Related Content: Understanding and Mitigating Manhole Explosions: An Interview with Stuart Hanebuth This should be of significant concern to engineers. When combined with the shrinking and sagging factors, the performance numbers for duct putty seals that start out relatively weak to begin with, tend to only go in one direction over time. Figure 4Īs evident in the “after” image, neither duct putty could hold water once they start to sag or shrink. Note that the “after” image shows both putties having sagged or shrunk and neither could support the short piece of cable in the duct. Figure 4 shows the before and after images of this test. Then the duct putty seals underwent accelerated aging for two weeks at 212☏ (100☌). Two different duct putties were tested and their results are seen in the infographic (figure 3) below: Figure 3 A column of water was then poured over the top of the seal, increasing by one inch (2.5 cm) per day until the seal failed. To test seal performance, a one-inch (2.5 cm) plug of putty was fitted into a two-inch (50 mm) PVC conduit. How strong is the bond between the putty and conduit wall or cable jacket? How much water head will a duct putty seal hold? How many psi of gas pressure will it resist? The answer to these last two very important questions, unfortunately, is not much. Here the question becomes, how good is a duct putty seal? That question leads to others. Another serious consideration is the performance of an otherwise intact duct putty seal that has not yet sagged or cracked. The development of gaps and cracks over time is only part of the problem. Related Content: Duct Seals Prevent Ruinous Water Damage.ĭuct putty seals are often temporary at best. Wire movement causes yet more gaps (see Figure 2). Putty also tends to settle under the crushing weight of cables, opening gaps above the cables. This slow flow manifests itself in sagging over time, which is another way for the material to separate from the top of the conduit wall and break seal integrity (see Figure 1). While it may move very slowly, putty does flow under the force of gravity. Sometimes the shrinkage results in cracks in the body of the seal itself, again breaking the seal and allowing ingress of water, gas, and other contaminants.Īnother problem is that while the volatile content is still present, the putty is essentially a thick liquid. The shrinking sealant pulls away from conduit walls and/or cable jackets, opening a pathway for water, gas, dirt, insects, snakes, rodents, fire ants, etc. Too often, initial installation results in no seal at all.Įven with adequate installation, when duct putty’s volatile content evaporates, it causes shrinkage that results in gaps. Even a rudimentary seal is difficult to craft. It is very sticky and hard to manipulate. Putty comes with no instructions on how much to use or how to properly seal a conduit. Related Content: Is Meeting a Standard Enough?-An Examination of Duct Sealing Code Compliance.Īpplication is one cause of failure. Specifiers and engineers should understand that this is problematic for reliable cable installations because duct putty fails as a long-term sealant solution. Certainly, it is frequently used as such. Despite this, duct putty is often recommended for and marketed as a conduit sealant. In the case of duct putty, the UL mark is generally for sealing junction boxes, not conduit entrances containing electrical or communications cables. It is important to note that a UL mark does not make a product suitable for a purpose other than the one it’s designed or marketed for. Many commercially available duct putties are UL approved.ĭuct putty is not, however, an excellent conduit sealant. It seals gaps and cracks and works well to reduce drafts, moisture, dust, insects, and noise. Uses in the electrical and communications markets include sealing around electrical boxes, flashings, junction boxes, pedestals, service mast entries, etc. It is also commonly used in the refrigeration and plumbing trades. It can be easily worked by hand to form an infinite variety of shapes that are pressed into place to seal HVAC ductwork-thus its name as a duct putty. Variations include mastic or oil and wax blends. Though formulations obviously vary by manufacturer, duct putty is typically a thick and pliable inert commodity made of clay and water. This article reviews duct putty and analyzes specifically why its use as a duct sealant can be problematic. Unfortunately, this seemingly ideal sealant too often falls short of expectations, resulting in expensive damage that far outweighs any perceived convenience or cost savings over high-performance, engineered sealing solutions. It ostensibly allows cable installers to comply with industry codes mandating that conduits are sealed. The product is cheap, safe, easy, and ubiquitous. Duct putty is arguably the most common product used to seal electrical and telecommunications conduits worldwide.
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