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Starting with the Basics
Every duct system has to be matched to the air conditioning requirements of the house and the equipment. If a mistake is made in determining the size system required then everything is designed off that mistake and the net result is a complete system that will not perform satisfactorily. Increasing the size of the equipment is not a solution at this point. In order to correct this type of problem, there are changes that need to be made to the duct system. Those changes are dependant on the increase needed for the system sizing. If the equipment was oversized then there are few if any changes needed to be made tot the ducting system.
But the equipment size cannot be increased without regard for the duct system as well.
There are some basic considerations for the duct system that should always be kept in mind. These are the so called basic rules. Never put return grilles in the kitchen or bathrooms to avoid dissipating odors through out the house. Always try to locate diffusers on the exterior walls when possible. Perfect location is under windows for heating. A duct system with high and low supplies is the perfect duct system. High diffusers for cooling so the air can fall and low diffusers so hot air can rise. Gentle is a word for air dissipation. Thinking of an air pattern that gently washes over the area is a perfect environment. Tornadoes and hurricanes are for outside conditions, not your house. You can almost never install more than enough diffusers. If the minimum required diffusers are 10 for the system then try to use 12 or more. For air supply the more the better the effect and the better air washing effect created.
Remember that duct systems can be perfect harboring grounds for molds, fungi and other bacteria and bugs that have even yet to be classified or determined. Being in the HVAC business for over 32 years our technicians have seen some of the strangest critters including molds etc., that are scary looking to say the least. Remember Legionnaires disease came form HVAC systems. Keep your HVAC system clean and dry to avoid growing of these critters. An HVAC duct system and the internal blower section and A/C coil are like Petri dishes if allowed to go unattended and permitted to be saturated with moisture. Cool dark moist duct systems are exactly what these bacteria thrive on. 
Clorox based or chlorine based cleaners are the most effective cleaners but elimination of the cause such as moisture must be eliminated as well. If for any accidental reason the duct system becomes saturated with moisture and there is insulation present, the best remedy is a mold remediation specialist.
And removal of the contaminated areas is the only solution. Some of these molds and bacteria have been known to cripple and kill humans. Use sufficient attention to these possibilities and have your HVAC system and the duct system inspected regularly.
One of the biggest energy wasters and again contributor to mold growth is leaky air duct systems. Aty joints, seams and where the duct system joins the diffusers are areas where moisture can enter and energy is wasted. It is of no value to install high efficiency HVAC equipment to a leaky undersized duct system. You gain nothing if the energy is wasted. Using proper tapes and duct sealants inspect and plug any leaking sources on the duct system. On leaking duct systems that are on the return duct system or in areas such as attics where the temperatures are very hot, make certain all the ducts are properly sealed and insulated. There is a myth that return duct systems didn't have to be insulated. All components of the duct system need to be insulated regardless of whether they're return or supply ducts.
Also remember that the tapes you may use are not the types of tape you may find at discount stores. Duct tapes should be UL and SMACNA rated so the material and glues can withstand the temperatures. Otherwise you're wasting time and money on tapes that lose their adhesiveness or become brittle in a short period of time and allow leaks to start all over again. If you do not have duct sealants available, use silicone acrylic mixed long life cauls as well as roofing sealants in lieu of the recommended products. Materials that are rated for roof applications are more likely to survive the adverse temperature conditions instead of basic household tapes and all purpose caulking.
Note that duct materials such as flex duct are delicate and can become ripped or gouged by any of numbers of ways. Inspect all areas where there may be storage boxes or other items that may cause punctures or cuts in the duct system. After time due to settling of the house and other remodeling duct systems and their joints can separate causing large losses of energy.
If you decide this project is more than you wish to handle, you will be far better off knowing what a proper designed and installed duct system is. DESCO is the only internet advertiser providing requirements and providing you with the factory warranties. To obtain the warranties you must also be willing to follow the requirements we've established. First we will size your system by you filling out the sizing form and returning the information requested. After this give us a call and we'll review the system requirements. With this information you can then begin to design the duct system or review your existing duct system. After you design the duct system send us a drawing and we will advise you of any changes necessary.
