Heating-system placement wipes out mold

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Dear Barry, My home has a forced-air heating system, with ducts installed beneath the concrete slab floor. When it rains for several days in a row, the vents get wet, and some have standing water in them. This has been bad for our family's allergies, and on occasion I can smell mold. I've tried bleach to kill the mold, and one year I had to use a small water pump to drain one of the vents. How do I address this condition? Could the drainage problem be caused by a crack in the slab? Should the vents be moved to the ceiling? Do you think I need a new furnace? –Barbara Dear Barbara, Installation of heating ducts beneath a slab floor is neither practical nor prudent. It exposes the warm-air distribution system to seasonal ground moisture; it subjects the ducts to water-related damage; and it can promote environmental health problems for occupants. Flooding of air ducts, such as you describe, is typically a matter of ground water infiltration and is not likely to be caused by a cr...