FAQs

How are Thermaflex®'s pressure ratings determined?

Thermaflex’s UL Listed pressure ratings are determined by testing duct in straight lengths, at ambient temperatures and witnesses by UL. Testing is done in a static condition (no airflow). The test sample is sealed airtight on both ends and pressurized to 2 times the manufacturers desired rated pressure for a test period of one hour. This applies to both positive and negative water column pressure claims. These tests can also be witnessed by UL.

Thermaflex performs some additional tests in accordance with the Air Diffusion Council, FD 72-R1 Flexible Duct Test Code. The purpose of these tests is to simulate a variety of realistic conditions, which may exist in actual installations. The ADC tests include a 90 degree bend, heated air at continuous and intermittent temperatures, for a test period of 168 hours. The resulting “Recommended Operating Pressures” are intended to give the contractor and/or engineer a responsible guideline within which to design the duct system.

What are Thermaflex's pressure ratings?

The Pressure ratings are listed in our SPECS for positive and negative pressures. Click here to view specs by product.

These are ratings for our standard residential products:

Maximum Positive Pressure 10˝ water column (4˝ – 16˝ diameter):
6˝ water column (18˝ – 20˝ diameter)
Maximum Negative Pressure 1˝ water column (4˝ – 12˝ diameter):
1/2˝ water column (14˝ – 16˝ diameter)
Velocity 5,000 (feet per minute)
Negative 1 inch (4˝-12˝ ID)
1/2 inch (14˝ – 20″ ID)
Flame Spread/ Smoke Developed 25 max/ 50 max

Can any of THERMAFLEX's Flexible Duct be exposed to ultraviolet sunlight?

The RKD product line can be exposed to direct sunlight. Click Here for product information and spec sheets. All other products lines must be protected from the weather. We manufacture duct with black polyester jackets and metallized polyester jackets to alleviate the break of chemical bonds from the ultraviolet rays. However, other than the RKD, we require that the duct should be installed in areas without direct or indirect sunlight.

What is the thickness of Thermaflex insulation?

Thermaflex provides insulated flex duct with three different R-Values (4.2, 6.0, and 8.0). The thickness of each is:

  • R-4.2 = 1.25˝ (Owens Corning)
  • R-6.0 = 2˝ (Owens Corning)
  • R-8.0 = 2.375˝ (Owens Corning)

All Density = 3/4˝ lb.

In an attempt to standardize, the flexible duct industry no longer states insulation values in thickness and density. The Air Diffusion Council, an industry organization of manufacturers, certifies R-Values based on insulation only, at installed wall thickness, based on ASTM C-518. Underwriters Laboratories classifies these tested R-Values based on plant inspections.

Does THERMAFLEX manufacture an acoustical duct?

At Thermaflex we strive to improve the quality of life for those around us, whether the classroom, boardroom, office or hospital. Our MKE acoustical duct offers not only Acoustical Benefits but is Greenguard Certified and mold and mildew resistant. Standard duct systems reverberate sound through the duct while the MKE’s superior inner liner transmits sound energy out into the plenum space while reducing noise levels in the occupied space.

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Does THERMAFLEX have a plenum rated duct?

In the Standard UL 181, Factory-Made Air Ducts and Air Connectors the Listed Systems receive a Class I rating when the flame spread index is not over 25 without evidence of continued progressive combustion and the smoke developed index is not over 50, based on the Standard UL 723, “Test for Surface Burning Characteristics of Building Materials” (NFPA 255, ASTM E84). These requirements are consistent for all pipe and duct installation materials as well as materials used for the fabrication of air duct and air connector systems used in air handling plenum spaces.”

Based on this definition and clarification all Class 1, UL181 Air Duct Listed products are considered “plenum rated.”

What materials are required/allowed to be used when hanging or supporting flexible duct?

Click here for ADC installation standards. For a full description of the material we offer, click here for our duct accessories.

How does duct condensation or sweating occur?

Condensation or “sweating” is a complex problem that is primarily seen in high humidity areas. There are a large number of factors that can lead to condensation. Condensation occurs on any surface that is colder than the wet bulb temperature of the air surrounding it. In air ducts it is most common to see this on the outer vapor barrier of the duct. However, it can also be on the inner core in flexible ducts, or the sheet metal of a sheet metal system. We also see condensation at the fittings or plenums. Any place where the temperature of a surface is colder than the wet bulb temperature of the surrounding air you will have condensation.

To prevent sweating it is necessary to either raise the temperature of the sweating surface, or lower the wet bulb temperature of the air. It is usually easiest to increase the temperature of the sweating surface by adding insulation. However by increasing ventilation it may be possible to lower the wet bulb temperature of the air.

When it is necessary to route an insulated duct through a truss/joist support, care should be taken to prevent tearing of the vapor barrier and insulation. If localized compression occurs, a 20% diametric compression of the duct core at the truss/support will result in only an approximate 5% change in cross-sectional area of the duct. This is little change compared to oval ducts, which are commonly used in HVAC applications.

In an insulated flex duct, much of the visible compression is actually compression of the fiberglass. Such localized compression of the fiberglass should have minimal, negative effect on the total, thermal losses of the duct. However, when the installation is in a critical condensation zone, localized compression could result in localized sweating. The A/C contractor or designer should consider this information in view of total design performance. Also, be advised the local authority having jurisdiction will have the final say in these matters.

Have Thermaflex's products been tested for mold growth and humidity?

All UL 181 Air Ducts and Connectors have been tested by Underwriters Laboratories, Inc. for mold growth and humidity, and are considered by the manufacturer as acceptable for use in conjunction with evaporative coolers. Please refer to section 3 Testing, Listing, Reporting and Certifying of the ADC Flexible Duct Performance & Installation Standards (Greenbook) www.flexibleduct.org for further information.

In addition to the UL 181 standards, Thermaflex meets Air Quality Sciences, Inc (AQS) requirements of ASTM Guideline D 6329-98. Testing of our duct work samples were conducted using static environmental chambers operating at normal and elevated humilities. Our duct was inoculated with two indoor molds and results showed that our material is resistant to mold colonization.

What is the difference between Air Ducts and Air Connectors?

UL (Underwriters Laboratories), in their 181 Standard for Factory-Made Air Ducts and Air Connectors, defines two categories of flexible “ducts”. The UL Listed Air Duct must pass all of the tests in the UL 181 Standard. Air Ducts are labeled with a square or rectangular shaped label showing their respective listing. There is no limitation on the length of runs when using UL Listed Air Ducts.

The UL Listed Air Connector must pass only a limited number of the UL 181 tests, and is labeled with a round shaped label, which states “for installation in lengths not over 14 feet”.

Does Thermaflex manufacture duct that can be used for dryer venting?

The International Mechanical Code states that exhaust ducts for domestic clothes dryers shall be constructed of metal and shall have a smooth interior finish. We do not manufacture any products that meet this code.

Thermaflex® is currently working to list our SLP, SLD, and STL as UL 2158A approved to vent both gas and electric dryers.

Can screws be used to fasten flexible duct core?

Thermaflex® does not recommend screws to be used to fasten the polyester core of the air ducts because they weaken the polyester. Polyester is a very strong material as long as there are no holes or tears in it. As soon as a tear or hole is introduced, the material weakens.

To maintain Thermaflex’s UL approval status, our air ducts must pass a tension test (25 pounds hanging from one end of the duct), a torsion test (one end rotated 180° or to 25 foot-pounds whichever comes first) and then a leakage test. In all of these tests, both ends must be connected to collars per our installation instructions. Flexible duct connected with screws would not pass these tests.

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