Do I need to test again after the system goes in?
The cadence we recommend
- Post-install short-term test. 48 hours minimum, ideally 5 to 7 days. Starts no sooner than 24 hours after the fan is energized. Confirms the system actually brought the number below the EPA action level of 4 pCi/L.
- Long-term test in the first year. A 90-day-or-longer measurement gives a more honest annual average than a short-term test taken on a single warm or cold week.
- Re-test every two years. The cadence the EPA and AARST recommend. The radon fan is the only moving part on the system; fan life is usually 8 to 12 years, and a quiet failure can go unnoticed for months.
- Re-test after any major home change. Basement renovation, new HVAC, new windows, sump pit work, or a fan swap. Any of these can move the number.
Why the manometer gauge is not enough
Every active mitigation system has a U-shaped tube on the pipe in the basement called a manometer. It is a pressure gauge. The two fluid levels sit at different heights when the fan is pulling air. That is useful. It tells you at a glance that the fan is still running.
What it does not tell you is whether radon levels in the living space are still under 4 pCi/L. The gauge is a one-bit signal. Fan on, or fan off. The actual radon concentration depends on the fan plus everything else around it. Slab condition, new penetrations, changes to the house envelope, weather, season.
We have walked into homes where the manometer was reading correctly for years and the basement was still over the action level because a sump pit cover had cracked or a slab penetration had opened up. The gauge said "running." The air said something else. The only way to know is to test.
How to do the post-install test
Closed-house conditions
Both for a real estate transaction and for a personal short-term test, keep windows and outside doors closed except for normal use for at least 12 hours before the test starts and during the test. Run heating and cooling as you normally would.
Where to place the test kit or monitor
Place the test at the lowest level of the home that is regularly occupied. For most Rochester-area basements, that is the floor below grade. Place it at least 20 inches off the floor, away from drafts, exterior walls, and the radon vent pipe itself.
What "passing" looks like
The EPA action level is 4 pCi/L. A post-install number well below 4, ideally below 2, is the goal. The Minnesota Department of Health recommends action when long-term results exceed 2 pCi/L. The World Health Organization sets its line at 2.7 pCi/L.
Short-term kit vs. continuous monitor
A charcoal short-term kit costs about $20 at the hardware store. It measures the average over the 48-hour or 7-day test window. A continuous radon monitor logs readings hour by hour and gives a much more honest picture, especially across a few weather cycles. We use continuous monitors for post-install testing because the data is better.
What changes the answer
- Season. Winter readings run roughly 26% higher than summer readings in Minnesota (state arithmetic mean: 4.3 vs. 3.4 pCi/L). A summer test that lands at 3.5 may be a winter test at 4.5.
- Sump pits and slab penetrations. A failed sump cover seal is the most common source of "the gauge looks fine but the number drifted up."
- Fan age. Radon fans are sealed inline blowers, and the bearings eventually fail. A failed fan reads as zero suction on the manometer but homeowners often miss the change for months.
- Renovations. Any work that breaks the slab or the under-slab vapor barrier can change the system's reach. Re-test after.
If your system was installed by someone else
We do post-install and follow-up testing on systems we did not install. The most common version of this call: the system has been on the wall for five or ten years, the homeowner has not tested since the original install, and now they are wondering. The right starting point is a free short-term test. From there, we have a real conversation about what the system is actually doing.
We also do diagnostic visits on systems that are over the action level. Sometimes the fan has aged out. Sometimes the original install was not a great match for the foundation. Sometimes the slab has changed since the system went in. The visit sorts out which it is.