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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