How a Sub-Slab Depressurization Radon System Works
September 1, 2026
A radon system does not filter air. It creates a small vacuum under the slab so soil gas flows to a pipe and out above the roof instead of into the basement. Everything about the design follows from that one idea.
The principle: lower the pressure under the floor
Radon enters because the air pressure inside a house, especially in winter, is slightly lower than the pressure in the soil beneath it. The house pulls soil gas in through every crack, joint and opening. Sub-slab depressurization reverses that. A fan connected to a pipe that penetrates the slab pulls a gentle vacuum on the gravel or soil underneath, so the pressure under the floor is lower than the pressure in the basement.
Once that is true, soil gas moves toward the pipe and up the stack rather than into the house. The fan is not moving much air; it is maintaining a pressure difference. That is why a well-designed system uses a small fan and stays quiet.
The parts
A complete radon mitigation installation has a small number of components, and each one is doing a specific job.
- Suction point: a hole cored through the slab, with a pit excavated beneath it so the vacuum spreads.
- Vent pipe: 3 or 4 inch PVC from the suction point, through the house or up the exterior wall, to above the roof eave.
- Fan: inline, mounted outside living space (attic, garage or exterior), rated for the pipe and the soil conditions.
- Manometer: a U-tube gauge on the pipe below the fan that shows the system is pulling suction.
- Sealing: floor-wall joints, cracks and sump pit sealed so the fan pulls on the soil, not on the basement air.
- Discharge: at least 10 feet above grade, away from windows and intakes, so exhaust does not re-enter.
Where the pipe goes
The route is chosen for two things: a short path to the roof and a fan location outside the living space. In many Lincoln homes that means up through a utility room and garage into the attic, with the fan in the attic and the pipe exiting through the roof. On homes without a convenient interior route, the pipe exits the basement wall low, runs up the exterior and terminates above the eave, with the fan mounted outside.
Pipe downstream of the fan is under positive pressure, which is exactly why the fan is never placed in a basement or occupied room. Any leak below the fan pulls air in; any leak above it pushes radon out.
What a working system looks like
The manometer shows two liquid levels offset from each other. Equal levels mean no suction: the fan is off or the pipe is blocked. The offset itself is set at install and noted on a label; a reading that has drifted toward zero is the signal to call for system repair and retest.
A working fan is audible only within a few feet of it and sounds like a bathroom fan behind a wall. Vibration through the pipe, humming in a bedroom wall or a whistle at a joint are installation problems, not normal operation.
Proof, and what happens after
A post-mitigation radon test is run after the fan has operated for at least 24 hours and within 30 days of installation. Most Lincoln systems bring a house from the 4 to 12 range to under 2. After that, the manometer is the daily check, a retest every two years is the long-term one, and the fan is the part that eventually wears out.
Anyone comparing quotes for a sub-slab radon system in Lincoln should hear the same design language from every installer: diagnostic test, suction point placement, fan outside living space, sealed pit, manometer, retest. An installer who skips any of those is describing a different, weaker system.
More from the blog
Quick Answers
Does a radon fan run all the time?
Yes. The system works by holding a pressure difference under the slab, and that difference disappears the moment the fan stops. Fans are designed for continuous duty and typically draw less power than a light bulb.
Can the vent pipe run inside the house?
It can and often does, as long as the fan itself is outside the living space. An interior route through a closet or utility chase to the attic is common in Lincoln homes and keeps the exterior clean.
How far does one suction point reach?
It depends on what is under the slab. Coarse gravel lets a single point depressurize a large floor; tight soil may need two. A pre-install diagnostic measures the pressure field and settles the count before any core is drilled.