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 cracked slab. Unfortunately, the solution to this problem is neither simple nor economical. To begin, it would be best to install a new air duct system, preferably in the ceiling, and the old duct system should be filled with concrete to prevent future water intrusion and ongoing mold infection. It would also be wise to improve the site drainage on your property, to eliminate ground water directly beneath the slab floor. A geotechnical engineer should be consulted for an evaluation of ground water conditions.

As to the advisability of furnace replacement, you’ll need to have the fixture evaluated by a licensed HVAC contractor to ascertain its general condition. Repair or replacement options and related cost estimates can be discussed with your HVAC contractor. And a professional mold survey of the unit would be an advisable precaution.

Dear Barry,

My bathroom has an exhaust fan in the ceiling, and I always thought that it vented to the outside of the building. Now that I’m selling, the buyer’s home inspector says that the fan is merely blowing into the attic, but that it does so at a screened exterior vent opening. Is this a problem, or does this venting arrangement comply with the building code? –Cindy

Dear Cindy,

According to the Uniform Building Code, a bathroom should be vented to the exterior by way of openable windows or by means of “a mechanical ventilation system connected directly to the outside…” At first glance, one would assume that “connected directly to the outside” would call for a vent shaft terminating at the exterior of the building. But building codes are subject to the interpretations of those agencies and individual bureaucrats empowered to affect their enforcement. When one considers the many new homes in which bathroom vent ducts terminate at screened attic openings, it would appear that many building authorities are opting for that interpretation. To determine the specific legal outlook governing bathroom ventilation requirements in your area, you’ll need to consult with your local building department.

To write to Barry Stone, please visit him on the Web at


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