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What is a passive radon system?

Passive radon systems or “radon-ready” systems—as they should be called—are pre-plumbed radon systems for a building. They do not function unless there is a radon fan added to them*. This is why it is my personal mission to remove the term “passive” from the nomenclature. It implies that the system works as is. If you have high radon levels, you’re gonna need a fan no matter what. See this new home my good friend built. He was under the impression that he had a radon system and almost didn’t test! Good thing he knew me. We tested his passive radon system and he had a whopping 11+ pCi/L, nearly three times the EPA action level! He could’ve lived decades in that home thinking he was safe.

Always perform a radon test on a passive home. This is why EPA recommended testing is important and can save your life. You don’t test a home once; you test at minimum every five years.

Why should you build a radon-ready home?

There are many benefits to building radon ready. In my opinion, the biggest benefit is: no loud, ugly radon fan on the side of your house. Yes, we know they’re loud, we know they’re ugly, but they’re better than stage IV lung cancer.

In Zone 1, high-risk states like Colorado, Iowa, or North Dakota, your chances of having dangerous radon levels are higher than having low levels. So if you’re a gambler, you’re betting against the house: literally. Even if you don’t have high radon levels, many homeowners choose to add a low-wattage fan to the system for the benefit of increased airflow under the slab. A dry slab is good for the home’s foundation and it can eliminate musty basement smells.

Other benefits of radon-ready homes are: more energy-efficient radon fans can be installed since you have perfect PFE; smaller fans are quieter fans; fan location is often in garages or attics away from occupants–again quieter; increased home value from having a radon system; and again, radon-ready systems are much more cosmetically appealing than post-construction radon systems.

Passive Radon-ready systems are the future of homebuilding. Don’t get left behind, ask your builder or realtor about radon-ready homes today! PDS offers free system designs at

*Passive radon systems can function in very specific weather conditions due to the stack effect. In North America, these weather conditions do not exist 12 months out of the year, which is why I believe the term is dangerous. If you insist on trying to use your system passively, I cannot recommend a continuous radon monitor more. Watch your radon levels fluctuate with weather patterns and see for yourself.  Passive radon systems include those called out in IRC Appendix F, ANSI/AARST RRNC 2020 and AARST-ANSI CC-1000-2018

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How to choose an energy efficient radon fan

Radon fans are already some of the most energy-efficient air movers in the marketplace.  However, a radon fan runs constantly, 24/7/365. Even the most efficient motors are going to use a lot of energy over their lifetime, so it’s in your best interest to choose an energy-efficient radon fan.

The first and most important way to get an energy efficient radon fan is to have it sized properly.  If you read my post on CFM and PFE, you’ll know that the marketplace drives radon pros to “poke and hope” in order to keep upfront costs low.  “Poking and hoping” may fix a radon problem, but it’s often going to do it with an oversized radon fan.  We’re talking 100+ watts versus 20-60.  Over the lifespan of the fan (5~15 years) that can mean hundreds of dollars in energy usage.

The next thing that an oversized radon fan can do is exhaust conditioned air from the home.  That’s air you paid for!! Radon fans are going to pull from the path of least resistance, and sometimes that means pulling from a tear in your crawl space barrier; or a hairline crack in a slab; or an expansion joint; or a poorly sealed sump; and so on, and so on.  When a large radon fan is moving 100+ cubic feet of air per minute and half of that is coming from inside your home, what do you think that does to your energy bill?  Your HVAC system has to replace that air and condition it to hospitable temperatures.  This is where the real energy costs come into play.

Case studies from leading radon professionals and educators show that merely sealing hairline cracks in a foundation on an average-sized single family home can save over $1,000 over the life of the radon fan.  If you choose a licensed radon mitigator that performs proper PFE testing and radon fan sizing you can’t go wrong.  The upfront cost of using an educated pro will be made up 10X over with the efficiency in your system.

Leading fan manufacturer, Fantech, has also introduced EC motors to the radon marketplace.  EC stands for electrically-communicated.  These motors operate more similarly to DC motors than AC.  They have built-in speed controls and run about 20% more efficiently than AC motors with similar fan curves.  With the speed control, you can also “dial in” the fan to exactly what your home needs.  A radon professional in Minnesota recently used an EC fan to depressurize a home using less than 5 watts of electricity!  EC radon fans can be twice the price of AC fans, however, over the lifespan of the fan you recoup this costs and then some.

For the most energy-efficient radon system possible: use a radon professional that specializes in PFE testing, proper system design, and sealing; and also uses Fantech’s EC fans.  Be sure to sign up for a professional maintenance plan or follow my guide to maintaining your system to keep it running efficiently for decades.

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How much CFM do I need for my radon fan?

We get this question a lot. The true answer is 1+. If you’ve depressurized your soil under your home, you’ve typically solved your radon problem. However, that’s not really what people are asking when they call. They want to know, “what radon fan should I buy?” Without doing PFE testing using professional equipment (pressure field extension), you can’t size a radon fan properly, and radon fan sizing is a complicated process. 

You see, in other industries that use in-line fans–like HVAC–fans work with known inputs and outputs. Duct size, wattage, amperage; all these things can determine a fixed CFM that a fan will run at to do its job. In radon mitigation, there are too many variables to post a consistent CFM. What is your radon level? What are your soils like? Are you using 3” PVC or 4” PVC? Are you connected to a perimeter drain, a crawlspace, a suction pit, or something else? The same fan in any one of these configurations will pull a different CFM every single time. This is why in radon we talk about the fan curve and pressure field.

A fan curve is a chart of airflow versus pressure. The greater the pressure (resistance) the lower the airflow. I like to use the “milk shake versus beer bong” example. A milkshake is thick and hard to pull (high resistance), whereas us college grads know the beer bong is fluid and easy to pull (low resistance). That’s kinda how the fan curve works. Using a manometer like the U tube, or digital professional grade micromanometers, you can estimate your fan’s current CFM by plotting it on a fan curve chart provided by the manufacturer. These charts are also listed on our How to Guides and Data Sheets page.

However, even if you know the CFM, you’ll still need to know how far your pressure field extends to know if you’ve gotten rid of the radon problem. Many radon pros can provide pressure field extension testing these days, however few provide it in every job due to the cost-competitive nature of the business.  It’s cheaper to “poke and hope” and provide a radon-reduction guarantee than it is to do extensive PFE testing on every home.  Ask your NRPP licensed mitigator if they can do PFE testing for effective radon fan sizing that will get the most economical energy use.  You may pay more up front, but having a more energy efficient radon fan will save you in the long run.  You can dive into the specifics of PFE testing by watching the instructional videos at