Duct blaster test?
Ducts in ground - no matter what it is in cased in - will eventually rot out and leak like a very large open window... unless the ducting is fiberglass, epoxy or some kind of plastic. Metal duct work rusts out in no time at all. Worse is some of the "cardboard" ducting that apparently was used in the midwest -- then concrete poured around.
You really can't seal the ducts -- you can have them re-lined -- with a similar process that is used for re-lining large drain and sewage pipes. That does work - it is expensive - but it will also last.
Even oven fired clay bricks will act as great sponges and move a lot of water through them when wet - I would not depend on a clay back fill to keep ducts dry...
I don't understand Dennis's prediction that underground ducts will eventually fail so as to leak air. I agree that their thermal conduction and moisture performance should be questioned, but not their air leakage, unless there is something fundamental I am missing here.
Unless you use a plastic like material - they rot out. HVAC also is used for air conditioning. Older 60's, 70's, 80's houses weren't as careful about insulating the foundation, drainage, etc. Even some of the ducts which had concrete poured around them seem to fail overtime. Soil conditions and subsurface water eat away -- and the next thing you see is dust being blown around the inside of the house.
You can move the ducts inside of the house - but that means moving them into the unconditioned attic...
If it's humid - you need the air conditioning. A better choice would be filling ducts, and going with mini-splits. But that still leaves you dealing with floor -- possibly carpet, or tile.
If they insist on duct work - then relining, or perhaps mini-splits and, relining the ducts and use the old duct work only to distribute air from ERV/HRV using smaller ducts to each rooms...
It really sounds messy -- I do not like slab on grade...and ducts below.
I have seen in a 30yr old condo below slab PVC pipe ductwork had major problems with the galvanized metal boots or couplings connecting the floor registers cut into the slab floor. The boots simply rusted out so most of the air escaped into the gap between the slab and the ground soil.- nasty! And just think about the distribution losses from the rusted boots coupled with the uninsulated PVC pipe below ground temperatures.
Kevin, I agree, he would be wasting his money to seal the sub-slab ductwork.
There is only one appropriate retrofit measure for ducts below a slab: fill with concrete and relocate ductwork within the building.
I've put a camera in a sub-slab duct. It aint pretty. I don't care what the duct material is, it will still collect stuff. And moving conditioned air through it will deposit moisture. And there will be much rejoicing among the microbials.
Delivering breathing-zone air through sub-slab ducts is not something one should do to people you love. It's not something one should do to people.
What about Mold???
Moldy cheerios, mice poop, marbles and matchbox cars. M-m-m-m! Plus lots of cat hair if the house has cats. I'm not a fan of floor registers even if they are not associated with below-slab ducts.
When considering where the bottom (air barrier) of this house is, the clay soil is it because of the relative leakage of the ductwork. Without the duct system, the clay soil could still represent that due to leakage through the concrete slab at the perimeter cold joint, cracks and other penetrations like under a tub or pipes. The gravel patch under the slab allows all slab leaks connection to the soil and the gravel, so duct leaks would in theory would leak back into the house unless the slab is well sealed. Energy loss to the soil from unsealed and uninsulated duct could be huge, depending on where you are. Using a blower door and hole drilled through the floor into the gravel can inform you of the series leakage across the slab, and the impact of the operating system on subsoil pressures and house pressure above grade.
But my main focus is IAQ and the connection to radon. Others have mentioned the issues with moisture so I will note the connection to radioactive air. Both clay soils and gravel bodies can produce enough radon atoms to represent a significant lung cancer hazard. Leaky duct systems under floors next to radon sources are problematic. Some duct systems can be overcome, some actually provide dilution, and others need to be abandoned. Test for radon. That result may lead the decision.
Wow. The radon monitor can be considered a good one but it is always best to use other devices that can be calibrated to compare the results. Pro-sirens seem to have a life and after 5 years the data can get iffy...especially when you are bouncing around the smaller numbers. Time of year and whether the system was heating or cooling will also make a difference. Is this return or supply ductwork or both?
The fact that a radon system was needed, and installed and exhausting conditioned air is a significant part of the energy efficiency equation. This is a fairly simple energy penalty calculation the radon system installer should have supplied. Depending on the footprint and gravel volume and house stack pressure, a small 20 watt fan moving less than 100 cfm could provide the radon control and operate a weaker pressures. Many times a radon mitigator will install stronger blowers that move more air and operate at stronger pressures to overcome leaky situations and larger footprints, and overcome the resistance of tighter leaks. Pipe sizes used for the radon tend to limit the flow of the fans to between 80 cfm for 3" pipe and 150 cfm for 4" pipe. Most radon folks only use those pipe sizes and nothing larger. Still, since radon is an issue and the duct system will allow directly conditioned air to be exhausted without getting to the house, the energy loss calc could be used to determine payback on the sealing vs installing an above grade system.