There are a number of potential problems when ductwork is installed under a slab. Perhaps of all the issues, the biggest is with water infiltration. A lot depends on the gradient where the home sits but water can run under the slab, slowly infiltrating the concrete that the duct is imbedded in and then into the duct itself. This process can take a long time but eventually the ducting rots out, damaging this extremely important component of a HVAC system.
In-slab ducting installation:
A series of trenches are dug following the details on the design drawings. A series of wooden blocks are set in the bottom of the trenches and sections of duct are set on them. Concrete is then poured into the trench, often the concrete does not fully encapsulate the duct and it sits directly on the soil. Once this is done the actual floor slab is poured. The majority of underground ductwork uses spiral or locking type steel tubes, when they are exposed to water they eventually rust and deteriorate.
Signs that there is a problem:
Water attracts mold, mold has a very distinctive odor, when this odor is noticeable it is fair to say there is a problem with the ducting system. There will also be evidence of rust around the registers and if the deterioration is bad enough there will be a noticeable decay in the air flow into certain parts of the house.
Correcting the problem:
Some contractors are of the opinion that the best solution is to simply abandon the underground ductwork and install new ducting above ground. This may be a practical solution if the problem is severe but may not be the most economical solution if the problem is isolated. Once the source of water is located and stopped it is possible to engage specialist companies that can restore deteriorating ductwork to better than new condition.
