May 1, 2011
In the article "Investigation Into the Failure of Chilled-Water-Pipe Insulation" (March 2011,, the author writes, "The nice thing about cellular-glass insulation on cold pipes is that damage to the jacket/facer has no effect on insulation performance."

Cold-Pipe Insulation
In the article "Investigation Into the Failure of Chilled-Water-Pipe Insulation" (March 2011), the author writes, "The nice thing about cellular-glass insulation on cold pipes is that damage to the jacket/facer has no effect on insulation performance."

That is true—to a point. You must properly butter the joints (longitudinal and radial). I prefer to use a thick mastic-faced insulation wrap over cellular glass and a heavy stainless-steel jacket held on with stainless-steel banding. You usually have to fight the "value-engineering experts" about "gold-plating" the job.

On smaller jobs (6 in. and below), I try to use copper chilled-water piping. When you consider the installation cost and not needing to prime and paint the piping before insulating it, the cost is not too bad. For chilled-water lines underground, where a failure would not flood a building, I have used PVC (polyvinyl chloride), with cellular glass on the PVC. On the Gulf Coast, I insist on having conditioned air-handler rooms.

The point about inspection is very important. Even the best designs can fail through lazy or incompetent installers.
Curtis E. Dyle, PE
Curtis Dyle & Associates
Lake Charles, La.

Flexible-Duct Systems
I enjoyed the article "Predicting the Performance of Flexible-Duct Systems" (December 2010); it makes many good points. However, I think we need to look at more factors to get the full picture.

First, if you use an elongated plenum (horizontally—the size of one end of the air handler or A-coil and about 2 ft in length for each ton of cooling), you will get better results than with a long trunk line because you won’t lose as much pressure through the full length of the system.

Second, the loss of airflow from louvered registers is surprisingly large—usually, 30 to 40 percent. If you test entire systems, where return-air suction plays a role, and the entire environment is pressurized, you will get better results testing the performance of flex duct. Also, you should consider the huge benefit that metalized or foil-covered flex duct has over cheaper plastic-covered flex duct. The ability of the foil covering to reflect heat has a significantly positive effect on output and efficiency—10 to 15 percent for both. Another benefit of flex duct over sheet metal is the fact you get less duct air leakage, especially if you use duct board for your plenum or trunk line and good sealing takeoffs.
Tom Becker
Climate Masters Inc.
Webster Groves, Mo.

I found the performance of flexible duct to be a good topic and the article certainly well-written. Two points I would like to add:

1) I found curious the apparent need to "stretch" the material because we always are fearful of placing too much strain on taped joints. The influence of ducting on wyes is another issue. Trying to keep the alignment of a wye to that of a "geometric wye only" can be a challenge. We have tried to hang wyes at their center of gravity only to have them roll, turn, or rotate once ducting was attached.

With "old school" fabric-style duct tape, there typically is looseness; occasionally, there are disconnections. The use of nylon tie wraps and mylar tape is the method these days.

2) I long have wondered why there are no pre-bent plastic sleeves that can be slipped into a duct prior to installation for the purpose of making consistent bends. Sure, there would be times when a bend would be in the middle of a run, but often, bends are required near plenums.
Bob Dorazio
R Dorazio Contractors and Builders
Avila Beach, Calif.

Author's response:
Mr. Becker's and Mr. Dorazio's solutions should be followed by others, and, it is hoped, the desired accessories for supporting duct will be marketed.

Engineers need to understand the limitations of flexible duct and modify their specifications and details to reflect exactly how it should be installed. The test data referenced in the article should be obtained to avoid underperforming systems. It is hoped the American Society of Heating, Refrigerating and Air-Conditioning Engineers and the Sheet Metal and Air Conditioning Contractors’ National Association will upgrade their standards to reflect this data.

Fabric-style duct tape will dry out and fall off. Use UL (Underwriters Laboratories) 181, Standard for Factory-Made Air Ducts and Air Connectors-listed aluminized "metal" duct tape with quality adhesive. LBNL (Lawrence Berkeley National Laboratory) 47214, "Residential HVAC and Distribution Research Implementation" (, shows how fabric duct tape fails to meet UL testing standards. Duct tape may be used to secure a vapor barrier; however, it never should be used to secure the inner core of a duct. Heavy-duty temperature-rated draw bands and metal bands should be used to secure the core of a duct to a fitting.

Fittings for flexible duct were not covered because this is an area where metal or possibly thermoplastic products can be made available by accessory manufacturers. The duct-board fitting shown in the LBNL articles is not engineered and is subject to failure if tape fails and duct connections do not direct air properly.
William Allen, PE
MACTEC Engineering and Consulting Inc.
Kennesaw, Ga.

Letters on HPAC Engineering editorial content and issues affecting the HVACR industry are welcome. Please address them to Scott Arnold, executive editor, at [email protected].

About the Author

Scott Arnold | Executive Editor

Described by a colleague as "a cyborg ... requir(ing) virtually no sleep, no time off, and bland nourishment that can be consumed while at his desk" who was sent "back from the future not to terminate anyone, but with the prime directive 'to edit dry technical copy' in order to save the world at a later date," Scott Arnold joined the editorial staff of HPAC Engineering in 1999. Prior to that, he worked as an editor for daily newspapers and a specialty-publications company. He has a bachelor's degree in journalism from Kent State University.