My house is on a slab. When it was built the ducts were metal tubes in the gravel that were covered with the slab. All the vents come together where a metal box was inset in the concrete where a downdraft furnace was placed.

That tin box is long since rusted and gone and ground water sits in the bottom of the gravel pit. There is a pump to eject any high levels in the rainy season but recently this water has a really bad odor. I figure if I pour dye down my sewer drain I can see if that is leaking under the house and mixing with the ground water. Should I not see any dye in the pit under my furnace, then I can assume it is the natural ground water with a foul odor.

My first question is is there anything I could do to stop the odor, and second, would reforming the rusted box with concrete stop the water coming into the pit? I’m sure the metal vents need a very expensive spin cast concrete lining eventually also, but I’m not having trouble with water in my vents. Just the pit causing a horrible smell.

Damn. That sounds like a HORRIBLE design. I'm guessing it would be a very large job to ditch the portion in the slab and reroute through the walls/ceiling?

In addition to the stink, I'd be concerned about mold.

Are these ducts still in use?

do you have good access to the entire "pit"?

If so, I would line it with either a PVC panels cut to fit and then "weld" them at the seams.
(in place repair)

Take measurements and cut the holes for the existing duct, then push in PVC tubes to fit and seal with butyl tape.

Both PVC and butyl are plenum safe for any "outgassing" and it being in the "slab" it is unlikely to pose any fire or noxious gas hazard from fire.

If it is not, that junction would require accurate "diagramming" in order to offer any other "fix".

If this or the vents are not in use, demo what you can, remove the vents, repair those openings and fill the pit with gravel and sand, then seal the top with vapor barrier.

Yes, it's a shitty design. I have a very similar design, except coated cardboard-like tubes under the concrete rather than metal. My house was built in 1972. In my case, when the concrete was poured they crushed the tubes. At some point the previous owners added electric baseboard heat to the rooms.

Technically it's called a downflow furnace, not a downdraft.

I found this out when I replaced the furnace (old one was non-operational). I wasn't getting forced hot air to any of the rooms. I hired someone with a camera snake to look at the branch lines. They were all crushed and no air would flow out to the rooms.

Since it was so expensive to heat with electric I abandoned the forced hot air furnace and added natural gas direct vent heaters. In essence each room is on it's own zone. Has worked great for the past 15 years. I ran gas line on the exterior of my house.

The heaters were a LOT less expensive when I installed mine, I think about $700 ea.
Williams Direct Vent Wall Furnace

Yes they are still in use. One neighbor who has the same model home put in an upflow furnace and had ducts installed in the attic. That cost him just over 10k probably 12 years ago. He pumped concrete into the old vents. That’s why I figured I’d try to restructure the pit and then have the ducts lined with concrete and then some sort of gum liner that lines the new concrete. Heck just the duct reclining is going to be over 10k. And I’m not sure I have the patience to pull out my furnace and try to build a pvc sink in there. I’ve never worked with pvc. I’d also hate to think what it would cost today to run everything through the attic.

First I need to pour some dyed water down my sewer to see if there’s a leak there. Any dye suggestions?

At the CE Firm I worked we had an Underground Utilities Locator service, smoke and dye testing of storm water and waste water systems.

This is one of the dyes used.
https://tinyurl.com/ycxjx6ne

FWIW, if you can measure and cut PVC (or plywood), you can do the liner.

(happy to coach you before your decide to try).

But you may want to get a "camera" inside look of the duct from the "box" to the vents to make sure they are not near failure, and also to see if there is any mold or other problems that contribute to the odors.

(My house is 85 years old, with a similar subfloor ducting that was "added" about 40 years ago. The floor is about 1 foot to 18" off grade making it about too much trouble for me to try and tackle at my age by crawling around under the house with fiberglass batting that has falling after the paper decomposed.

When it gets to the point I need to replace the HVAC/GAS, I will be going with mini splits and install them myself.

Until then, I can do all my own HVAC/GAS repairs at a fraction of paying anyone, and I know how to do all of it, have all the tools and equipment so I can keep it going until the ductwork finally fails.

quote:

But you may want to get a "camera" inside look of the duct from the "box" to the vents to make sure they are not near failure, and also to see if there is any mold or other problems that contribute to the odors.

In the San Diego area, I called a company that checks for leaks in drains to see if we had sewer water leaking into the slab. For a couple of hundred bucks they did a great job. Could use a camera I think for only part of the job; but it was well worth it.