The short answer
A passive system uses the stack effect — warm air rising through a vertical pipe — to draw a small amount of soil gas out from under the slab. An active system adds a continuously running inline fan to that same pipe. Passive systems reduce radon modestly and inconsistently. Active systems reduce it reliably and by a large margin. If your home has tested at or above 4.0 pCi/L, you need an active system.
Side by side
| Passive | Active | |
|---|---|---|
| Driving force | Natural stack effect | Continuously running inline fan |
| Typical reduction | Modest and variable | Large and consistent, commonly 80–99% |
| Performance in mild weather | Weak — depends on indoor/outdoor temperature difference | Unaffected |
| Electricity | None | Roughly 40–90 watts continuous |
| Noise | None | Low hum at the fan |
| Maintenance | Visual inspection | Fan replacement every 8–12 years |
| Verification | Test required | Test required; manometer shows operation |
| Best for | New construction rough-in, marginal levels | Any home measured at or above 4.0 pCi/L |
Why passive systems underperform in New Mexico
The stack effect scales with the temperature difference between inside and outside. On a 25°F January night it is meaningful. On a 70°F spring afternoon it is nearly zero — and that is exactly when a passive system stops drawing. A passive stack also has no way to overcome the resistance of tight, compacted sub-slab soil, which is common across the Albuquerque metro.
When a passive system makes sense
- During new construction, as a rough-in that makes later activation trivial and cheap.
- In a home that tests between 2.0 and 4.0 pCi/L where the owner wants a first step.
- Where the design intent is 'ready for a fan' rather than 'finished system'.
Converting passive to active
- 1Confirm the existing riser is continuous, sealed, and terminates above the roofline.
- 2Cut the pipe at the attic or exterior location and install an inline fan with couplings.
- 3Connect to the junction box roughed in during construction, or add a circuit.
- 4Install a manometer and label the system.
- 5Run for 24 hours, then perform a 48-hour verification test.
Activation is typically a one- to two-hour visit when the passive components were installed correctly — one of the strongest arguments for roughing a system in during construction.
Related services
Keep learning
- How Radon Mitigation Actually WorksA mitigation system does not filter radon. It reverses the pressure difference that pulls soil gas into your home in the first place.
- What Radon Mitigation Costs — and What Drives the PriceRadon mitigation is priced on how hard it is to move air under your specific foundation — not on square footage. Here is what each line item actually represents.