Air Ducts

Old 02-01-02, 12:49 PM
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Question Air Ducts

I am in the process of remodelling my house and replacing the existing HVAC unit as well as adding an additional unit. The current air ducts are "hardpipe" sheet metal with insulated wrapping. I have noticed that the majority of new homes have the "flexpipe" for air ducts. What is the advantages/disadvantages of one verses the other? Should I specify "hardpipe?" It seems that I recall hearing something about the "flexpipe" restricting the flow of air due to its flexibility and "kinking" at the turns. Does anyone have any information to clear up this confusion? Or should I not be concerned about it? Thank you for your help!
Old 02-01-02, 02:58 PM
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One restriction with air flow is kown as surface tension. It goes without saying that the smoother the surface the less surface tension. HVAC people use calculators, tables and slide rulers to determine the size of ductwork. There is also a difference when they are calculating ductwork for sheet metal verses flexible. I believe it's multiplied by 2 with calculators. This is because of the increased surface tension with flexible ductwork.

There is a move in this country to make new homes more energy efficient, that is known as Energy Star Homes, where flexible ductwork will disqualify this rating for the new home. Besides the concern with surface tension, another problem with flexible ductwork is sagging and coupling. This has more to do with workmanship than with the product. In most cases, the hangers used to suspend the ductwork is installed too far apart, causing sagging. When two pieces of ductwork are joined, there not sealed properly and separate shortly after installation.

In all fairness, this workmanship problem usually does not apply to independant contractors, but is found mostly in developments, where many homes are built at once. You'll find most independant contractors are aware of this and they express their concerns to you by explaining how they install the product and methods used to avoid such concerns. Those that do, are in my opinion, worth hiring.

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