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Thread: Underground air return ducts for HVAC

  1. #1
    DIY Junior Member
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    Cool Underground air return ducts for HVAC

    I have leaking underground return air ducts outside the perimeter of the house that leak when it rains, inducing a musty odor in the house. I need a solution to replace/mitigate (flex hose?) the old ductwork. Replacement has been quoted at enormous sums and requires digging up patio areas and walkways. The entire vent is about 60 feet long and has three intakes along the course. The furnace/AC is located in an exterior room at the corner of the house (1950s style Adobe home). Air exhaust is through the ceiling. Due to the configuration and fairly new, energy efficient system, replacing with new modular A/C systems is not my first choice. Anyone have suggestions? One company told me his dad used to work on this type of setup and they used cement piping for underground sections. Vent from exterior wall into ground appear to be galvanized sheetmetal. What is underground here is unknown.
    Last edited by dslove; 08-06-2008 at 04:51 PM.

  2. #2
    DIY Hillbilly Southern Man's Avatar
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    Quote Originally Posted by dslove View Post
    I have leaking underground return air ducts outside the perimeter of the house that leak when it rains, inducing a musty odor in the house. I need a solution to replace/mitigate (flex hose?) the old ductwork. Replacement has been quoted at enormous sums and requires digging up patio areas and walkways. The entire vent is about 60 feet long and has three intakes along the course. The furnace/AC is located in an extrior room at the corner of the house (1950s style Adobe home). Air exhaust is through the ceiling. Due to the configuration and fairly new, energy efficient system, replacing with new modular A/C systems is not my first choice. Anyone have suggestions? One company told me his dad used to work on this type of setup and they used cement piping for underground sections. Vent from exterior wall into ground appear to be:confused galvanized sheetmetal. What is underground here is unknown.
    You might be able to line it with a thin walled plastic pipe, if you can find a large enough diameter. That way you would only need to dig up at bends and tees. This stuff here is fairly inexpensive:http://www.ads-pipe.com/en/index.asp

    They sell a flexible pipe in diameters 3-6" in smalll quantities, and up to 15" in larger quantities. Having to buy too much pipe might be your problem.

  3. #3
    Like an engineer alternety's Avatar
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    If you can find a plumber with a camera that can be pushed through the duct you can get some idea what is in there before you decide to push a liner through. It could get real annoying to get halfway through and find there is a partial collapse of the duct. You can also get accurate measures of where you have to dig for connections. Then maybe you could even precut holes in the liner.

    Thre are high pressure distribution systems that use much smaller pipe sizes. I believe that would be pretty pricey.

  4. #4

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    Most of the system is exactly the same. The Cold Air Return ducting is all absolutely standard, as is the furnace and the Warm Air Ducts.

  5. #5
    Moderator & Master Plumber hj's Avatar
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    Default duct

    Usually you can just look down one of the intake vents to see if you have PVC, galvanized, or concrete pipe. PVC would be the least likely to leak unless it was damaged after installation. The other two are extremely prone to leak and there is no good, or easy, way to repair them. A sewer pipe relining company could do it if there are no pipe size changes, but it would be fairly expensive for those size pipes.

  6. #6
    DIY Senior Member burleymike's Avatar
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    Here is a simple solution, you said the supply ducts run through the attic? Why not run a new return line through the attic? Of course you will need a tech to figure out what size you need and if the air handler needs to be up sized.

  7. #7

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    There's a relatively new process for fixing inground air ducts that have rotted out or have water problems. Check out www.rustyducts.org or www.ductsealtechnologies.com. They have a process that was developed just for this kind of problem. I have many homes in So Cal that have the exact same problem. Good luck!

  8. #8

    Default Underground Air Duct problems

    If anyone has problems with underground air duct systems that have deteriorated, filled with water, or simply rusted out, I have a system that can repair them successfully. We spray a latex-based liner material into the ductwork with a live video and most homes can be completed in one day.

    The liner is sprayed directly into the ductwork through the registers and plenum and the heating & air system can be run immediately after the spraying process. Its a great alternative to more expensive solutions such as remodeling or demolition of the slab.

    My website is www.Enviroductseal.com and I would welcome any questions that anyone may have. We have successfully been restoring galvanized, transite, AK and even fiberglass ductwork encased in the foundation.
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