Active vs Passive Radon Systems: What's the Difference?
If you bought or built a home in central Iowa in the last fifteen or so years, there's a good chance the builder roughed in a passive radon system during construction. You might have noticed a length of white PVC pipe running up through a closet or utility chase and exiting above the roof. That's not plumbing or HVAC — it's a radon vent.
The question almost every Iowa homeowner with a passive system eventually asks is: "Is this thing actually doing anything? And if not, what does it cost to make it work?" This guide explains the difference between passive and active systems, when conversion is needed, and what the process looks like.
What is a passive radon system?
A passive radon system is a vent pipe — usually 3-inch or 4-inch schedule-40 PVC — that runs from a sealed point under the home's foundation slab, up through the interior of the house, and out above the roof. It relies on natural draft to vent radon gas from the soil under the slab to the outside air.
The natural draft works because the air column inside the pipe is generally warmer than the outside air, especially in winter. Warm air rises, creating a slight upward flow that draws radon-bearing air from beneath the slab and releases it harmlessly above the roof line.
Passive systems became standard in new Iowa construction starting in the late 1990s and early 2000s as building codes recognized Iowa's elevated radon profile. They're cheap to install during construction (the pipe mostly hides inside framed walls) and they require no electricity, no fan, and no maintenance.
What is an active radon system?
An active system is the same vent pipe with one important addition: an inline radon mitigation fan. The fan is mounted in the pipe, usually in the attic or on an exterior wall above the roof line. It runs continuously, pulling a measurable vacuum on the soil under the slab and forcing radon gas out the top of the pipe.
Active systems are quieter than people expect — most fans run at about the volume of a refrigerator and are inaudible inside the house. They use roughly $40–$80 of electricity per year, depending on the fan model and local electric rates.
Active systems also include a manometer (a small fluid-filled vacuum indicator) mounted on the pipe in the basement. The manometer shows the homeowner at a glance that the fan is still pulling vacuum and the system is operating.
When does a passive system actually work?
Sometimes — especially in homes where the radon source is mild and the sub-slab gravel is well-connected, a passive system pulls enough natural draft to keep indoor levels below the 4.0 pCi/L EPA action level. When that happens, no further action is needed beyond periodic re-testing.
But in Iowa, "sometimes" isn't most of the time. Iowa has the highest average residential radon levels in the country, and many homes need mechanical assistance — a fan — to get below the action level. The only way to know whether your passive system is doing the job is to test the home.
How to test a home with a passive system
Testing a home with an existing passive system is no different from testing a home without one. The standard options are:
- Long-term test kit (90+ days): available at most hardware stores for $20–$30. Most accurate reading; provides a true average over multiple seasons.
- Short-term test kit (2–7 days): also available at hardware stores; cheaper and faster, but less accurate. Best run during cold months when radon levels tend to peak.
- Professional short-term test: a licensed mitigator or home inspector runs a continuous-monitoring test for 48–96 hours. Faster turnaround, more reliable than DIY kits.
Place the test in the lowest livable level of the home (typically a finished basement). Keep windows closed and HVAC running normally during the test. After the test, compare the result to 4.0 pCi/L:
- Below 2.0 pCi/L: passive system appears to be working well; retest in two years.
- 2.0–4.0 pCi/L: below the action level but elevated; consider a long-term test or proactive conversion.
- Above 4.0 pCi/L: the EPA recommends mitigation. The passive system needs to be converted to active.
What does conversion to active actually involve?
Converting a passive system to an active one is generally the cheapest form of radon mitigation, because most of the work was already done during construction. The mitigator's job is to:
- Confirm the existing passive vent is properly sealed at the slab and meets current code.
- Cut the vent pipe at a location above the home's living space (usually in the attic or just above the roof line) and install an inline mitigation fan.
- Run electrical wiring to the fan, typically from a nearby junction box or new dedicated circuit.
- Install a manometer on the pipe inside the basement so the homeowner can monitor system function.
- Run a post-mitigation test (24+ hours) to confirm the level has dropped below 4.0 pCi/L.
Total time on site is usually a few hours. The home's appearance changes very little — most of the new components are inside the attic or on an exterior wall in an unobtrusive location.
Typical cost of conversion
Industry pricing data for Iowa puts most passive-to-active conversions in the range of $500–$1,500. The lower end of that range applies when the existing passive system is in good shape, has accessible electrical, and the conversion is straightforward. The higher end applies when the existing system needs adjustment (slab sealing, sump cover, or a new electrical run) before the fan goes in.
This is meaningfully less expensive than a full new mitigation install (which typically runs $1,000–$2,500 in Iowa), because the passive infrastructure is already built into the house. If you have a passive system and your test came back high, getting an estimate is the next step — see our full Iowa pricing guide for context.
What if my home doesn't have a passive system?
If your home was built before the late 1990s, or the builder didn't follow the radon-rough-in code, you may not have a passive system. In that case, mitigation means installing a complete new system from scratch — slab penetration, vent pipe, fan, and post-test. Cost is higher, but the result is the same: indoor radon below the action level, monitored by a manometer.
How to tell if you have a passive system
Look for:
- White PVC pipe (3" or 4") running vertically up through an interior closet, utility chase, or HVAC chase, exiting through the roof.
- A label on the pipe near the basement: "Radon Vent — Do Not Block" or similar.
- A capped opening or visible suction point near the floor in the basement (sometimes hidden behind a sump cover or in a mechanical room).
- No fan in the pipe, no electrical wiring near it, no manometer.
If you see vertical PVC running up out of the basement and out through the roof, with no fan attached, you almost certainly have a passive system.
Common questions about passive systems
If my passive system worked when the house was built, why might it not work now?
Several possibilities. The seal at the slab may have degraded. Cracks or new penetrations in the slab may have developed (e.g., a sump pit added later, plumbing work that opened gaps). The home's air sealing may have improved (which paradoxically can make passive draft worse). Or the test conditions may differ — winter readings are usually higher than summer readings.
Can I just leave the passive system in place if my reading is 4.5 pCi/L?
You can — the law doesn't require mitigation. But the EPA recommends action at any reading at or above 4.0 pCi/L because radon is the second-leading cause of lung cancer in the U.S. (and the leading cause among non-smokers). Most homeowners in this range opt for conversion given the relatively low cost.
How long do mitigation fans last?
Most are rated for around 10 years of continuous operation. Replacement is straightforward — the fan is unbolted from the existing pipe, the new one bolted in. Replacement parts run $200–$500; installed cost is usually $400–$800.
Will a fan make my home noisier or use a lot of power?
Modern radon fans are quiet (about the volume of a refrigerator) and inaudible inside the home. Electrical use is roughly $40–$80 per year in continuous operation.
Get an estimate for conversion
If your test came back high and you have a passive system, the conversion estimate is free and on-site visit takes about thirty minutes. The mitigator can confirm the existing passive system is in good shape and quote the conversion specifically for your home.
This article describes typical Iowa residential practice for passive radon systems and conversion to active systems. Specific system requirements depend on your home's individual configuration, the existing passive infrastructure, and current Iowa code. Always obtain an on-site estimate from a licensed Iowa mitigator before scheduling work.