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Flexible Duct Sizing

Frequently Asked Questions

Flexible duct sizing sometimes can be complicated for non-professionals. Below we have compartmentalized the ideas in a question and answer scheme. We hope that this article answers you questions about flex duct sizing!

What is diameter?

Diameter is defined as the distance of a straight line passing from one end of a circle or sphere to the other end. In other words, it goes straight across the circle. The diameter is a very important unit of measurement when it comes to flexible duct sizing because all duct sizes are measured according to their diameters. Flexible ducting is normally sized according to the internal (or inside) diameter, often labeled as ID.

Do not mistake circumference for diameter!

It is quite easy to confuse circumference for diameter because they are both units of measurement relating to circles. It is very important to know the difference between the two, as they are completely different. Circumference is the distance around the circle. Circumference is not normally asked for in relation to flex duct sizing as diameter is more widely used for measuring flexible ducting.

What is OD?

OD stands for outside (or outer) diameter. It is the measurement of the diameter of the circle from one outside section to the other outside section of a circle. In relation to flexible duct sizing, the total OD measurement will account for the hose’s wall thickness. As a result, OD measurements tend to be slightly greater than the ID. OD size has no real relevance in most flexible ducting sizing, since these hoses do not have a real thick wall thickness and do not require hoses assemblies. OD is important when measuring the outside of the pipe on which your duct will be installed upon. As the flex duct is pulled over the pipe for installation.

What is ID?

ID is the abbreviation for internal (or inner) diameter. It measures the inside of a circle from one point on the inner wall to the other point. All flexible ducting hoses have an ID size. The specific measurement will depend on the type of hose in question, but ID measurements in flex duct sizing typically range from 1” to 24”. The inside diameter of flex hose is important because, this size fits over the OD of the pipe that it is connected to. At that point the flex duct is clamped down.

How are ID and OD related?

Both ID and OD are measurements of a circle’s diameter, but the real difference lies within the points of the circle they measure. The inside diameter (ID) is strictly the measurement of the inside sections of the circle from one point to the other. Outside diameter (OD) is the same measurement taken from the outer edge of that circle. The OD also accounts for the wall thickness of a flex hose. It tends to be greater than the ID because it takes into account the wall thickness of a hose.

How is the inside diameter of a flex duct measured?

It is easy to find the inside diameter for flexible duct sizing. One simply needs a ruler or measuring tape along with the end of the hose or outlet. Keep in mind that you have to begin the measurements from the interior wall of the circle. The distance to the opposing point of the circle is the measure of the flex duct sizing for the inner diameter.

What should I know when measuring the length of flexible duct hoses?

Flexible duct sizing relates to more than just the ID sizes of a hose. You should also keep in mind to consider the overall length of the hose you want for your application. Hose lengths can vary from one class of flexible ducting to another. Solid body and non-compressible hoses are easier to measure from one end to the other. Compressible hoses can cause some confusion in length measurements due to the fact that they compress down into smaller sizes. However, these hoses are measured when they are stretched out from one end all the way to the other. Length measurements for flexible duct sizing tell you the greatest possible distance your hose can stretch. In other words if two people pull the ends of the hose to its most stretched point, that is its actual length, and the length that it is sold at.

What does a compression ratio refer to?

In the context of flex duct sizing, compression ratio refers to the actual length that a flexible hose can compress. It is the measurement of exactly how long the hose will be in its smallest possible form. For example, if a flex hose is listed as having a 6:1 compression ratio, that hose can be compressed from a full 6ft down to 1ft.

What lengths are flexible ducts sold in?

It ultimately depends on the specific class of hose one needs for their application. The lengths seen in flexible duct sizing tend to range from 5ft to 50ft. The application being performed also plays a role in the length of a hose. For street sweeping, the hoses attached to the heads of street sweeper vehicles tend to be short in length. A flexible hose for an air ventilation system, however, is usually found in longer lengths that can range up to 50ft.