Radon Levels Rising Even With a Mitigation System? Run This Checklist

Here's a call a Colorado mitigator gets all the time: "We have a mitigation system. The fan is definitely running — we can hear it. But our monitor says the 7-day average is 4.75 now, and it used to sit between 3 and 4. What's going on?"

If that's you, this post walks the same diagnostic sequence the pros use, in order. Most of it you can check yourself in ten minutes, and each step narrows down what (if anything) needs fixing.

First: your monitor is doing its job

If you're watching this happen on an Airthings or a similar consumer monitor from the hardware store, good — that's exactly what those are for. They aren't certified measurements (no consumer monitor is), but they're reliable trend-spotters. A sustained climb over weeks is a real signal, not noise.

One calibration note before you worry: radon readings bounce around day to day. What matters is the 7-day and 30-day average, not this afternoon's number.

Step 1 — Rule out weather

Radon levels spike temporarily during:

• Wind storms (pressure swings pull more soil gas)
• Heavy rain and snowmelt (saturated ground pushes gas toward your foundation)
• Extreme cold (a bigger indoor/outdoor temperature difference increases the stack effect drawing air upward through the house)

If your spike lines up with a storm cycle, wait a week and watch the average. But a sustained rise in calm, mild weather — the kind where nothing about the season explains it — points at the system itself.

Step 2 — Read the manometer (the tube with the colored liquid)

On the pipe near where it enters the floor, there's a U-shaped tube with colored liquid — the manometer. It measures how hard the fan is pulling.

• The two liquid columns should sit clearly offset from each other — that offset is the suction.
• If the columns are barely offset, your fan is spinning without moving much air — fans wear out gradually, and "I can hear it running" doesn't mean it's pulling like it used to.
• If the columns are level with each other, the fan is effectively doing nothing. That's your answer.

Ten seconds, no tools, and it's the single most informative reading in this whole checklist.

Step 3 — Check the sump seal (especially after other work)

If your system draws from the sump basin — very common along the Front Range — the seal on that sump lid is everything. Sump-tied systems are the most effective installs there are, but only while the lid is airtight. When the seal is intact, the fan pulls soil gas from under your foundation. When it isn't, the fan pulls basement air instead, and your actual radon extraction quietly collapses.

Here's the pattern that catches people: someone opened the sump and didn't re-seal it. A plumber servicing the sump pump. A sump-pump replacement. A home inspector taking a look. If your levels started creeping up weeks or months after any work near that basin, the seal is the prime suspect — it's the most common cause of a slow rise we see.

Look for cracked or missing silicone around the lid, a lid that shifts when you press it, or penetrations (cords, pipes) that aren't sealed where they pass through.

Step 4 — Identify your fan

Go outside (or to the attic) and find the fan — the bulge in the pipe with rubber couplers top and bottom. The model is printed on the housing, usually near the electrical box.

Why it matters: fans are tiered. Many older or builder-grade installs run standard fans that move meaningfully less air than current mid-tier equipment. If your system was always marginal — passing at 3.5 instead of dropping to 1 — a standard fan on an older sump is often why. An upgraded fan plus a properly sealed sump is the difference between "hovering just under the action level" and comfortably low.

The EPA's action level is 4 pCi/L, but their guidance is to keep reducing where you can — there's no known safe level, and a system that only ever got you to 3.5 has more to give.

What the fixes cost in Colorado

• Inspection + minor repairs (re-sealing a sump, small fixes): from about $199
• Fan replacement/upgrade: roughly $499–$749 installed, depending on tier — and a reputable mitigator credits the inspection into it
• Either way, the job should end with a certified test — not just the monitor — so the fix is documented, not assumed

Full pricing context: what radon mitigation costs in Colorado.

The short version

1. Spike during storms/snowmelt? Wait a week, watch the 7-day average.
2. Manometer columns level or barely offset? Fan problem.
3. Anyone touched the sump since it was last fine? Seal problem (most common).
4. Standard-tier fan that never got you far below 4? Capacity problem.

Two or more of those at once is typical — a weak fan and a leaky sump seal compound each other.

If you're in the Denver metro or Front Range and want it diagnosed properly, get a free repair quote — most rising-level calls are a seal-and-fan visit, not a new system. And if you want a certified number instead of a monitor reading, join the free test-kit waitlist.