Renovation & Design

Heating ducts in concrete may present unique challenge

Question: I am hoping you can tell me how a repair can be made to the heat vents which are laid into the cement slab. I think this winter my house shifted in such a way as to crack a vent, which filled with water recently, after all this moisture. Needless to say, I was in shock, as I have had this house for 25 years and never had such a thing happen. Is this repairable? Thank you, Sandy Brow Thiessen.

Answer: Specialized repairs to heating ducts, especially those located in hard-to-access areas, may only be attempted by very experienced HVAC contractors. There may be several different methods for this repair, and only those with many years of direct experience will be the ones able to answer this question for you.

Repairing heating ducts embedded within a concrete floor slab may be a unique challenge, especially in our area. While there are a few older neighbourhoods where slab-on-grade homes were built, most houses have full basements or crawlspaces. The reason that slab homes are not common is due to our harsh winter and soil conditions. Because slab homes are literally built with the foundation on, or just above, the grade they are more subject to seasonal movement. Because of our extremely cold weather, our expansive clay soil may significantly swell in the spring, when the weather warms and the frozen ground thaws. This may vary from year to year, depending on the severity of the winter and the soil moisture content.

With a full basement concrete foundation, the footing support for the house is typically one to two metres below grade. While the soil outside the home may freeze this deep in a very cold winter, in some milder years it may not. The deeper the footing is, the less chance it will be affected by frost-related heaving, as the frozen moisture in the ground expands. Houses built right on grade, like yours, will have little protection from this movement. Also, if your grading is low around the house, moisture from rain and snowmelt will easily penetrate the soil underneath the perimeter of the home. The wetter the soil under this area, the more chances of seasonal movement of the home.

As you have recently experienced, excessive moisture from very snowy winters and heavy rainstorms may seep further underneath the concrete slab. While this may also be a major problem for homes with basements, they are equipped with a floor drain, and often a sump, to drain away any basement water. You probably don’t have any such mechanism in place to prevent the accumulation of water under the slab foundation. Unfortunately, embedding the heating ducts in the slab may have been necessary to install forced-air heat in our climate, but it can have very negative consequences.

While I have seen these older ducts with significant corrosion near the boots below the floor registers, specialized equipment would be required to fully investigate the condition of the entire duct system. This would likely begin with a scope of the ducts with a snake camera, often employed by rooter technicians and plumbers to inspect potentially damaged plumbing drain pipes. I suspect that your ducts may already have had some damage, simply from years of contact with the cold concrete floor slab and the soil beneath. However, most of this deterioration would have been hidden. If there are any areas that have major rust, holes, or damage, replacement would definitely be in order. There may be the possibility of installing a non-metallic liner inside the existing ducts, but I have no personal experience with such a system.

The main concern I would have with the recent flooding of your heating ducts, and possible pre-existing conditions, would be mold and other contaminants inside the ducts. If the water remains inside the ducts for more than a very short time, mold would likely begin to grow. It will use any dust, dirt, mouse droppings, or soil inside the ducts as a food source for the mold. By wetting this material, conditions would be ideal to support mold growth, with the byproducts able to circulate through your home via the HVAC system. This could be a significant health concern should anyone in your home be sensitive or allergic to molds. For that reason, I would immediately try manually removing the water, with a shop vac, utility pump, old towels, or any other method available. Once it is almost all gone, running your furnace blower continuously, and renting commercial dehumidifiers, will help dry the ducts further.

As far as removal and replacement of the ducts, that would present an extremely difficult challenge. The only way that would likely be possible would be by cutting the concrete floor slab, in several areas, and chopping out those sections. After removal, the damaged ducts could be pulled out and eventually replaced, once no more moisture was present under the area. After that, the openings could be filled in with new concrete, to patch the slab. This would have to be completed in a manner that would not compromise the structural integrity of the slab, which is the foundation of the house. Removal of too much concrete could subject the home to even more movement than usual. Hiring a structural engineer, prior to any work, would be a prudent choice to provide a plan for safe removal of any portions of the floor.

After getting the water out of the flooded heating ducts embedded in your home’s concrete floor slab, proper planning and experience by a professional structural engineer and very experienced, licensed HVAC contractor will be required. Only those professionals will be able to fully answer your questions, and devise a method for this extremely difficult repair.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and a Registered Home Inspector (RHI)( Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at


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