How to Tell If Your Radon Mitigation System Is Actually Working

1. The three checks that tell you a system is working

A radon mitigation system is mostly invisible once it is installed. A fan runs continuously, pulling soil gas from beneath the slab and venting it above the roof. Because it works silently and out of sight, most homeowners never think about it until something prompts the question: is this thing still working? The good news is that you can answer that with three simple checks, two of which take less than a minute.

The first two checks tell you the system is running. Read the manometer on the vent pipe to confirm the fan is creating suction, and confirm the fan itself is spinning by sound and airflow. The third check is the one that actually matters for your health: a follow-up radon test. A system can run perfectly and still leave a home elevated if it is undersized or if conditions changed, so the test is the only true confirmation. The sections below walk through each one.

2. Read the manometer (the U-tube gauge)

Almost every radon system has a small clear plastic gauge attached to the vent pipe, usually near where the pipe runs through the basement, garage, or utility area. That gauge is a manometer. It is a U-shaped tube partly filled with colored liquid, and it measures the pressure difference the fan is creating inside the pipe. It is the single most useful at-a-glance indicator of whether the fan is pulling a vacuum.

To read it, just look at the two legs of the U. When you first installed the system, the installer should have noted where the liquid sat. The thing to watch for is whether the two columns are at different heights. The fan pulls the liquid down on one side and pushes it up on the other, so a working system shows the two sides clearly out of balance. For a deeper walkthrough of exactly how to interpret the numbers and markings, see our radon manometer reading guide.

3. What a level vs unlevel reading means

Here is the simple version. An unlevel reading is good. A level reading is bad. When the two columns of liquid sit at different heights, the fan is creating suction and moving air through the pipe, which is exactly what you want. The bigger the difference between the two sides, the more vacuum the fan is pulling, though the important thing is simply that the sides are not equal.

When the two columns sit at the same height, level with each other, the fan is not creating a pressure difference. That usually means the fan has stopped, lost power, or worn out, or that there is a leak or blockage in the pipe. A level manometer is the most common visible sign of a failed radon system, and it is the reading that should prompt you to investigate right away.

4. Confirm the fan is actually running

The fan is the heart of the system. It is a sealed inline unit on the vent pipe, usually mounted outside the house, in an unconditioned attic, or in the garage, never in the living space. A working fan runs continuously and makes a steady, low hum. You should not hear rattling, grinding, or loud vibration. A healthy fan is quiet but never completely silent.

To check it, put a hand or an ear close to the fan housing and listen for the hum. You can often feel a faint vibration through the pipe. If the fan is silent and the pipe is still, the fan may have stopped. Two quick things to rule out are power related: confirm the fan is plugged in or wired in and that its circuit breaker has not tripped. A silent fan paired with a level manometer is a strong sign the fan has failed or lost power.

5. The only true confirmation: a follow-up radon test

The manometer and the fan check both tell you the system is running. Neither one tells you what the radon level in your home actually is. The manometer measures pressure, not radon, and the fan running does not guarantee the system is removing enough soil gas to keep the home below the action level. The only way to know the system is doing its job is to test the air your family breathes.

The benchmark is 4.0 pCi/L. That is the action level used across US radon programs, and a working system should keep your home below it. Run a radon test in the lowest lived-in level of the home, follow the test instructions for closed-house conditions, and compare the result to 4.0 pCi/L. A result below that confirms the system is working. A result at or above it means the system needs attention even if the fan is running and the gauge looks fine.

Keep in mind that the action level marks where action is required, not a sharp line between safe and unsafe. Radon risk is continuous, so lower is always better, and many homeowners aim to keep their post-mitigation level as low as the system reasonably allows.

6. Warning signs your system has a problem

A few clear signals tell you a radon system is no longer doing its job. Watch for these, and treat any one of them as a reason to investigate or test.

• The manometer columns are level. Both sides sitting at the same height means the fan is not pulling suction. This is the most common sign of a failed or unpowered fan.
• No fan noise or airflow. A silent fan with no vibration or exhaust airflow suggests the fan has stopped, lost power, or worn out.
• Rising radon readings. If a retest comes back higher than before, or above 4.0 pCi/L, the system is no longer keeping up, even if the fan still runs.
• New noises. Rattling, grinding, or loud vibration points to a fan that is wearing out and may soon fail.
• Visible pipe damage. A cracked, disconnected, or crushed section of vent pipe can break the suction the system depends on.

7. When to retest or call for service

Match your response to what you see. If the manometer is level or the fan is silent, first rule out power: check that the fan is plugged in or wired and that the breaker has not tripped. If power is fine and the gauge stays level or the fan stays silent, the fan has likely failed and needs to be replaced. Replacing a worn fan restores suction and is the most common service a long-running system needs. Our radon fan replacement guide explains what that involves.

If the fan runs and the gauge looks healthy but your radon readings are rising, that is a sign the system needs evaluation rather than just a restart. The fan may be weakening, a suction point may no longer cover the whole slab, or a new entry path may have opened. A technician can measure the suction and decide whether the system needs a stronger fan, an added suction point, or sealing. For routine upkeep that keeps a system healthy between issues, see our radon system maintenance guide.

8. How often to retest a working system

Think of your checks on two schedules. The quick visual checks, a glance at the manometer and a listen for the fan, take seconds and are worth doing about once a month so you catch a level gauge or a silent fan early. The full radon test is the periodic confirmation, and a common rule of thumb is to retest at least every two years.

Retest sooner whenever something changes that could affect radon entry or air movement: major foundation work, a new HVAC system, significant air sealing or renovation, or any warning sign from the gauge or fan. Also retest whenever you notice the manometer has drifted toward level or the fan sounds different. The point is not to test constantly, but to confirm with a test rather than assume. A system that ran well for years can quietly fall behind, and a fresh reading below 4.0 pCi/L is the proof that it is still protecting your home.

9. Frequently asked questions