How to Read Your Radon System’s U-Tube Manometer

If you have a active radon mitigation system running in your home, you’ve likely noticed a clear, plastic U-shaped tube attached to the white PVC vent pipe in your basement or utility area.

That little gauge is called a U-Tube Manometer.
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A lot of homeowners call us thinking it’s a dynamic digital radon monitor that shows their daily air levels. It isn't. The manometer is simply a pressure gauge designed to tell you one critical thing: is your system's inline exhaust fan actually running and pulling air out from beneath your foundation?

Checking this gauge takes less than ten seconds, and it is the absolute easiest way to ensure your family remains protected from radioactive radon gas. Here is exactly how to read it, what the numbers mean, and how to spot a system failure before it becomes dangerous.

The 10-Second Inspection: Running vs. Failed

The manometer works using static pressure. One end of the clear tube goes inside the PVC pipe to measure the suction created by the fan, while the other end remains open to the ambient air in your room.
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The gauge contains a colored liquid (usually red or blue). The difference between the fluid levels on the left and right sides of the tube indicates the amount of suction your system is pulling.

Scenario A: The Liquid is Unequal (System is Working)

​Look closely at the fluid columns. If the liquid is higher on one side than the other, your fan is successfully creating negative pressure. The vacuum pressure inside the pipe is physically drawing the fluid up the side connected to the PVC.
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• What to look for: A difference of anywhere from 0.5 inches to 2.5 inches on the printed scale is completely normal for most residential setups.

Scenario B: The Liquid is Level at Zero (System is Broken)

​If the fluid levels on both sides of the U-tube line up perfectly at the 0.0 mark, there is zero vacuum pressure inside the pipe. Your system is not pulling sub-slab gases out of your home.
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• What to look for: If the liquid forms a flat, even line across both columns, your inline exhaust fan is turned off, has thrown a breaker, or has failed completely.

How to Calculate Your System's "Normal" Operating Pressure

Every house sits on different soil conditions—some clay or packed dirt underneath your slab creates high resistance, while loose sand or gravel allows air to flow easily. Because of this variation, every radon system has its own unique "baseline" operating pressure.
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To figure out your specific system's reading, look at the numbered hash marks on the gauge:

1. Step 1. Locate the high and low lines - Find the top point of the fluid on the higher side, and the top point of the fluid on the lower side.
   
   
2. Step 2: Read the markings - If the left side sits at 0.5 inches above zero and the right side sits at 0.5 inches below zero, your total differential pressure is 1.0 inch.
   
   
3. Step 3: Write it down on the pipe - Take a permanent marker and draw a small line on the PVC pipe right next to the current fluid level. This is your home's unique baseline. Check back once a month to ensure the fluid hasn't dropped down to zero or significantly shifted.

Troubleshooting a "Zero" Reading

If you walk down to your utility room and find the fluid sitting perfectly flat at zero, don't panic. Before calling out a certified mitigation specialist, run through this quick checklist:
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• Check the Power Switch: Active systems feature an electrical shut-off switch located near the exterior fan housing or in your attic space. Make sure a family member didn't accidentally flip it off thinking it was a spare light switch.
• Inspect Your Breaker Panel: Find your home's main electrical panel and check for any tripped circuit breakers labeled for the garage, attic, or outdoor outlets.
• Inspect the Vinyl Tubing: Sometimes the small plastic tube connecting the top of the manometer gauge into the PVC pipe gets bumped loose or cracked by storage boxes. Make sure the hose is pushed securely into the rubber grommet on the pipe.

Important Note: If the power is on, the breakers are clear, the tube is connected, and the gauge still reads zero, your inline fan motor has likely burnt out. Because radon fans run continuously 24 hours a day, 365 days a year, their bearings typically give out after 3 to 5 years of operation.

What to Do Next

If your system passes the manometer check but it has been more than two years since your last air test, it's still highly recommended to perform a short-term charcoal canisters test. A running fan proves the mechanics are functioning, but a certified testing kit is the only way to verify that your structural sub-slab depressurization is effectively keeping the radon levels below the EPA action threshold of 4.0 pCi/L.
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If your system's gauge is sitting completely flat at zero and you can't restore power, get in touch with our team at Wichita Radon (316-799-3393) to diagnose the system and get a professional fan replacement scheduled.