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Radon issue with crawlspace and no return ducts

Gba755 | Posted in Mechanicals on

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.

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  1. walta100 | | #1

    How many return vents connect the house to the crawlspace? When the fan is running how strong is the flow thru the return vents?

    Is there any HVAC ductwork or equipment in your attic?
    Leaky supply ducts in an attic would depressurize the house sucking in the radon.

    Got any thing else that might depressurize the house fireplace, wood stove, gas furnace, gas water heater, 6 burner stove with vent hood attic fan, leaky ceiling in a 3 story house?

    If you had your home blower door tested that could help pinpoint the leaks where radon enters the crawl space and where air leaks into the attic that creates the pressure differential pulling in the radon.

    You could install a return duct system but you would need to install a separate small HVAC system for the crawl space.


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