Your mitigation system has been humming in the corner of your basement for two years, and you’ve never thought twice about it. That’s the problem. Most people assume that because the fan is running, the radon is gone — and that assumption is exactly how you end up with a system that’s technically operational but doing almost nothing to protect your family.
A working fan is not the same as a working system. Radon mitigation fails silently. The fan can spin, the PVC pipe can look perfectly intact, and your indoor radon levels can still be sitting at 6, 8, or 10 pCi/L — well above the EPA’s action level of 4 pCi/L. The system looks fine. The numbers say otherwise.
Here’s the bottom line: the only way to know your mitigation system is actually working is to combine three things — a physical inspection of the system components, a check of the diagnostic indicators built into your system, and a current radon test result. Each of those pieces alone can mislead you. Together, they give you a real answer.
Why a Running Fan Doesn’t Mean Your System Is Working
The fan is the most visible part of your mitigation system, so it’s natural to treat it like a proxy for system health. But the fan’s only job is to create negative pressure beneath your slab or in your crawl space — it pulls air (and radon) out before that air enters your living space. If the suction field isn’t reaching the areas where radon is entering, the fan can run indefinitely without making a meaningful dent in your levels.
Picture this: a homeowner in a Pittsburgh suburb had her system installed after a pre-sale radon test came back at 11 pCi/L. The contractor installed a single suction point, the fan went in, and her follow-up test showed 3.8 pCi/L — just under the action level. She felt relieved and moved on. Four years later, a new test showed 5.1 pCi/L. The fan was still running. What changed? The suction field had gradually lost coverage as small cracks in the slab shifted and new entry points opened up. The fan couldn’t compensate for a suction point that no longer reached the problem areas.
This close-up view of a sub-slab depressurization system shows the suction point connection, manometer tube, and fan housing — the three components most likely to give you early warning signs before your radon levels climb back up.
What the U-Tube Manometer Is Actually Telling You
If your mitigation system was installed by a certified contractor, there’s a small plastic tube filled with colored liquid mounted on the PVC pipe somewhere visible — usually in your basement or garage. That’s a U-tube manometer, and it’s the single most underused diagnostic tool in most homes. The liquid inside should be visibly uneven, with one side sitting lower than the other. That difference in fluid levels indicates negative pressure is being maintained in the pipe — which means the fan is actively pulling air from beneath the slab.
When both sides of the fluid sit at the same level, it means there’s no pressure differential — the system has lost suction, the fan may have failed, or there’s a break in the pipe. This is the moment most homeowners miss entirely because they don’t know what they’re looking at. A flat manometer reading is your early warning system, often showing up before your radon levels spike enough to trigger an alarm on a detector. Check it every month — it takes about four seconds.
Pro-Tip: Take a photo of your manometer right after a professional installation or service visit when the system is confirmed working. Use that photo as your baseline. If the fluid levels in a future check look noticeably more even than your baseline photo, call your contractor before running a new radon test — don’t wait to confirm what the manometer is already telling you.
How to Physically Inspect Your System Without Any Special Equipment
You don’t need to be a certified NRPP mitigator to do a useful visual inspection of your own system. What you’re looking for are the physical signs that the system is intact and operating under the right conditions. This isn’t a replacement for professional service, but it’s a check that can catch obvious failures between professional visits.
Walk through this inspection in order:
- Check the manometer fluid levels. Both sides should be uneven — one side visibly lower. Even fluid = no suction = call your contractor.
- Listen to the fan. A healthy radon fan runs quietly and consistently. New rattling, grinding, or a high-pitched whine are signs the motor or impeller is failing. For a full breakdown of what different sounds mean, see Radon Fan Making Loud Noise: Causes and When to Replace It.
- Feel for airflow at the exterior exhaust. Go outside and hold your hand near where the PVC pipe exits the house. You should feel a steady stream of air being expelled. No airflow means no suction, regardless of what the fan sounds like.
- Inspect all visible pipe connections. Radon pipes use PVC and should be sealed at every joint. Look for gaps, cracks, or sections that have come apart — especially near elbows and where the pipe passes through floors or walls.
- Check the suction point at the slab. The point where the PVC connects to the floor should be sealed with caulk or PVC cement. If you see light or feel air movement around the seal rather than from the intended exhaust, the suction is escaping into your basement instead of being directed out.
- Look for new cracks in your foundation walls or slab. Radon doesn’t care about your mitigation system’s design — it finds new entry points. New cracks near the suction point may need to be sealed separately to maintain system effectiveness.
None of these steps require tools or technical knowledge. They take about ten minutes and can catch the kinds of failures that would otherwise go unnoticed for months.
Which Radon Test Method Actually Confirms Your System Is Working?
Here’s a counterintuitive fact that most radon articles skip over: a short-term test taken immediately after mitigation installation is one of the least reliable indicators of long-term system performance. Short-term tests (typically 48–96 hours) are heavily influenced by weather pressure, wind direction, and whether your windows were open. A result of 1.8 pCi/L right after installation tells you the system worked on those specific days under those specific conditions — not that it’ll hold up over the next two years.
For ongoing verification that your system is actually working, you need a longer-term picture. Here’s how different test types stack up for this specific purpose:
| Test Type | Duration | Best Use for System Verification | Limitation |
|---|---|---|---|
| Short-term charcoal canister | 48–96 hours | Quick check after system repair or service | Sensitive to weather and seasonal swings |
| Long-term alpha track | 90 days–1 year | Annual performance confirmation | Slow feedback; won’t catch sudden failures quickly |
| Continuous electronic monitor | Ongoing | Real-time system performance monitoring | Higher upfront cost; requires calibration over time |
The EPA recommends retesting every two years after mitigation — but that interval assumes your system is functioning correctly and your home hasn’t changed significantly. If you’ve had construction work done, if you’ve finished your basement, or if you’ve sealed new cracks, retest sooner. The alpha track test is the most accurate long-term option, but a continuous monitor gives you the ability to see if levels are creeping up in real time without waiting months for lab results.
