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What are transite ducts?

Transite ducts consist typically of 10- to 12-inch round PVC tubes or pipes that run under a concrete slab in the ground to provide heating and cooling to a house. Typical transite, or "in-slab" ductwork consists of a single long tube that runs the perimeter, along the outside walls. While most ductwork relies on upflow furnaces, which push air upward toward the ducts to heat the home, transite ducts, because they are located in the floor, rely on downflow furnaces, pushing air downward into the underground transite ducts.

Why do transite ducts get so dirty?

That these ducts are located underground makes them ripe for many problems. Though the ducts themselves are made of PVC, the elbows (for whatever reason) are made of metal, which, in the moisture-rich underground environment, leads them to rust. And often, as a result of this rusting, they separate. Once the elbows have separated, the ducts themselves are vulnerable to the entry of sand, soil, and most detrimentally, water.

Also, because of their position in the floor (unlike traditional galvanized sheet-metal ducts, which are often situated in the ceiling), debris frequently falls into the vents or gets swept inside. And because all the vents are connected (rather than a trunk-and-branch system), something dropped or spilled into one vent can easily spread into the others. Further, the environment below ground is very damp, and that creates a haven for insects that favor a moist, cool environment. Our technicians have found all manner of insects, including centipedes and spiders, and even "balls of flies," dead and clustered together, during the cleaning of transite duct systems. By contrast, in typical sheet-metal ducts we find dust, debris, dander—dry stuff, basically—but in transite ducts, discoveries within tend to be much nastier, including mold.
 
What is the best way to clean transite ducts?

The location of transite ducts underground and the all-too-frequent separation of the elbows leaves the system vulnerable to penetration by sand, dirt, and water. This moist, muddy residue is impossible to clean with air pressure and suction alone (the primary method of most standard air duct cleanings) and requires more firepower, so to speak.

Even when transite ducts are intact and the elbows are free from rust, because of the way the system is designed (a large, long pipe with holes in the top serving as vents), they are a challenge to clean effectively. In a standard air-pressure cleaning, with the technician on one end of the system, blowing to the other end where the vacuum is (with venting in between), the vacuum suction is severely diminished. Additionally, the debris being pushed from one end of the system toward the other is liable to simply fly out the other vents. Even with the vents covered, when only air pressure is utilized, the space between vents is great enough that some of it would stop short and come to rest in the middle, between the two vents. 

For these reasons, the recommended method of air duct cleaning on a transite duct system would be a high-level cleaning utilizing an extreme agitation tool, such as the Viper Clean Sweep or a cable brush down every vent. The technician would begin by finding the "end point" of the system, attaching his vacuum at that point, and covering every vent he is not working on. Then, starting at the other end of the system, he would insert the agitation tool and work the debris toward the vacuum, proceeding from one vent to the other in a systematic manner (again, leaving all vents covered except the one he is working on). At times it will be necessary to use a shop vac on some of the heavier debris (concrete-chunk remnants from construction, etc.) that tends to cluster immediately beneath the vents.

A final note: if the system has been penetrated by water, which is a frequent occurrence, the water needs to be removed first before any cleaning can begin, and an assessment should be made for the presence of mold. A restoration company should be consulted, so that the water (and possibly mold) can be removed and the system repaired, or even replaced.

Wanna dig deeper? Download our free tipsheet: 10 Questions to Ask Before Hiring an Air Duct Cleaning Company. 

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Many thanks to our technicians Roy S. and Ben S. for lending their expertise to this article.