Our home is slab-on-grade and some of the air ducts are under the slab.
Yesterday we detected a moldy/musty smell, and traced it to one of the in-floor supply grilles. I don’t think we were running the heat or the fan at the time (no AC). I removed the grill and discovered two bad things:
a small amount of standing water there
the duct itself seems to be made of cardboard tube
I theorize this is a kind of freak temporary thing because the slab/soil is unusually cold compared to the outdoor air. For reference, we had snow on the ground a week ago, and its been in the 80s for the past few days. This is a very dry climate & we haven’t had such a problem before.
So:
-is my theory plausible?
-any corrective measures until the soil and air come into closer equilibrium?
-anything to do about the larger problem of having cardboard ducts under the slab that get damp once in a while?
Yep,not only plausible but likely.
Can you run the fan solely,to circulate air?
They may look to be cardboard,but most such ducting isn’t.That aspect I wouldn’t be concerned about if you can rectify the standing water.
This could be condensation but it may also be water under the slab from the snowmelt. I think water under the slab is more likely. Depending on how the house was built, the drainage may be poor. I’d get a professional opinion including a test of the gravel bed under the slab because over the long run it will be a recurrent problem. You may need a sump or some other remedy.
Cardboard tube? Not good. Does it degrade with moisture? Potential problem.
It’s possible that your duct has collapsed in the slab, something that not’s entirely uncommon. (although your duct is almost certainly not made of cardboard; maybe clay based on the age of the house)
It’s more common that it is condensation from your evaporator coil/pan, especially if the water is in close proximity to the furnace. If it is, this is a refrigeration problem, and it may be a dirty coil, dirty filter,leaking evaporator pan, low of Freon, etc. Have you been running the A/C? Did the problem surface only after the A/C had been running?
I’d be hard pressed to believe it’s snow melt—snow melt underneath 4+ inches of concrete, and 4+ inches of gravel and several feet (at least) from the perimeter of your house. (!) I would imagine that for this to be true there would be standing water all around the perimeter of the house.
Do you know the quality of your drainage tiles, or how close your foundation is to the water table? Are you in a flood zone?
I would call a qualified HVAC technician and have your A/C system serviced and have them inspect the evap coil and evap drain pan.
Yes, I did that as soon as I noticed the problem, but it seemed to cause more condensation. I stopped, because I assumed I was bringing in hot outdoor air and running it through the cold ground, amplifying the problem.
The area in question is quite some distance from the furnace (horizontally and vertically - the furnace is in the attic). There is no A/C or refrigeration of any kind.
Actually the duct in question runs along the perimeter, but I don’t think it’s snow melt either. Although we did have above-average snowfall this year, the soil is very dry in general. I think flooding is basically unheard of here.
I remove the in-floor grille, and right there beneath the floor is a ~8" horizontal (apparently) cardboard tube. It doesn’t appear to be collapsed in any way. I reach in, and it’s thoroughly damp.
KLG,don’t give up on running the circulator either alone or with some heat.I believe the situation is caused by a lag in the slab warming to ambient,the reverse of what happens with radiant heated floors when cooling is desired.
There is a mediation method of stabilising slab temperature involving vertically placed foam board around the perimeter of the slab, but I can’t provide a link as I read about it in a builder’s periodical some years ago. If this is a chronic problem you might look into it.