Messages and Online Forum Q&A

Re: HVAC Ducts

From: info@builderswebsource.com
Category: HVAC
Remote Name: 63.198.181.43
Date: 25 May 2001
Time: 08:53 PM

Comments

If you are not experiencing any issues with your heating system, I see no reason to switch from flex ducts to metal ducts. Having said that, if I were building a custom home, I would generally specify insulated flexible aluminum ducts for overall longevity and lifecycle performance.

Flexible aluminum ducting works well in both low and high velocity HVAC systems, making it more forgiving over changes in pressure. Negative pressure can be as high as 3" W.G., with positive pressure as high as 8" W.G. It also performs well over a wide temperature range (10-250 degrees).

According to Casco, a supplier of both metal and flex ducting, flexible aluminum duct is leak proof, fire resistant, bendable, light weight, self-supporting, and provides superior air flow. It also dampens vibrations, is vermin proof and does not support mold growth, which can be a source of airborne allergens.

Furthermore, since the insulation is fully isolated from the air stream, the chance of fiberglass erosion into the air stream is nil.

Of course, if aluminum duct is accidentally crushed during installation or routine maintenance, it's difficult to restore the round shape, possibly requiring replacement of the damaged section.

On the other hand, less expensive flex ducting is more susceptible to damage or puncture during installation and requires more frequent supports to avoid sagging or deformation. Flex ducting has a galvanized spring steel wire with a nylon or polyester film lining. Some flex ducting has thicker insulation available, which actually reduces heat loss through the system (R 4.2, 6 or 8) making it more energy efficient. Flex duct can support typical sustained velocities of 4000 FPM, negative pressures of 0.5-1" and positive pressures of 1" W.G. to as high as 6" W.G. depending on the manufacturer. Flex duct generally has a good temperature range tolerance comparable to metal ducts (typically from 0-20 degrees on the low end to 200-250 degrees on the high end).

Some high-performance flex duct is designed for very low velocity HVAC systems where acoustical noise abatement is important, such as home theaters, studios or reading rooms.

The friction loss of either type is comparable.

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