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: 

1. 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.
   
   
   
2. 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. 

    Also see Frequently Asked Questions (FAQ) about Air Conditioning Troubleshooting.

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  To learn more about heating and air conditioning systems, see these other blog posts:

• What is the average life expectancy of house a/c and heating ducts?• How can I find out the SEER of my air conditioner? 

• My air conditioner won't turn on. What's wrong? 

• How can I find out the size of my air conditioner? 

• How can I tell whether the condenser (outdoor unit) is an air conditioner or heat pump? 

• Where is the air filter for my central air conditioner and furnace? I can’t find it? 

• Does an old air conditioner use more electricity as it ages? 

• How did homes stay cool in Florida before air conditioning? 

• What is wrong with an air conditioner when the air flow out of the vents is low?

• Why has the thermostat screen gone blank? 

• Why does it take so long to cool a house when an air conditioner has been off for a while? 

• Why is my air conditioner not cooling enough? 

• What are the most common problems with wall/window air conditioners?  

• Will closing doors reduce my heating and cooling costs? 

   Visit our HEATING AND AIR CONDITIONING and RADON pages for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.