What CFM radon fan do I need?

So the useful numbers for a radon fan come in pairs: how much air it moves (cubic feet per minute, CFM) against how much resistance (inches of water column). That is why the guidance figure below pairs an airflow with a resistance.

Those figures describe the typical case, not your house. The fan runs all the time, so its power use is part of the running cost: fan systems slightly raise the electric bill, and most systems also lose some heated or cooled air.

The number and placement of suction points depend on how easily air moves through the crushed rock or soil under the slab and on how strong the radon source is. Often one suction point is enough. Those same two things decide how hard the fan has to work.

Contractors start with a visual inspection and design a system around the home's features. If that is not enough, they run diagnostic tests early in the installation. One is a soil communication test: a vacuum cleaner hose in one small hole and chemical smoke in another show how easily air moves under the foundation, and whether a given system will work.

1. 1

   Inspect

   Look at the foundation, what is under it and where suction pipes could go.

2. 2

   Test the soil

   Where needed, run diagnostic tests such as a soil communication test.

3. 3

   Choose suction points and fan

   Pick the number of points and a fan that can hold the pressure field; one state lists diagnostic testing to ensure the proper fan size as a normal step.

4. 4

   Seal and install

   Seal cracks and openings, which limits radon flow and makes the system more efficient, then install the fan in a vertical run of pipe outside the living space.

5. 5

   Test

   A post-mitigation test shows whether the fan choice worked.

Some states require radon fans to be installed by a certified mitigation contractor, and doing the work without the right knowledge can raise the radon level instead of lowering it. See do-it-yourself radon mitigation.