Abandoning under-slab HVAC ductwork which fills with groundwater seasonally.

Our company is building a custom home in Wyoming on a site with high ground water. The client wanted to have supplemental forced-air heating and air conditioning. The design called for "waterproof" direct burial ductwork which we placed in good faith and according to the manufacturer's direction last fall. After that, we shaded it with gravel and poured structural slabs over it.

This past spring, our site experienced high ground water and all of the major supply/return ductwork filled with water. We have looked into various means of repair, but none are proven or guaranteed. At this time we are moving ahead with a retrofit overhead conveyance system. I need to fill the abandoned ductwork with something that will prevent them from filling with water next spring. A major concern is the environmental/organic contaminant risk.

Does anyone have a similar previous experience?

Tags: HVAC, abandon, ductork, underground

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Homes in Willingboro, NJ were built that way 30-40 years ago using a ductile concrete pipe with ductwork openings broken into the pipe prior to pouring the slab.  If I were you and the homeowner, I would fill the ducts with a slurry-type concrete & sand mixture so that the ductwork piping was filled to the top of the interior slab. Unfortunately there is no way that I know of to seal the ductwork itself from moisture.

Public water systems regularly "reline" the interior of existing 50 year old water mains with an epoxy that is sprayed. This is usually done by a contractor that specializes in that work. It must be safe enough for public water systems !

We looked into this. Quite expensive and the applicator would not make any guarantees. Looking at  pumping cementious slurry to fill the voids. Have given up on trying to seal the ductwork.

That is what I was thinking. Looking into Flo Fill. Probably using two pours - the first would be at the source of the largest diameter ducts. Self-leveling pour as high as possible. Second pour would be from the register boots protruding through the slab and downward - hopefully meeting the top of the initial pour.

Thanks for your advice. I'll keep posting as we proceed.

In Kansas City a lot of "cardboard" for supply was used in slab on grade.  1950-1970 If very well drained - no roof leaks no waste water back ups, could still work.   Thats a lot of If's.    Last job I drilled to drain the "tube" in to the low spot  then put sheet of steel - drilled 3/8 rebar then poured  4" of grout to level the floor.   If in dry dry high area with PVC tubes this can work well but not must of time.

I have used black Max for supplies/return  under slab http://www.general-plastics.com/gallery but if ground water is close will steal the heat from PVC.   Also it will take some time for the smell to go from PVC glue  pre made system using SDR 81 pipe  thin wall.   I do have a large house with lowest level with PVC duct work for 25 yr and works fine with a system eff of 84% on Heat side or 16% of heat when 0' outside stays in ground with 4" of foam around it

All duct work needed to be in the building!  

My largest concern is moisture. I have seen mold forming on interior walls in the heating season wicking moisture from the concrete then blowing onto outside walls. In addition bacteria could also be an allergen that could coming into the home as well. If you see silt or standing water then there is a bigger problem. At CI our recommendation is always abandon and put in over head duct work. As the others have stated fill the existing ducts with concrete. I realize that some homeowners will not want to do this and we can't make them. But that is the correct way to go.

Usually, it’s something like that , but that great you’ve started that kind of project. Constriction are the most difficult, so that’s a bravery of yours. You could check for additional reinforcement bar’s coupling to make it more solid. If it's not solid enough for you - just add more concrete But it's for you to know that eventually it's gonna crush and you have to be ready for it. Like, mentally ready, because you will get frustrated with that but it will happen. If i were you, i would prefer doing it with http://hardman-de.com to make it more safe and solid

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