Radon Mitigation System Maintenance: Reading the U-Tube Gauge

TL;DR: The U-tube gauge on your radon fan should show a visible height difference between its two liquid columns, usually somewhere between 0.5 and 2.0 inches, which confirms the fan is pulling suction under your slab. If the liquid sits level in both columns, the fan has likely stopped working and radon levels in your home can climb back above the EPA's action level of 4 pCi/L within days.

_Last reviewed: August 2026 · 6 min read_

You paid a few thousand dollars for a radon mitigation system, and now there's a little plastic U-shaped tube on the pipe with colored liquid inside it, and nobody explained what you're supposed to do with it. Here's the plain-language version: that gauge is your only free, instant way to know if the fan is still doing its job.

Okoniq Property Hub lets you log a monthly photo of your U-tube reading alongside your other maintenance checks, so you have a dated record if you ever need to prove the system was running.

What does the U-tube gauge actually measure?

The gauge measures suction, or negative pressure, inside the vent pipe that runs from under your foundation up through the roof. The radon fan sits in that pipe and continuously pulls soil gas — including radon — out from under the slab before it can seep into your living space. That pulling action creates a pressure difference, and the U-tube gauge shows it as a height difference between two columns of colored liquid, usually red or blue oil.

When the fan is running normally, one side of the liquid rises and the other drops. The gap between them, measured in inches, tells you the fan is actively moving air. No gap means no suction, which means the system isn't protecting anything. This is the same logic behind checking whether your bathroom exhaust fan is actually doing its job — a fan that spins but doesn't move air is just noise.

How do I read the U-tube gauge correctly?

Look at the two liquid columns and note the difference in height between them, not the absolute position of either one. Most residential systems run in the 0.5 to 2.0 inch range on the gauge, though the exact number on your system depends on the fan model and pipe layout your installer used. What matters more than hitting a specific number is consistency — write down the reading when the system is installed, then compare future readings against that baseline.

If your installer left a paper tag near the gauge with a written baseline number, use that instead of a generic range. A gauge reading a full inch lower than it was at installation is worth investigating even if it's still technically "in range," since it can signal a slow leak in the pipe or a fan losing strength before it fails completely.

What does it mean if the gauge shows no difference at all?

A level gauge, where both columns sit at the same height, means the fan has stopped creating suction and your system is not currently reducing radon. This can happen because the fan motor burned out, a breaker tripped, someone unplugged it, or a crack developed somewhere in the pipe network that's bleeding off the vacuum. Radon fans typically last 5 to 10 years, and motor failure is the single most common reason for a level gauge.

Don't assume the problem is minor. Radon levels can rebound to pre-mitigation levels within 24 to 48 hours of a fan stopping, according to EPA guidance on system performance. If you have a basement or crawlspace with foundation issues, it's also worth checking for foundation cracks that let more soil gas migrate upward, since a failing seal around the slab can undercut even a properly running fan.

| Gauge Reading | Likely Meaning | What to Do | |---|---|---| | Same as install baseline | System running normally | Check again next month | | Lower than baseline but showing a gap | Fan weakening or minor leak | Monitor weekly, plan for fan replacement | | Level, no gap | Fan has stopped | Check breaker/plug, call installer if unresolved |

How often should I check the gauge, and when do I retest the air?

Check the U-tube gauge once a month with a quick visual glance, and do a full radon air test every two years even if the gauge looks fine. The monthly gauge check catches sudden failures, but it doesn't tell you the actual radon concentration in your home, only whether the fan is moving air. A short-term or long-term radon test kit, the kind sold for $15 to $30 at hardware stores, is the only way to confirm the number itself.

The EPA recommends retesting after any major renovation, after replacing the fan, or every two years as routine maintenance, since seasonal ground moisture and foundation settling can change how radon enters even with the fan running well. If your home also has moisture issues near the foundation, pair your radon check with a look for signs water is undermining your foundation, since the same cracks that let water in often let soil gas in too.

Do I need a professional for radon fan maintenance?

Call a licensed radon mitigation contractor for fan replacement, pipe repairs, or any reading you can't explain, but the monthly gauge check itself is something any homeowner can do without tools. Fan replacement typically runs $300 to $600 for parts and labor, and most licensed installers can swap a failed unit in under two hours since the piping is already in place.

If you're an owner-operator managing a rental with a radon system, treat the fan the same way you'd treat a carbon monoxide detector — a silent safety device that needs a documented check schedule, not just a "set it and forget it" install. Tenants generally won't report a level gauge because they don't know what it means, so the monthly check has to be on the owner's calendar, not the renter's.

FAQ

What is a normal U-tube gauge reading for a radon system?

There's no single universal number, but most systems show a 0.5 to 2.0 inch difference between the two liquid columns, and the exact baseline should be the reading recorded at installation for your specific fan and pipe layout.

How long do radon mitigation fans last?

Radon fans typically last 5 to 10 years depending on the model and how continuously they run, and a sudden level gauge reading is often the first sign a fan has reached the end of its life.

Can I fix a radon fan myself?

Checking the breaker and making sure the fan is plugged in is fine to do yourself, but motor replacement and pipe repairs should go to a licensed radon mitigation contractor since improper venting can push radon back indoors instead of out.

How much does it cost to replace a radon fan?

Replacing a radon fan generally costs $300 to $600 including labor, since the existing pipe and vent routing usually don't need to change, only the fan unit itself.

Do I still need to test for radon if the gauge looks normal?

Yes — the gauge only confirms the fan is running, not the actual radon level in your home, so a retest every two years with a radon test kit is the only way to verify the concentration is still below the EPA action level of 4 pCi/L.

This is educational information, not medical or contractor advice. Call a licensed radon mitigation professional for fan replacement, pipe repairs, or any gauge reading you can't explain.