Ductwork comes in all shapes and sizes; galvanized metal ductwork allows the most air capacity to be delivered to your rooms per foot because it has the least friction resistance.
Flex ductwork is easy to install, but it's a treacherous road should your contractor have taken it. There are many pitfalls were simply unbeknown to many HVAC contractors on the implications of their work. Here is a short list of pitfalls Green ID's energy auditors typically find in an attic.
1. Improper size
"I'm supposed to put a 18" duct but I only have a 14", I'll just make it fit!" I feel that's a common thought HVAC contractors have when doing these horrible installs. Not only does undersized ductwork chock off your airflow, robbing your home of its full air capacity, but it makes your unit work harder and drives up your energy bills.
2. Too many wye splits.
Every wye split adds 10 feet in length to your duct runs, so after the 4th wye split don't be surprised that the airflow is weak when your heating and cooling system needs to add an extra 40 feet of run to its load. By rerouting the ducts to come right off the main supply plenum you will likely get much better airflow and your HVAC system won't have to work as hard.
3. Grey ductwork.
Yes the color of your ductwork matters. In the 1970's home's were built without good attic ventilation and with a horrible duct product that actually deteriorates, becomes brittle and cracks open. That sounds like a horrible product and it is. This would be one time where a full duct replacement is in order and possible a better design done for proper airflow.
4. Evap cooler (swamp cooler).
If your home ever had a swamp cooler, the swamp cooler ductwork may still be connected to the main ductwork of the house. Contrary to many beliefs, homeowners will save more money by capping off the evap duct and sealing the entire duct system than homeowners do by using the swamp cooler 3 months out of the year.
5. Abandoning an old metal trunk duct system when doing additions or remodeling.
Even though metal ductwork is actually great for airflow and moves air with much less resistance than flex ductwork, if a home had an addition done to it, that ductwork is probably not the best design to help keep the homeowners comfortable. One bad thing about metal trunk ductwork is the location of the registers is always close to the doors of rooms which can cause comfort issues and poor air circulation. By installing a properly designed duct system, we can locate the ducts in the proper place in each room depending on the rooms heat gain and energy model.
Other signs of ductwork wear are:
Arizona home performance standards require that every flex connection be sealed with mastic for a 50 year plus lifespan. If your ductwork is not properly sealed, every time the HVAC unit kicks on you are throwing money out the window by heating and cooling the attic. In homes older than 20 years duct tape may have been used as a "sealer." Duct tape may be a universal fixer of all things except when it comes to your ductwork. It doesn't take long for duct tape in the attic to become loose and its adhesive deteriorated.
Metal ductwork has a great life as long as no moisture or water has gotten in it and can last for a hundred years or more. Flex duct shelf life can vary greatly depending on the type of ductwork installed. Silver lining ductwork can last more than 50 years.