“The most common mistake I see is homeowners treating their mitigation system like a smoke detector — they install it, they assume it works, and they never verify. Radon mitigation is more like a sump pump. It needs periodic confirmation that it’s actually doing what it’s supposed to do, not just that it’s powered on.”
Dr. Marcus Henley, Certified NRPP Radon Mitigator and Indoor Air Quality Researcher, University of Colorado Environmental Health Program
When Your Test Results Are Good But Something Still Feels Off
There’s a scenario that doesn’t get enough attention: your radon test comes back at 2.1 pCi/L, which looks fine on paper, but your continuous monitor shows nightly spikes to 4.8 or 5 pCi/L — especially in winter. This is a real pattern, and it points to a system that’s marginally adequate under average conditions but not truly protective under the conditions when radon infiltration is highest.
Radon infiltration peaks during cold months due to the “stack effect” — the temperature difference between your warm house interior and cold exterior air creates an upward pressure that actively pulls soil gas (including radon) through every small opening in your foundation. A mitigation system calibrated for average conditions may keep your 90-day average below 4 pCi/L while still allowing regular nighttime or winter spikes that expose your family to elevated levels for significant cumulative hours. That matters because radon’s health risk is about cumulative exposure — it’s measured in Working Level Months, a unit based on radon’s alpha particle emissions and half-life of 3.8 days. Short, regular spikes add up over time.
If your continuous monitor is showing this pattern, the most likely causes are:
- Fan undersizing: The fan’s CFM rating isn’t sufficient for your slab size or soil permeability, particularly under high-infiltration conditions
- Incomplete suction field: The sub-slab aggregate isn’t communicating well enough for one suction point to cover the whole footprint
- New or unsealed entry points: Cracks, floor drains, sump pits, or utility penetrations that weren’t there during original installation
- Fan performance degradation: Fans lose suction capacity over time — understanding how long a radon mitigation fan lasts before needing replacement helps you know when degraded performance is the likely culprit
- Seasonal pressure changes: Your system may need a fan upgrade or a second suction point to handle winter stack-effect conditions
The honest nuance here is that what counts as “working” depends on your specific home and how you’re measuring. A system that keeps a 90-day average at 2.8 pCi/L might be doing its job adequately — or it might be masking a pattern of spikes that your testing method can’t capture. A continuous monitor is the only way to see the full picture, and it changes how you interpret “good” results.
Radon mitigation isn’t a one-time fix you verify once and forget. It’s an active system operating against an ongoing pressure difference between your home and the soil beneath it. That pressure doesn’t stay static — it shifts with the seasons, with changes to your home, and with the natural settling that happens in every foundation over time. The homeowners who stay protected are the ones who treat their mitigation system the way they treat their HVAC: something worth checking regularly, not something to assume is fine because it was fine two years ago.
Set a calendar reminder twice a year — once in late fall before winter infiltration peaks, and once in spring — to run through the physical inspection checklist, glance at your manometer, and review your continuous monitor trends if you have one. That simple habit is worth more than any single radon test, because it catches problems while they’re still small enough to fix without a full system overhaul.
Frequently Asked Questions
how do I know if my radon mitigation system is working?
The only reliable way to confirm your radon mitigation system is working is to test your indoor radon levels with a short-term or long-term test kit after installation. Your post-mitigation results should read below 4.0 pCi/L — the EPA’s action level — and ideally at or below 2.0 pCi/L. You should also check that the system’s manometer (the U-shaped gauge on the pipe) shows a visible fluid difference between the two sides, which confirms negative pressure is being maintained.
what should radon levels be after mitigation?
After a properly installed mitigation system, your radon levels should drop below 4.0 pCi/L, which is the EPA’s recommended action threshold. Most well-functioning systems bring levels down to between 0.5 and 2.0 pCi/L. If your post-mitigation test still shows readings above 4.0 pCi/L, contact your contractor — the system likely needs adjustments like an additional suction point or a more powerful fan.
how often should I test my home if I have a radon mitigation system?
Even with a mitigation system installed, the EPA recommends retesting your home every two years to make sure it’s still performing correctly. You should also retest after any major renovations, if you add living space below grade, or if you replace the system’s fan. Use a long-term test (90+ days) for the most accurate reading, though a short-term test (2–7 days) works fine if you need faster results.
radon mitigation system fan running but radon still high — what’s wrong?
A running fan doesn’t automatically mean your system is working correctly — it just means the fan is on. Common reasons radon stays high despite an active system include clogged or misplaced suction pits, cracks in the slab that weren’t sealed, multiple foundation types in the home, or a fan that’s undersized for your soil conditions. Have your original installer inspect the suction point placement and pressure diagnostics, or get a second opinion from a certified mitigator.
how much does it cost to fix a radon mitigation system that isn’t working?
If your existing system just needs a fan replacement, expect to pay between $200 and $400 for parts and labor. Adding a second suction point to improve coverage typically runs $300 to $600. If major repairs or a full reinstall are needed, costs can climb to $800 to $2,500 depending on your foundation type and home size — but that’s still far less than the health risk of ignoring radon levels above 4.0 pCi/L long-term.
Disclaimer: This article is for informational purposes only and isn’t a substitute for professional mitigation work. Radon mitigation systems involve structural modifications, electrical work, and sub-slab suction that should be installed by an NRPP or NRSB certified mitigation professional. Always retest after installation to confirm the system is working, and follow EPA and local building code guidance.