Reviewed and updated September 13, 2026. This article provides general public-health and referral guidance, not testing, medical, legal or engineering advice.
New York gives a clear window for the first post-mitigation test
After a radon system is installed, homeowners naturally want to know whether they should test the next morning, wait a week or wait for a particular season. New York State Department of Health guidance says a post-mitigation test should be performed within 30 days of installation, but no sooner than 24 hours after an active system has been operating with the fan on.
For homes in Elmira, Horseheads and Corning, that first test is the basic performance check. A manometer can show that a fan is creating pressure difference, but it cannot tell you the indoor radon concentration. The test is what establishes whether the system actually reduced the level in the occupied space.
Use closed-house conditions for the short follow-up measurement
NYSDOH recommends a two- to seven-day measurement to check system effectiveness and specifies closed-house conditions for the follow-up test. Windows and doors should remain closed except for normal entry and exit, and the system should operate normally throughout the measurement.
Do not make the house artificially favorable by opening windows before the device is placed or changing the system during the test. If construction is still underway, doors are being left open or the fan is being switched on and off, wait until normal test conditions can be maintained or ask the testing professional how to document the situation.
Independent follow-up testing can strengthen the record
New York notes that a contractor may perform a post-mitigation test but recommends an independent follow-up measurement as a way to avoid a potential conflict of interest. That can mean using an independent tester or conducting an appropriate measurement yourself according to the applicable instructions and guidance.
Keep the pre-mitigation test, the system proposal, installation documents and the post-mitigation result together. If the property is sold later, that record is far more useful than saying only that 'a radon system was installed.' It shows what problem existed and what the measured result was after correction.
One good post-test does not eliminate future retesting
Radon systems are mechanical building systems. Fans age, components can be damaged and renovations can change pressure or foundation conditions. EPA recommends checking the system warning device regularly and retesting periodically to make sure radon levels remain low. New York also recommends retesting after mitigation and after relevant remodeling or weatherization work.
If a basement becomes a regularly occupied office or bedroom, test that use condition as well. If the fan is replaced, a suction point is added or a crawlspace membrane is repaired, another measurement gives you evidence that the changed system is still controlling radon.
If the first post-test is still high, do not just wait for another season
A valid post-mitigation result that remains elevated deserves a system review. Waiting months while assuming the weather will fix it can delay correction of a design, fan or foundation-zone problem. Check the warning device, preserve the test report and contact the installer or another qualified professional.
Use the radon-still-high-after-mitigation guide to organize the information before the service call. If the question is whether a monitor's temporary spike is meaningful, the continuous monitor guide explains how to separate a peak from an average.
Sources
Primary guidance used for this article
We use government and public-health sources, distinguish historical datasets from current household measurements, and do not invent local results. Read our editorial policy.