Beyond the typical HVAC service checks for your customers, you might need to tackle issues with sub-slab HVAC ducts. In contrast to in-slab ductwork built into a concrete floor, sub-slab ductwork is below the slab. Here are some of the common issues you may encounter while assisting people who reach out to your HVAC business.

Concerns Surrounding Asbestos With Sub-Slab Ductwork

Discussions about sub-slab ductwork often contain the word “transite.” A common misconception is that if ductwork is the sub-slab variety, it is also transite. However, that’s not necessarily the case. Transite is a previous brand of cement-asbestos products and pipes from Johns Manville that came onto the market in 1929.

Although transite ductwork was obviously popular in the 1960s and 1970s, most of the sub-slab ductwork used today is not the same. It contains PVC materials instead of the cement-asbestos mix.

However, due to how some HVAC contractors using the words “sub-slab” and “transite” interchangeably, you might find that some clients unnecessarily fear they’re getting exposed to asbestos. A telltale sign of true transite ductwork with a cement-asbestos mix is ragged edges. This means it is dangerous to have the ductwork cleaned because the process could release tiny particles of asbestos into the air.

Besides the potential for transite ductwork to leak or become moldy, the material normally contains between 15 to 25 percent asbestos fibers. These fibers could be released into a building whenever the HVAC system is switched on. For that reason, its best to recommend sealing off that ductwork and installing a new HVAC system elsewhere.

Water Entering the Sub-Slab Ductwork

Another common issue with sub-slab HVAC ductwork is that it often floods. The moisture can pull millions of mold spores into a home or business as the air moves through the building, which can pose extreme health risks to building occupants

Why does this kind of water buildup happen with sub-slab ductwork? For starters, soil contains a tremendous amount of water. Moreover, the liquid often moves sideways instead of down. Water failing to move downward is especially problematic in structures built on soil containing a substantial amount of clay.

Now, how can you help homeowners and businesses remedy the issue? One way is to build a trench around the building and put gravel and a perforated pipe inside. It’s easier for water to pass through gravel than soil, and when it does, it will flow into the perforated pipe and away from the sub-slab ductwork.

Difficulties With Repairing Sub-Slab Ducts

Because sub-slab ductwork is under concrete, it can be difficult to assess problems without destroying the slab. Sometimes, that means issues can be present for a while without homeowners or building owners noticing something is wrong.

When sub-slab ductwork gets rusty, it can become weakened. Some homeowners or building managers initially notice the problem if there are visible areas of the ductwork passing through the slab.

Two of the more labor-intensive methods to deal with this are to stop using the sub-slab ducts and run a new duct system elsewhere. You could also cut into sections of the slab and replace the damaged ductwork with a more rust-resistant option.

Lastly, it is also possible to re-line the ducts with a zinc-oxide filled coating. The substance covers the rusted areas and fills holes. Although it sounds labor intensive, it is still not a complete duct system overhaul, and your customers will thank you for any costs saved in the process.

However, thoroughly investigating the problem is the ideal approach to take before proposing any type of solution as a completely new duct system may be required — especially if you find true transite. 

If you do need to replace a concrete floor after performing an HVAC ductwork repair, always instruct the customer to let it cure. Concrete gets progressively harder with each passing day. For example, it’s at about 70 percent of its strength after a week and considered fully hardened after 28 days.

Radon Coming Into the Building Through the Ducts

Radon is a radioactive gas that results from decaying uranium in rocks and soils. It’s not a health hazard outdoors because it disperses so quickly. However, the Environmental Protection Agency (EPA) warns that radon gas is a top cause of lung cancer. It’s particularly a concern in nonsmokers.

Radon often becomes a problem indoors after it seeps in through cracks in the foundation. Professional testing can confirm if levels are too high.

Additionally, sub-slab ducts can introduce radon inside if mud gets into them. The approach, then, is to stop using the compromised ductwork and install new ducts at a higher point in the building. However, the design of some structures makes that option impossible. If that’s the case, installing HVAC units in each room may be unavoidable.

Offering Feasible Solutions

The problems covered here can cause significant challenges, making them worrisome for homeowners and building owners alike. However, you can help customers navigate these sub-slab ductwork scenarios by educating yourself on the most effective fixes that will leave you and your customers breathing easy.