Not All Ducts Are Created Equal...

Unlike our Declaration of Independence, which declares “all men are created equal”, all ducts are definitely NOT created equal.

In a typical residential heating & cooling system, you may find three major types of duct materials;

1. Sheet metal

2. Fiberboard

3. Flexible ducts

Each has its advantages and applications.

Sheet metal ductwork is smoother, and has less resistance to system airflow than the other two. It’s also harder to fabricate and install, so it takes longer and costs more too.

Fiberboard ductwork has more resistance to airflow than sheet metal, but not as much as flexible ductwork. It’s relatively less expensive and simpler to install than sheet metal, but doesn’t always have the same durability.

Flexible ductwork has the highest resistance to system airflow, making it generally the worst choice for comfort and efficiency. The primary advantage to flex duct is that it’s “cheap and easy”, which is rarely an expression found in the company of “good” or “quality”.

An easy way to think about these three types of duct materials is to equate them to type of roads:

1. Sheet metal is like a smooth paved highway

2. Fiberboard is like an old, beat-up highway that could use resurfacing

3. Flex duct is like a bumpy, gravel road

Think about cruising down each of these roads; what comes to mind? Well, that’s pretty much what’s happening with your comfort and efficiency too. Just like your car’s MPG on that bumpy, gravel road, improperly sized or installed flex duct is a comfort and efficiency killer.

Flexible duct comes in 25-ft lengths, that are compressed to less than 1/8 that length into a short, approximately 3-ft long box. If you were to take a look inside that box when you opened it, you’d see that when flex duct is compressed, it’s severely restricted. Unless flex duct is properly cut to length, then pulled tight to decompress it to full extension, it could potentially have up to 10x more resistance to your system’s airflow than expected. 10x!

Generally, flexible ducts need to be at least one size larger than if the same duct was constructed from sheet metal. This rarely, if ever, happens, which is a major factor in why over two-thirds of systems suffer from low-flow.

The most common flexible ductwork mistakes:

• Improperly sizing it like it was one of the other type of materials

• Not cutting off the excess length

• Long lengths that “snake” back & forth in an S-shape

• Squeezing or compressing under, over, or through obstacles or holes

• Kinking around corners or construction assemblies

• Lack of support on 90-degree bends

• Hangers or supports spaced too far apart so that the flex duct sags

• Hangers or supports that are too tight so they pinch or compress the duct

With all that said, flexible ductwork is not only a reality of modern HVAC systems, when it’s sized and installed properly, it could actually be an advantage all the way around. A quick visual inspection, combined with some basic tests and simple engineering calcs, will confirm if you have a problem and what to do in order to solve it. Anything else is just guessing, leaving you wide open to nothing but headaches and problems!


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