What are the problems with underground return air ducts?
Wednesday, October 9, 2019
Under-floor-slab return air ducts were briefly popular in our North Florida area during the late 1950s and early 1960s. They were usually in upscale, midcentury modern homes. We guess it was considered cutting edge technology at the time, but now these old underground ducts are a liability for two reasons:
- Corrosion and settlement have created openings in the ducts over the years. Because the return air is a suction, it pulls any gases and moisture out of the soil. Several areas around Gainesville and Ocala have elevated radon in the ground, and the ducts essentially suck the radon into the air flow, elevating interior radon levels.
- During the wet season, when the water table rises, we have inspected houses with standing in these ducts.
The ducts terminate directly under the furnace/air handler in most homes. So they can be abandoned and sealed at the floor level, and the return air rerouted to the attic—except that many midcentury modern homes are flat roofed and have no attic. The solution then is typically adding a soffit along the ceiling to enclose the new duct.
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