Does air leak in/out of HVAC ducts located under a concrete slab?
The supply HVAC ducts in my house are under the slab (flex return ducts are in attic), which I know most building scientists recommend against. The supply ducts are made from 10′ long sections of PVC pipe which connect to galvinized steel floor outlets that are generally located under windows on the perimeter of the house. As part of my house air sealing project, I applied duct mastic to the seams of one the floor outlets and by reaching my hand down to where the PVC connects to the galvinized steel outlet. By liberally applying the mastic to the seams which I can feel (but can’t see), I think I have a pretty good seal at this junction of the pipe and floor outlet.
Recognizing that there are several other joints in the PVC tubes between the floor register and the furnace plenum that are impossible to access, is there enough possible air leakage from a floor register & PVC duct imbedded in/under a concrete slab to be worried about?
I will mention that I have never had any water in these ducts (common problem with under-slab ducts) in 20+ years and the area I live (Oklahoma City) is supposed to be very low risk for radon gas. I also purchased/used a radon gas tester recommended by Consumer Reports and the levels were very low.
My wife thinks I am nuts for spending time sealing these floor registers. Is she right?
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Replies
I'm with your wife, may I suggest yoga?
Pat,
If you haven't noticed any moisture in the ducts over all these years, you probably don't have to worry about moisture and mold.
Is it possible that your duct system isn't airtight? Of course it's possible.
One other point: no one should use PVC for supply ducts on a forced air furnace, because of concerns that the warm PVC will off-gas fumes.
It doesn't sound like you have a good duct system. You have two options: relax and don't worry about it; or replace your current HVAC system with a couple of ductless minisplits.