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posted: 6/28/2013 10:47 AM

Water in forced air heating ducts

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Q. We have an old forced air heating system, with the air ducts in the ground below the slab floor. Every year, during the rainy season, the ducts become half filled with water. Sometimes the air from these ducts smells moldy, and my kids' allergies always get worse when this happens. We've tried using bleach to kill the mold, but this just makes things smell worse. We've tried pumping the water from the ducts, but they just fill up again. What can we do to solve this problem? Should we install a new heating system with air ducts in the ceiling, or should we keep the old furnace and just install new ducts? Please help!

A. Installing forced air ducts below the slab floor was a bad idea when your home was built. Most warm air duct systems are installed in attics or under wood frame floors in homes with raised foundations. When sheet metal ducts are installed in the soil, ground water is likely to penetrate during wet weather. This can lead to air contamination and related health problems, as in your home, and can cause the metal ducts to disintegrate from rust.

New air ducts in your ceiling would be a good solution, as you suggested. Whether you also need a new furnace is a question for a licensed HVAC contractor. If the system you have is very old, a newer one is likely to operate with greater efficiency. More importantly, the old furnace should be inspected for conditions that might be unsafe.

The mold issue is also something that warrants professional attention, especially if your children have had allergic reactions to the moldy air from your wet ducts. Hire a certified mold expert to take air samples and material samples for lab analysis to determine if any remediation is warranted.

Finally, if ground water under the slab floor is flooding the ducts, there may be a site drainage problem on your property. You can hire a drainage specialist to determine if improvements are needed.

Q. We have an exhaust fan in our bathroom ceiling, and we thought that it was vented to the outside of the house. Now that we're selling the house, the buyer's home inspector says the vent duct blows into the attic, right next to a screened vent opening. Does this need to be fixed, or is the method of venting acceptable?

A. According to the building code, a bathroom should be vented to the exterior by way of openable windows or mechanical ventilation system and "shall be exhausted directly to the outside." That description, " ... directly to the outside," sounds quite definite. Nevertheless, building codes are subject to the interpretations of state and local building officials, and many municipal building inspectors approve bathroom venting that is installed the same as in your home, with the duct terminating at a screened vent in the attic. To determine how this installation measures up to local building standards, you should consult one of the inspectors at your local building department.

• To write to Barry Stone, visit him on the web at, or write AMG, 1776 Jami Lee Court, Suite 218, San Luis Obispo, CA 94301.

Action Coast Publishing

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