Radon Evacuation System: Active Suction Mitigation Guide
The phrase radon evacuation system is an alternative name for what most contractors and EPA documents call a radon mitigation system. The “evacuation” terminology emphasizes the active mechanical principle: drawing soil gas out from under the foundation before it can enter the home. This is fundamentally different from passive venting, which relies on natural stack effect alone, or from indoor air filtration, which tries to remove radon after it is already inside. An active evacuation system uses a continuously running fan to maintain negative pressure under the slab, producing reliable radon reduction that passive methods cannot match. This guide explains the active-suction principle, compares it with alternative approaches, and walks through the design choices that distinguish a quality evacuation system from a marginal one.
This guide summarizes EPA, CDPHE, and CDC guidance current as of 2026. Consult a certified radon mitigation professional for design and installation decisions.
The Active-Suction Principle
The physical principle behind every effective radon mitigation system is creating a pressure differential that draws soil gas away from the home instead of toward it. Without intervention, soil gas flows toward the home because indoor air is typically warmer and at lower pressure than the soil gas, especially during winter when the stack effect pulls air upward through the building. Cracks, joints, and porous concrete provide entry pathways. The result is steady infiltration of radon-bearing gas into the lowest level.
An evacuation system reverses this dynamic. A fan creates a small but persistent negative pressure under the slab, on the order of one-quarter to one inch of water column. The pressure differential pulls soil gas toward the suction point rather than letting it diffuse upward through the slab. The gas exits through PVC piping that vents above the roofline where it disperses harmlessly. The slab itself becomes a barrier with the fan ensuring that the pressure gradient always favors evacuation over infiltration.
For the same architecture described from a different angle, our radon vent system architecture guide walks through the component-level engineering.
Active vs. Passive: Why Active Wins
Passive radon mitigation uses the same piping layout as an active system but without a fan. The vent stack relies on stack effect: warm air rising through the home creates suction at the lower end of the pipe, which draws soil gas up the stack and out the roof. Passive systems are commonly installed in new construction because the cost is low. Some achieve adequate radon reduction; many do not.
The problem with passive systems is unpredictability. Stack effect depends on indoor-outdoor temperature difference, building envelope tightness, and HVAC operation. On a calm summer day with the air conditioner running, there is little stack effect and minimal radon evacuation. On a cold winter day with the furnace running, there is strong stack effect and substantial evacuation. The system performs well when not needed and poorly when needed most.
An active system runs a fan continuously regardless of weather, season, or HVAC operation. The negative pressure under the slab is maintained around the clock. This consistency is what produces reliable radon reduction. EPA, CDPHE, and the American Association of Radon Scientists and Technologists all reference active sub-slab depressurization as the dominant approach for retrofit mitigation in elevated-radon homes.
Why Indoor Air Filtration Is Not the Answer
A frequent question is whether HEPA filtration or other indoor air cleaning can remove radon. The answer is no for two reasons. First, radon itself is a gas and passes through any conventional air filter unchanged. Second, the radon decay products that cause health risk are particles, but they form continuously in indoor air as radon decays. Removing the existing decay products does nothing about the radon gas still in the air, which produces new decay products immediately.
The only effective approach is to prevent radon from entering the home in the first place. That is what an evacuation system does. Sealing the slab alone is also insufficient because soil gas finds new pathways through any opening. Active suction under the slab addresses both: it pulls gas away from the slab and depressurizes the gravel layer so any remaining slab openings flow soil gas away rather than into the home.
System Components in an Active Evacuation System
Every active evacuation system has the same core components, sized and configured to the specific home.
Suction Point
A four- to six-inch hole through the basement slab connects the vent piping to the gravel or void layer underneath. Material is removed to create a small chamber that the fan can pull from across a wider area than a point connection.
Vent Piping
Three- or four-inch PVC pipe rises from the suction point through the home to the roof. Joints are sealed with PVC primer and cement. Pipe is supported at code-required intervals.
Radon Fan
A continuously running fan installed above the conditioned envelope (in the attic or on the exterior). The fan is sized to the home’s sub-slab characteristics based on a pre-installation communication test. Common models include RadonAway GP series, Festa AMG, and similar.
Manometer
A U-tube or digital pressure gauge mounted on the vent pipe that displays the negative pressure created by the fan. The manometer is the homeowner’s at-a-glance indicator that the system is working.
Roof-Line Discharge
The vent terminates two to three feet above the roof surface and at least ten feet from any operable window. A rain cap that does not restrict airflow caps the pipe.
