A whole-house fan or evaporative cooler was running
“The attic fan was on for part of the test — nobody thought about it.”
These appliances move large volumes of outdoor air through a building. Outdoor radon concentrations are low, so the effect on an indoor measurement is dilution.
That means the reading is likely to understate the house under ordinary conditions, which is the direction that matters: a result that came back high despite dilution is telling you something clear.
Why this matters
What it changes about the decision in front of you.
- Closed-building conditions exist precisely to prevent this, and a whole-house fan is the largest single departure from them a household can make.
- The dilution is not constant — it applies only while the appliance runs, so the effect depends on hours nobody logged.
What it could influence
Where the effect is likely to show, and in which direction where that is knowable.
- The reading probably sits below what closed conditions would have produced in the same period.
- A result near a decision threshold is the case where this matters most, because the true closed-conditions figure could sit the other side of it.
What this does not settle
What still stands. Noticing a problem is not the same as losing the evidence.
- A reading above the action level taken while the house was being ventilated is a stronger indication than the same figure under closed conditions, not a weaker one.
- The device worked and the measurement happened. What is in question is the conditions, which are describable and repeatable.
- It does not mean permanent HVAC should have been switched off — a furnace or air conditioner running normally is part of the protocol.
What a professional is likely to ask
Worth having answers ready. None of these is a test you can fail.
- Which appliance ran, and roughly for how many hours of the test.
- Whether it is on a thermostat or a manual switch, since that affects how predictable the running was.
- Whether the household can hold closed conditions for a repeat measurement.
What you can check yourself
Things you can establish today, without anyone visiting.
- Which ventilation appliances the house has, and how they are controlled.
- Whether a repeat test could realistically avoid running them, given the season.
What genuinely needs somebody on site
Where this platform stops. These are not questions a website can answer about your house.
- The air-change rate the appliance actually produces, which depends on the building and the appliance together.
Categories a next step could fall into
Unordered, and none of them is a recommendation. Which applies depends on things this page does not know about your situation.
Related reading
- How testing works — Devices, placement, closed-building conditions, and what invalidates a result.
- False positives — Most disputed results are not false — they are invalid, mismeasured, or comparing two different things.
Sources
- U.S. EPA — A Citizen’s Guide to Radon
When you are ready to talk to somebody
Tell us about your home and what you are trying to work out. We will tell you honestly whether we can route it to a provider in your area today.