as the boot. The grate the air flows out of is called a "register".

The Other Scenario is the return side. This is the duct work which returns air from the structure back into the furnace. The air begins it's journey by entering the "return grille" Then into the boot or wall cavity or some other cavity. From there the air will flow down the return branch. The return branch run may be pipe, square duct, flex, or evan a stud or joist space. The air will the n flow through the "return trunk" and into the "return plenum". The return plenum is where the air filter is located. If the furnace is a air handler rather than a furnace the air will pass through the indoor coil before entering the fan.

As you can see, in most cases the air has a long way to travel even if the ducts are not very long. And I would like to reiterate on the benifit of keeping the duct  work in conditioned space. by doing so reduces installation costs, reduces cost associated with loss and gain of heat into unconditioned spaces and is usualy results in fewer changes in direction of the duct work. Changes in direction add major resistance to air flow and add cost to the installation. I have seen many cases where a structure will require larger equipment to accommadate the load added by duct work in unconditiioned spaces. 

It is not always posible to install the duct work in a conditioned space so sometimes and probably more often than not ducts are located in unconditioned spaces.  The most economic materials to use in these scenarios are duct board and flex. Problems can arise if some details are not addressed. 

Flex duct needs to be pulled tight and in a straight line and when hung should be strapped every 4' with 3" fabric strap material. The inner liner should be pulled tight and attached with at least two laps of duct tape and a large plastic zip tie. Flex duct should not snake around or droop. Metal elbows should be used to change direction. Air will not turn in a pipe like water. And flex duct has much more resistance to air flow than metal. If you have some of this in your home compare yours to this description.

Duct board has the advantages of being preinulated and maintaing a superior seal due to it's nature. It also has better sound deading properties than metal duct work. However it has a higher resistance to air flow than metal and therefore sometimes needs to be larger than metal duct. It is normally 1" thick as a board and when fabricated into a duct adds 2" all the way around the outside dimension. An other problem associated with duct board is loose fibers from the material. The surface of the duct board is sealed from the manufacturer however loose fibers can propagate from the corners of the duct where the board is cut to fabricate it into a box. I seal the corners and joints with silicone to seal off any fibers and to super seal the duct. See the pictures to the left. Another issue that needs to be addressed is changing direction or changing dimensions of the duct while minimizing pressure drop and a loss in velocity. This is a major concern no matter what material is used for the fabrication of a duct system but is probably more prevalent in systems utilizing duct board because most installers will meerly butt one duct into another at right angles to change direction or dimensions. They may go one step further and cut a section of duct at an angle of 22.5' and then rotate one piece 180' then tape to the two halves back together to form a 45' fitting, they may do this twice to form a 90' turn. This alone is not enough in some cases to maintain the required air downstream. A change in direction like this will dramaticly reduce the amount of air leaving the fitting. The air strikes the back wall of the duct and is stopped while more importantly the air on the inside side of the elbow will pass the the inside angle and continue to the back wall. Air leaving the fitting is then only occuring due to be being displaced (pushed to the side) by the pressure force of the air entering the elbow. A fitting like this can have an equivalent  length of over 100'. You only have 250 to 500 equivalent feet to begin with when designing the duct system. Which you can automaticaly deduct around 100 equivalent feet just for the take off and boot. Of course you must allow for actual length also. That is another topic and will be addressed in the design / analysis section. There is two ways to deal with this. You can fabricate a long radius rounded elbow out of metal. This requires a large amount of space and labor. The reason the radius needs to be very large is becuase of aerodynamic effect. You would think that if the outside dimension of the elbow was round the air would follow it. It does not . It just slams into it. The inside radius does all the work. Think of an air plane wing. The air will follow the inside radius if it is long enough.The other option is to istall turn vanes. Turn vanes act as many wings or insinde radius's for the air to follow. ACCA Manual-D duct sizing method shows an equivalent length for a square elbow with turn vane as 10 feet. This is as good as it gets. Even though turn vanes show up in any literature about duct work that you could pick up it is not widely used. It may be that it not understood or that it is not widely available. I am able to by the rail the vanes attach to but the vane is not available therefore I have to fabricate it. This requires the use of a special metal forming machine known as a slip roll machine. below is a picture of assembled turn vane.

 Supply Trunk T w/ inside radius

Warehouse

Fabrication Shop

Air Handler for 3 ton Heat Pump

100KBtu Furnace/ 4 ton A/C

Supply trunk splits 2 ways

3 ton Westinghouse Heat Pump

Insulated metal duct

Duct board and Flex in attic