Q&A: Under-Slab Heating Ducts & Air Quality

A.If water leaches into the ducts, molds can grow, particularly during the summer cooling season. As far as radon is concerned, as long as the blower is running, the supply ducts are pressurized and no radon can enter the system. Return ducts, on the other hand, would be prone to suck radon gas in. Also, if there are leaks, radon gas could enter the ductwork when the blower is off. To prevent this, the under slab should be well-drained, and the ducts should be well-sealed with a high-quality, foil duct tape such as Nashua 324A (Nashua Tape Products, 2600 7th Ave., Watervliet, NY 12189; 800/258-1342) or Polyken 339 (Polyken, 690 Canton St., Westwood, MA 02090-9167; 800/248-7659).

I have heard of some problems with termite treatment chemicals that were accidentally injected into the ductwork rather than under the slab. Many pest control operators will not treat a slab with heat ducts in or under it, because they could hit a duct when drilling through the slab. This can be easily avoided by starting the blower after the holes are drilled and before any chemicals have been injected; any holes in the ducts could then be easily detectable. There may also be a problem with chemicals leaching into the ducts with groundwater. Again, if the fill under the slab is well-drained, and the ducts are sealed, there shouldn’t be a problem.