The importance of 1.5″ wire pitch, heavier wire gauge and positive wire cast
The minimum wire helix in a Class 1 air duct is determined by UL-181 impact and collapse tests. The impact test requires dropping a 15-pound sandbag on the duct from a measured distance of 10” above the duct. To pass this test, the duct may not be crushed or deformed more than 20% of its total cross-sectional area. The collapse test determines if deformity occurs when a duct is operating under negative pressure. Together these tests reduce the possibility of duct deformation during installation, and provide adequate duct performance once installed.
JP Lamborn Co. believes that simply meeting the minimum requirements of these two tests does not produce a quality flex duct.
3rd party independent testing indicates that 1.5″ wire pitch (spacing), which creates a smoother inner core surface optimizing friction loss. A 1.5″ wire pitch also enhances a duct’s bending radius and maintains the integrity of the duct circumference. A wider wire pitch (over 1.5”) may constrict the inside diameter of a duct when compressed, bent or elbowed. JPL produces all of our flex duct core diameters with a wire pitch of 1.5” or less.
A quality flex duct should also be constructed with appropriate gauge wires so that the duct does not kink or distort when installed in 45° or 90° elbow. JPL uses six (6) distinct wire gauges in the manufacture of our duct core ranging from .041” to .065” in thickness.
In addition to proper wire pitch and wire gauges, the JPL manufacturing process also delivers another important feature to our inner core. The JPL process places a positive cast on the wire, which means that the wire helix in the finished duct expands outward and flat (not inward and compressed). Negative or neutral wire casts can cause a core to drop inside the insulation when removed from a finished package. A series of special pulleys and spindles deliver the wire to a mandrel that creates JPL’s unique positive wire cast.