The HVAC “return” in our house is actually the crawl space – no ducts at all! The crawl space is unvented, sealed, and generally below ground so I can see why they might have considered this acceptable – except that we have a radon issue in the basement that the mitigation system cannot fix.

The radon levels spike into the 30s when the HVAC is running. This is a vacation house so the thermostats are dropped back when we are not there and the radon results reflect this.

We have spent a lot of time in the crawlspace fixing where the radon barrier failed and trying to ensure a tight seal, plus we’ve tried larger fans (big house, so there are two fans), but no luck. I think the system was generally well designed (two fans, perforated pipe embedded in gravel and connected between the two fans).

So now I’m kind of stuck. I guess I could perform a vacuum test to see how well the mitigation system is working, but maybe the real answer is to add return sealed return ducts so the crawlspace isn’t depressurized whenever the HVAC kicks on. 

Looking forward to your thoughts, I really enjoy learning about these systems on GBA.