Slab Sealing
Cracks, control joints, plumbing penetrations, and the slab-to-foundation-wall perimeter are sealed with polyurethane caulk or radon-specific sealant. Sealing reduces leakage from indoor air into the depressurized zone, allowing the fan to focus on evacuating actual soil gas.
How Suction Reaches the Whole Slab
A single suction point can effectively depressurize a surprisingly large area when the gravel or void layer under the slab has good permeability. The fan creates a pressure gradient that propagates outward through the gravel. Communication testing during pre-installation measures how the pressure decreases at distance from the suction point. If the pressure differential at the slab’s far edge is still on the order of one-quarter inch of water column or more, the single point is adequate.
Homes with poor sub-slab permeability or unusual slab geometry may need two or more suction points connected to a common vent stack. The total airflow capacity must be sized to the combined demand. Two-point systems are common in homes over 2,500 square feet or in homes with finished basements where a single point cannot reach the entire slab.
Why Continuous Operation Matters
The fan runs continuously, twenty-four hours a day, all year. Cycling the fan on and off saves no meaningful electricity and produces unstable performance. When the fan is off, soil gas accumulates under the slab and begins infiltrating the home. When the fan turns back on, it takes hours to re-evacuate the accumulated gas. The slab pressure differential is also slow to re-establish.
Fan power consumption is small. A typical RadonAway GP501 draws about ninety watts continuously, costing two to ten dollars per month in electricity depending on local rates. Compared with the cost of furnace or air conditioner operation, the radon fan is negligible. Treating it as an always-on appliance is the correct mental model.
Monitoring System Performance
The manometer is the primary daily indicator. Homeowners should glance at it monthly. A reading consistent with the post-installation baseline indicates normal operation. A zero reading or a substantially changed reading indicates the fan has failed, a pipe has cracked, or sub-slab conditions have changed. A reading dropping to zero usually means fan failure or power loss to the fan circuit.
EPA recommends retesting indoor radon levels every two years and after any major home renovation. A test result consistent with the post-installation baseline confirms the system is still performing. A test result that has crept upward indicates the system may need service even if the manometer reading looks fine. Our how to test radon levels walkthrough covers the homeowner retesting procedure.
Maintenance Over Time
Radon fans are rated for five to ten years of continuous operation. Some last longer; some fail earlier. Replacement is straightforward and costs three to five hundred dollars including the fan and labor. The vent piping and slab sealing rarely need attention unless the home experiences major renovation that disturbs them. Roof-line discharge points should be checked annually to confirm the rain cap is intact and the pipe has not been blocked by nests or debris.
Long-term maintenance is minimal. A budget of three to five hundred dollars per decade for fan replacement covers the major recurring cost. Operating electricity adds the few dollars per month already noted.
When an Evacuation System Is Not Enough
Most homes achieve their target radon levels with a properly designed active evacuation system. Some do not, even with two suction points and high-performance fans. Possible reasons include unusual geological conditions producing very high soil gas flux, complex foundations with multiple disconnected slab zones, and homes with extensive crawlspaces that need separate sub-membrane mitigation.
In these cases, additional measures may include adding suction points, increasing fan capacity, addressing crawlspaces separately with sub-membrane depressurization, or adding heat recovery ventilation to dilute indoor air. A certified mitigator can design the appropriate combination. Our Colorado radon testing pillar covers the broader decision framework.
Front Range Application
Colorado’s combination of granite-based geology and tight winter construction produces elevated indoor radon across the Front Range. Active evacuation systems are the dominant mitigation approach in Denver, Boulder, Jefferson, El Paso, Larimer, and Douglas counties. Certified mitigators are widely available, and most installations are completed in a single day. CDPHE maintains a list of NRPP and NRSB certified mitigators serving the state.
Climate considerations for Front Range installations include condensate management on exterior pipe runs (cold winters can freeze water vapor in soil gas), fan models rated for cold ambient temperatures, and attention to slab cracking from dry-climate shrinkage.
Cost and Timeline
A typical Front Range active evacuation system installation runs twelve hundred to twenty-five hundred dollars. The job takes four to eight hours on-site. Post-installation testing confirms performance, usually within a few days of completion. The total timeline from signing the installation contract to confirmed performance is typically one to two weeks. Compared with the cost of long-term radon exposure or with the cost of unsuccessful passive mitigation, active systems are the standard cost-effective approach.
References
- EPA radon mitigation standards — U.S. Environmental Protection Agency
- CDPHE radon program for Colorado — Colorado Department of Public Health and Environment
- CDC radon information — Centers for Disease Control and Prevention
- American Lung Association radon resources — American Lung Association
Front Range homeowners considering radon mitigation after a high test result can reach out through our contact page for a vetted local inspector and certified mitigator referral.