Flexible air ducts shall not be limited in length. ❖ The length of flexible air ducts is not limited, unlike flexible air connectors, which are limited to 14 feet (4267 mm) (see commentary, Section 603.6. 2.1).
There is a difference between "flexible air duct" and "flexible air connector." Flexible air duct has no limit to allowable length, while flexible air connectors are limited to a maximum distance of 14 feet per IMC.
Flex duct runs should be as short as possible (5 to 6 ft. max.) and should be stretched as tight as possible.
My colleague John Puryear teaches the simple “2 Foot Rule” for takeoff placement in his classes. When there is a change in air direction in a duct, it takes roughly 24 inches for airflow to restore its pattern. Because of this, John recommends keeping takeoffs 24 inches away from any turns, transitions, or the end cap.
All ducts should be sealed using UL-181–rated duct mastic, and flexible ducts should be held in place with tie wraps at all connectors. Ideally, all ducts should be located within the conditioned space.
Flexible air ducts shall not be limited in length. ❖ The length of flexible air ducts is not limited, unlike flexible air connectors, which are limited to 14 feet (4267 mm) (see commentary, Section 603.6. 2.1).
One of the major disadvantages of flex duct is that it lacks any form of structural support. Because of its soft, bendable make-up, flex duct can easily sag or compress over time due to its own weight. This lack of structural support can lead to reduced air flow and ineffective HVAC performance.
1. Flexible duct shall be supported at manufacturer's recommended intervals, but at no greater distance than four feet. Maximum permissible sag is 1/2 inch per foot of spacing between supports.
When setting up a ventilation system we typically figure a 7% loss in CFM for every 25 ft. when using flexible ducting (a 3% loss if using metal duct pipe). You should also factor a 3% loss for every 90 degree bend in your ductwork.
The HVAC Duct Sizing Rule of Thumb. As a general rule of thumb, the diameter of your HVAC duct should be no less than 125% of the width of your furnace outlet. This is especially important when it comes to square and rectangular outlets, which tend to severely restrict airflow.
1) Length of duct run up to 3 metres – if your duct run is longer than 3 metres consider using a centrifugal or mixed flow impeller fan which are designed for this situation.
Excessive Length
Longer ducts can increase the size of the air handler needed to supply the same cubic feet per minute (cfm), or the HVAC system may not be able to supply the designed cfm to all locations.
When your ducting is oversized, it will struggle to deliver that temperature-controlled air to various areas of your home. In fact, you'll find that your HVAC system has to work much harder to produce enough hot or cold air to get your entire home up to your desired indoor temperature.
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.
Following these concepts can help with any flexible ductwork installation job. However, the IAPMO passed a ruling stating that flexible piping should be no longer than five feet for residential properties.
The longevity of HVAC flex ductwork hinges on several key factors, including material quality, installation, and maintenance. Typically, well-installed and properly maintained flex ductwork can endure for 15 to 25 years.
The room's square footage will tell you what the CFM should be (12 x 12 = 144).
The maximum airflow for a 10” flex duct is 270 cfm.
At 1,500 cfm, 14-inch duct is required; at 5,000 cfm, 22-inch duct is required; and at 15,000 cfm, 33-inch duct is required. In each case, the duct will carry the specified volume with a pressure loss of approximately 0.20 inches wg per 100 feet.
Another common mistake is using too small of a duct or underestimating the amount of airflow required. This can result in excessive noise and vibration, as well as decreased efficiency. If your ductwork is not properly insulated, it can cause the air temperature to fluctuate, resulting in hot or cold spots.
The maximum length of flexible ducts in the UK is, 1500 mm. This is specifically regarding installing a flexible duct, which should only be used as the 'final connection'. In terms of purchase, the ducting we sell can be bought for up to 10 meters of length, depending on the ducting you buy.
No restrictions on flex length for residential, and 6'-0" is considered a good maximum for commercial. However a system with a very short metal duct supply plenum and flex everywhere (“octopus system”) is considered by many (myself included) to be poor practice.
Steel duct systems are the easiest to clean and will not rip or tear in the process, as polyester flexible ductwork would.
There are two primary causes of sweating ductwork: poor duct insulation and excess indoor humidity.