When a home already has an active radon mitigation system, is it even worth testing for radon? That's a great question. To answer that, allow me to share a quick story.
Back when I answered the phones and did all of the home inspection scheduling for Structure Tech, I used to be in the habit of telling people that radon gas was mostly something that affected basements. I'd tell people that if they didn't have a basement, there was probably no need to test for radon. That was my message... until I got some doctor buying a slab-on-grade townhouse in Plymouth. He wanted a radon test no matter what. I told him it probably wasn't necessary, but he wanted the radon test anyway, just to be safe. I shrugged and thought to myself "It's your money."
I did the radon test, and it came back at 8.7 pCi/L. Yeah. That's over twice the EPA recommended action level of 4.0 pCi/L for a real estate transaction. Insert foot in mouth.
"An expert is a man who has made all the mistakes which can be made, in a narrow field." - Niels Bohr. I'm getting there.
After that experience, I stopped telling people that they probably don't need to test if they don't have a basement. I started saying that the only way to be sure of low levels of radon is to test. That's all. If you don't have a basement, your potential for high levels of radon will go down. If you have a Minnesota home that was built after 2009, it should have a passive radon mitigation system. Passive radon systems will lower the potential for high radon. We've found that about 20% of homes with passive systems have high levels of radon, compared to about 40% of homes without any system. See Radon in New Minnesota Homes.
Active radon mitigation systems
So what do I tell people about active radon mitigation systems? I tell them the same thing. The only way to know if you have a high level of radon is to test. To date, I've never found radon levels over 4.0 pCi/L at houses with properly installed, active radon systems running. If I were buying a home and the radon mitigation system appeared to be properly installed and functioning, I'd probably skip the radon test... but I'm a risk-taker. I may have a higher tolerance for risk than others.
To help determine if a radon mitigation system is properly installed, check out this checklist developed by the American Society of Home Inspectors (ASHI): Radon Checklist. Additionally, a few hardworking folks at ASHI have put together a proposed set of standards for inspecting radon mitigation systems. This standard has not been made official yet, but here's a draft: Radon Inspection SOP Draft.
The quick and dirty way to see if a radon mitigation fan is running is to take a peek at the radon pipe in the basement. If it's an active system, there should be a monitoring tube installed. This tube gets filled with a liquid which gets offset by the suction created by a fan. It'll tell you if the fan is running or not. If the liquid levels are even, there's a problem. If the levels are offset, that's good news.
According to the "authorities that be", you're supposed to test for radon every two years. You're also supposed to test your GFCI devices monthly. We all do this, right? ;-)