How much CFM do I need for my radon fan?

Not as much as you think

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.

Every house is different

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.   Charts and TDS sheets are normally found in the additional product photos section on each of our product pages.

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 PFEDK.fantech.app

Where is the air actually coming from?

The most overlooked part when people are trying to get “enough CFM” is: where the is air coming from.  In a perfect radon system, 100% of your mitigated air is soil air.  However, most systems are imperfect.  Many things can short-circuit your radon system: cracks in your foundation, rips in your barrier, shower pans, floor drains, etc..  If you begin drawing air from inside the home, then your radon system is no better than an open window.

Inside air being exhausted by a radon system is very inefficient and leads to a vampire drain on your HVAC system.  Many homeowners do not diligently inspect their utility bill.  One presentation I saw, from a well-respected radon mitigation instructor, empirically proved that 10′-20′ of a hairline foundation crack led to over $1,000 in extra utility bills for a typical Nebraska home with the radon system running.  Now look around your foundation and think of your average winter temps.  Are you removing air from inside?  Don’t.

A big fan also takes a big electrical current to run.  Have you read my post on fan power?  See here

In conclusion, collect data and ensure you’re using just the right fan size and nothing more.  A “Cinderella” system is better than a monster truck system.  Bigger is rarely better when it comes to radon fans.

Thanks for reading!