Besides looking for leaks and separations also look for areas where flex duct could possible collapse causing the air flow to be restricted. This can result also in high energy bills as the blower needs to work harder to supply air to the house.
When having an inspection of the duct system also check the interior for dirt build up and moisture. If someone is suffering from allergies and asthma, the culprit could be right in your house growing in the duct system. The tiny microscopic animals or bugs, molds and bacteria excrete allergens that can cause reactions. Want the best filtering system? Don't think it's an electrostatic filter. The best filtering system is one which is placed at the return diffusers such as a return air filter grill. When you install a 3M 1" pleated filter at this point you stop all dirt from entering the duct system to begin with.
If you have a tight duct system then you'll have provided the best filtering system possible by keeping not only the supply ducts clean but also the return ducts as well. Never use any type of filter that can be seen through such as standard fiberglass filters, washable filters made of foam or aluminum or reusable filters. If you can see through the filter, then many particles can bypass the filter and not only begin the breeding ground for all types of growth but the equipment such as the coils will soon become loaded with dirt and cause the system efficiency to decline rapidly. In cases such as high efficiency furnaces, this dirt will bake on to the condensing coil inside the furnace and cannot be removed.
The second best filtering system is the high media filter. Place this filter again as close to the return air diffusers as possible to begin filtering the air at the closest location to where the duct system begins to bring air back to the blower.
Electrostatic filters have been over marketed and highly over rated. Most don't work efficiently and even when inspected they are impossible to tell if they are operating properly. When do you tell when they are no longer working? When it's too late and the duct system and HVAC equipment have become coated with dirt. By then it's too late.
Another major problem in duct systems is poor lay out. Diffusers that are improperly placed or at poor locations without regard to the functionality of the house create major comfort problems as well as waste energy. As you see ion this picture the diffuser or grill is located at a very poor location. When you're inspecting the house for leaking ducts also check the area where the sheet metal boot joins the diffuser as in this picture. Removing the diffuser and visually inspecting how the boot is joined to the diffuser you will most likely find the connection is not joined by sheet metal screws and sealed. This is the area where most duct systems leak profusely.
There is little that can be changed without major renovations when the diffusers are laid out poorly. So if your project is new construction take your time and lay out the system taking all the negative possibilities into account so you don't end up with another new home that has a poor duct system and can't maintain comfort and robs energy. For those of you who are
working on a new house, or if you're adding a duct system to an existing house, we will review your duct system layout and assist you in the design so you don't end up with a poor design. Remember DESCO will provide sizing of the equipment as well as information for the Do It Yourselfer. We're interest in making certain your new system provides you with the best performance, maximum comfort and minimum energy consumption so you will have many many years of trouble free service.
Besides having a duct and HVAC system that can provide the best comfort levels you're also looking to avoid the most common problems that are running at epidemic levels today by contractors: Oversizing of the HVAC equipment, Undersized Duct system, Poor layout of the duct system, The Spaghetti poor man's duct system where a plenum box is plopped on the end of the unit and mega feet of flex duct is slapped into place wherever it will fit to the easiest location for the installer, too few diffusers and a duct system that leaks so badly that 25% of the energy produced is lost in leaks.
Remember that the fiberglass duct system we provide is a premium duct system that provides the most energy efficiency, tightest sealing, energy efficient, quiet and safeguard against condensation and moisture on the market. Fiberglass is by far the best duct system on the market today and with metal prices increasing daily, a fiberglass duct system is no more expensive than a metal system of equal insulating value and performance. Plus you will find it easier to install a fiberglass duct system. With DESCO providing duct tool rental, you can custom make any duct on the job. You can't do that with sheet metal.
Some say that size doesn't matter but it does here. A duct system purpose is to properly deliver the right amount of air to each room in the proper distribution. Size of a duct system can be too large and too small. The majority of systems suffer form inadequate size of the ducts. The other problem is how the air is distributed when it finally reaches the room is an area seldom addressed at all. Insufficient air flow is the resulting symptom of a poorly installed and designed system but the damage goes far beyond just not enough air flow.