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Owning and renting5 min read

How long after radon mitigation can you retest?

By Radon411 Editorial TeamUpdated How we research

Short answer

Wait at least 24 hours after the system starts running with its fan on, and test within 30 days of installation. A test of two to seven days is recommended, with windows and doors closed from 12 hours before. After that, retest at least every two years and after any remodel.

Next step: check the rules where you rent or let. Radon rules for rentals by state

The contractor may test to check the work, but an independent follow-up test, by a separate tester or by you with a kit, removes any conflict of interest. Keep the fan running for the whole test.

If the level is still high, more work may be needed. Many contractors guarantee to adjust or modify the system to reach a negotiated level, so read the guarantee in your contract.

What to do next: plan an independent between 24 hours and 30 days after installation, and keep the result with the system's paperwork.

The window: 24 hours to 30 days

The first test after a radon system goes in has a set window. It should start no sooner than 24 hours after the system is running with its fan on, and be done within 30 days of installation. Minnesota writes the same window into its rules: a between 24 hours and 30 days after the system is installed.

Timeline after a radon mitigation system is installed: the fan runs continuously, test no sooner than 24 hours and within 30 days, then retest at least every two years.
Read this figure as text

  1. Day of installation

    The system goes in

    Keep the fan running at all times: it must run continuously for the system to work correctly.

  2. No sooner than 24 hours

    Earliest post-mitigation test

    Test no sooner than 24 hours after your system is in operation with the fan on, if it has one.

  3. Within 30 days

    Post-mitigation test done

    A two- to seven-day measurement is recommended, and EPA recommends an independent follow-up radon measurement as well as any test the contractor runs.

  4. At least every two years

    Retest

    Retest your home at least every two years to be sure radon levels remain low.

Test conditions: Windows and doors must be closed 12 hours before and during the test, except for normal entry and exit. Not drawn to scale.

After a remodel. After you remodel, retest in the lowest lived-in area to make sure the construction did not reduce the effectiveness of the radon reduction system.

Testing after installation: no sooner than 24 hours, within 30 days, then at least every two years and after a remodel.
The testing schedule for a home with a radon system
WhenWhat
Day of installationThe contractor explains and demonstrates the system and hands over written operating and maintenance instructions and any warranties
After the fan has run at least 24 hoursThe post-mitigation test can begin
Within 30 days of installationThe post-mitigation test should be done
At least every two years after thatRetest to be sure radon levels remain low
After a remodel or other changeRetest in the lowest lived-in area

How to run the post-mitigation test

The test that checks a system is a short one. A measurement of two to seven days is recommended for judging the system's effectiveness, and like every radon test it runs at least 48 hours. Windows and doors stay shut from 12 hours before the test until it ends, apart from normal coming and going.

Conditions for the post-mitigation test
ConditionWhat to do
LengthTwo to seven days; never less than 48 hours
Closed houseWindows and doors closed from 12 hours before and during the test, except for normal entry and exit
The fanWorking properly, and running for the entire test; if it is not working, have it repaired first
LocationThe lowest level of the home that could be used regularly
Who testsThe contractor, an independent tester or you; an independent test removes any conflict of interest

The contractor may run a test to check the work and how well the system performs at first. An independent follow-up measurement is still recommended, either by a separate tester or by you with a kit. See can I test for radon myself for how a home test works.

Reading the result

The number is read against the same guidance as any other test. EPA recommends fixing a home when the radon level is 4 or higher. EPA recommends considering a fix when the level is between 2 and 4 pCi/L. A good system usually does much better than the line: radon levels in many homes can be brought to 2 pCi/L or less, and a quality system may bring year-round levels below 2 pCi/L.

Scale of radon levels from 0 to 10 pCi/L marking EPA's 2 pCi/L level, where EPA says to consider fixing, and its 4 pCi/L action level, where EPA says to fix the home, with each mark also in Bq/m³.
Read this figure as text
  • Below 2 pCi/L (below 74 Bq/m³)Below both of EPA's guidance levels.
  • 2 to 4 pCi/L (74 Bq/m³ to 148 Bq/m³)EPA recommends considering a fix when the level is between 2 and 4 pCi/L.
  • At or above 4 pCi/L (148 Bq/m³ and up)EPA recommends fixing a home when the radon level is 4 pCi/L or higher.
The two guidance levels a post-mitigation result is read against.

For what each number means in practice, see how to read a radon test.

Retests in the years after

One good result does not settle the matter for good. Radon levels can change, so a home with a system is retested at least every two years, and if possible in winter, when the heat runs day and night and the result is more likely to be accurate.

Changes that call for a retest of a mitigated home
ChangeExamples
AdditionsNew adjoining additions, structures or parking lots
RemodelingReconfiguring or rehabilitating the building, or occupying a ground-contact area not tested before
Heating, cooling and ventilationSystems altered in ways that change air distribution or pressure, or extensive weatherization
New openings to the soilNew or altered sumps, drain tiles, sewer lines or shower and tub retrofits, or major settlement cracks
Ground disturbance nearbyEarthquakes, blasting, fracking or sinkholes
The system itselfAny alteration to the installed system

A sale of the home is another moment to test, and a of more than 90 days gives the truest year-round picture at any point. The full schedule is in how often should I test.

Between tests: the warning device

Every active system has a warning device, often a u-tube manometer, to show whether the fan is pulling. Look at it regularly; Minnesota asks owners to check their monitors monthly. If the system's monitor shows a problem, call a contractor to have it checked.

What the warning device can and cannot tell you
The device showsIt does not show
Whether there is suction in the pipeHow much radon is in the house
Even fluid levels in a u-tube manometer: the fan stopped or something else is not workingWhether the level is still below the guidance levels
An alert in very cold weather can mean ice in the pipe; radon levels will increase while it lastsWhether the home's level has drifted since the last test

That is why the gauge never replaces a test. Like a furnace or chimney, a radon system needs occasional maintenance, and the fan must run continuously for the system to work. More on keeping a system working is in can I turn off my radon system.

Questions about testing after mitigation

Can I test the same day the radon system is installed?

No. Wait until the system has been operating with the fan on for at least 24 hours, then test within 30 days of installation.

How long should the post-mitigation test run?

Two to seven days is recommended for checking the system, and no radon test runs less than 48 hours.

Should the radon fan be on during the test?

Yes. Make sure the system is working properly and keep it running for the entire test; the result then shows what the system achieves.

Who should do the test after mitigation?

The contractor may test to check the work, but an independent follow-up measurement, by a separate tester or by you, removes any conflict of interest.

What if the radon level is still high after mitigation?

Additional work may be needed. Check your contract for a guarantee to adjust or modify the system to reach a negotiated level, and call the contractor.

How often should I retest after the first test?

At least every two years, and after a remodel or other change to the house or the system. Test in the lowest lived-in area.

Owning and renting

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