The Physics of Active Soil Depressurization (ASD)
Radon-222 is a naturally occurring, odorless radioactive gas released by decaying uranium in bedrock and soil. It enters homes through micro-cracks in basement slabs, foundation block voids, and sump pits driven by the stack effect (warm rising indoor air creating slight negative pressure in the basement). Active Soil Depressurization (ASD) reverses this pressure gradient, creating a continuous vacuum beneath the concrete slab so radon gas is sucked into a sealed PVC pipe and exhausted harmlessly above the roofline.
Why 3" vs. 4" PVC Matters: Air Velocity and Noise
Both 3-inch and 4-inch Schedule 40 PVC are approved under ANSI/AARST standards, but air velocity makes a massive acoustic difference:
- Air Velocity Formula:
Velocity (FPM) = Flow (CFM) / Cross-Sectional Area (sq ft). - 3-Inch Pipe: In a high-flow gravel application (e.g. 90 CFM), 3-inch pipe forces air through at over 1,800 feet per minute, creating audible hiss and whistling through walls.
- 4-Inch Pipe: The same 90 CFM flows through 4-inch pipe at under 1,000 feet per minute, virtually inaudible to home occupants.
Reading the U-Tube Manometer
The clear fluid-filled U-tube mounted on your radon vent pipe does not measure radon gas levels! It measures differential static vacuum pressure in inches of water column ("WC). When both fluid columns are level at 0.0, the fan has failed or lost electrical power. A typical healthy system displays a differential between 0.5" and 1.8" WC depending on soil tightness.