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.
Properly installed and well-maintained sheet metal, PVC, or insulated ducts can last 20 to 30 years, while flexible ducts have a shorter lifespan of 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.
Flex ducts are less durable but more cost-effective and easier to install. Rigid ducts are long-lasting but can be trickier to install due to their inflexibility and weight.
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.
While it's clear that sheet metal ducts may be the most energy-efficient ductwork due to the lower chance of damage and deterioration, each home has unique needs when it comes to its HVAC setup. Some homes may not be able to accommodate the space needed for sheet metal ducts, for example, due to tight spaces or bends.
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.
For example, a 90-degree bend in a flex duct adds an additional 15 feet of equivalent length to the duct system at a velocity of 700 feet per minute. (See Manual D, Third Edition, Appendix 3, Group 11, "Flexible Duct Junction Boxes and Radius Bends," [ACCA 2009].)
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.
Steel Duct: A Superior Alternative
When sealed and insulated properly, steel duct is the most energy-efficient means for air distribution systems, as metal duct maximizes HVAC system performance.
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.
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.
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.
USING THE CFM/FPM CHART
Referring to the chart, a 4" pipe will convey 395 CFM at 4500 FPM.
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.
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.
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.
Just as oversized ducting can wreak havoc on your heating and cooling systems, so can undersized ductwork. When the ducting is too small for the specific HVAC system that you have, air flowing through it will start to back up. The air pressure within the ductwork will become higher.
The compressor is the most expensive component to replace in an AC unit, and it can cost up to $4,000. It's expensive for two major reasons. First, it's responsible for circulating the refrigerant between the evaporator and condenser coils. Secondly, it's a highly complex equipment with intricate details.