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.
Easy installation, low cost and reduced HVAC noise transfer are the primary advantages of flexible ducts. Additionally, they are pre-insulated, which is a great way to save money on utility bills. In comparison to sheet metal ducts, the expected lifespan of flexible ducts usually hovers around 10 to 15 years.
Flexible ducts are an increasingly popular option due to their simple installation process and lower cost compared to metal ducts. They are created with materials like fiberglass, vinyl, or reinforced fabric, making them lightweight yet durable enough for most residential applications.
However, due to its thin material, flexible ductwork is more likely to become damaged due to crushing or tearing. It may require replacement after 10-20 years. Fiberglass ductwork has become less common due to concerns about its potential to deteriorate and release fiberglass particles into the air.
Flexible, non-insulated ducts cost $1 to $3 per linear foot; insulated ones cost $2 to $4 per linear foot. Fiberglass ducts range from $4 to $6 per linear foot, while sheet metal is the most expensive at $7 to $13 per linear foot.
Disadvantages of Flex Duct
Flexible ducts are more prone to air leakage around the seams, which can lead to lower HVAC system performance. Additionally, the pliable nature of flex duct tends to make them prone to tangling or crushing when they pass through tight spaces or fixtures.
Ductwork that Delivers. Use the “Rule of Two Feet” • No take-offs within 24” of trunk end-cap, • Never locate a takeoff in the end-cap, • Limit trunk-lines to 24 ft in length.
Friction is the enemy of airflow. Unlike rigid ducts, which are cut to length within a tolerance of 1 inch or less, it's easy to cut a length of flexible duct that's several feet longer than is needed to get from point A to point B. That creates slack in the duct, which reduces airflow for two reasons.
However, the IAPMO passed a ruling stating that flexible piping should be no longer than five feet for residential properties. After review, it was extended to six feet, but it could still cause issues with a project if you can't use flexible ducts.
Over time, dust and debris can build up in the ducts, affecting the efficiency of your system. While you may be familiar with cleaning rigid ducts, you may wonder if flexible air ducts can be cleaned. The answer is yes, but to avoid damaging the ducts, you should hire a pro to clean them.
Sheet metal ductwork is the most common type because it's tough and long-lasting. Plus, it has a smooth surface for easy cleaning.
Steel duct systems are the easiest to clean and will not rip or tear in the process, as polyester flexible ductwork would.
How long should ductwork last? You should replace your ductwork every 15 years because the materials deteriorate over time, impacting HVAC performance and indoor air quality.
Polyurethane flex ducts exhibit the best abrasion resistance and are recommended in applications that demand a durable flex ducting. This material handling hose exhibits superior flex fatigue and is recommended for applications that require continuous flexing.
The flex hoses can handle both positive and negative pressures. Each industrial duct has a working temperature range of -40 F to +250 F and is listed as a UL 94 V-0 flame retardant product.
ATCO Rubber Products is the worldwide leader in flex duct systems. With a complete line for residential and light-commercial heating, ventilation and AC applications, ATCO builds products that are built to last.
Pull it tight and keep it straight
If your contractor uses a flex duct for straight runs, they must pull it tight when they install it. Otherwise, the inside of the duct will be kind of bumpy and compressed. Air won't flow through it properly, and it will be more subject to sagging over time.
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.
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.
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.
Though there are different types of ductwork available in the market today, Sheet metal is most recommended as this type of air duct is non-porous in nature and thus does not allow mold or bacteria to breed in the air ducts. Sheet metal ductwork also helps you prevent the potential health risks of fiberglass air ducts.
While there are older versions of the material that may not be able to withstand the process and need to be replaced due to already existing damage, but most flex ducts are made with a metal spiral support to allow for contact vacuuming and other cleaning methods.
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.