What Your Radon Manometer Reading Actually Tells You
A radon mitigation system is something most homeowners in St. Louis never think about until they have a problem. When you buy a house with an existing system or have one installed, the installer usually points to a small gauge on the wall and says something like "that tells you the system is working." But what does a radon manometer reading really mean, and what should you do when the numbers start to change?
After years of maintaining and repairing these systems, I have seen how quickly a small change in pressure can lead to elevated radon levels inside a home. The manometer reading is not just a number. It is the single best indicator of whether your sub-slab depressurization system is actually pulling radon out from under your foundation and venting it safely above the roofline.
How a Manometer Works in a Radon System
Most residential radon systems use a U-tube manometer filled with a colored fluid, usually red or blue. One side of the tube connects to the radon vent pipe, and the other side is open to the air inside the basement or crawl space. The pressure difference between these two sides pushes the manometer fluid up on one side and down on the other, giving you a reading in inches of water column. That number is your radon manometer reading, and it tells you how much negative pressure your radon mitigation fan is creating at the suction point.
A typical reading for a properly working system in the St. Louis area falls somewhere between 0.5 and 2.0 inches of water column. But that range depends on soil type, the depth of the suction point, and the specific radon fan installed. What matters more than the absolute number is that the reading stays stable over time. A sudden drop usually means something changed, and that change often means radon is finding its way back into the living space.
What a Low or Zero Reading Means
If you look at your manometer and see the fluid level sitting flat, with no difference between the two sides, you have zero negative pressure. That is a clear sign that your radon mitigation system is not moving air. The most common cause is a failed radon mitigation fan. Fans run continuously, often for years, and the bearings wear out. When the fan stops spinning, the system loses its ability to create negative pressure, and radon can build up under the slab again.
A zero reading can also mean there is a blockage in the radon vent pipe. I have seen nests, debris, and even collapsed sections of pipe prevent airflow. In those cases, the fan may be running fine, but the path for the air is blocked, so the pressure never builds. A radon system inspection should always include checking the manometer reading at the point of installation and comparing it to what the system showed when it was first installed.
Another possibility is a leak in the suction piping system. If there is a crack or a loose joint somewhere between the suction point and the fan, the fan pulls air from that leak instead of from under the slab. The manometer reading will drop, and the system will not effectively remove radon from the soil.
What a High Reading Can Indicate
A radon manometer reading that is higher than normal can be just as concerning as a low one. Some people assume more pressure is better, but that is not always true. A high reading can mean the fan is oversized for the soil conditions under your home. In St. Louis, the soil is a mix of clay, loess, and fractured bedrock, and not all of it allows air to move freely. If the fan pulls too hard against a tight soil, it can actually create a vacuum that pulls radon up from deeper layers rather than venting the shallow soil gas.
High negative pressure can also cause the system to pull conditioned air from inside the house down into the sub-slab area, which wastes energy and can make the basement feel drafty. A properly sized radon mitigation fan should move enough air to keep radon levels below the EPA radon guidelines without overworking the soil or the structure.
Digital vs. U-Tube Manometers
Many newer systems use a digital manometer instead of the traditional U-tube style. Digital manometers from manufacturers like Dwyer Instruments give you a precise readout in inches of water column and often include a memory function that stores peak readings. They are more accurate and easier to read, especially for a homeowner who does not want to stare at a column of colored fluid.
But digital manometers have their own drawbacks. They run on batteries, and when the battery dies, the reading disappears. I have walked into basements where the digital display was blank, and the homeowner had no idea how long the system had been running without a functional gauge. A U-tube manometer, by contrast, never needs power. The fluid level will always show you the pressure difference as long as the tube is intact and the fluid has not evaporated. For that reason, many installers, including Air Sense Environmental, still prefer the U-tube design for long-term reliability.
Whichever type you have, the most important habit is checking the reading regularly. Once a month is enough for most homes. Write down the number or take a photo, so you can spot a trend before it becomes a problem.
What a Manometer Reading Does Not Tell You
A radon manometer reading tells you about pressure, but it does not tell you about radon concentration. You can have a perfect reading of 1.0 inches of water column and still have elevated radon levels inside the house if there is a separate entry point for soil gas that the system does not cover. That is why the EPA radon guidelines recommend testing with a radon test kit at least every two years, even if your mitigation system seems to be running fine.
I have seen homes where the manometer reading looked textbook, but a radon test came back high because the system was only pulling from one suction point and the radon was entering through a different part of the foundation. In those cases, the fix often involves adding a second suction point or extending the existing piping to cover more area under the slab.
The manometer is a diagnostic tool, not a complete safety guarantee. It tells you that your radon mitigation fan is running and that the system is depressurizing the soil under the slab. But it does not tell you whether that depressurization is happening in the right places or at the right rate for your specific home.
Common Manometer Problems and Fixes
Over the years, I have seen a few recurring issues with manometers in St. Louis radon systems. The fluid can evaporate over time, especially in systems that are exposed to heat or direct sunlight. If the fluid level looks low, the reading will be off. Replacing the fluid is simple but requires the right type of colored fluid, usually a red oil that is lighter than water and resists evaporation.
Another common problem is the manometer itself getting knocked out of level. If the bracket that holds it shifts, the zero point changes, and the reading becomes inaccurate. A quick check with a small level can confirm whether the manometer is still mounted correctly. If it is off, loosening the bracket, leveling it, and retightening is a five-minute fix.
Sometimes the tubing that connects the manometer to the radon vent pipe gets pinched or clogged. A kink in the line can cause a false reading, making it look like there is more or less pressure than there actually is. Tracing the line from the manometer to the pipe and checking for obstructions is part of any thorough radon system inspection.
Finally, the manometer itself can fail. The U-tube can crack, or the seals can leak. In digital models, the sensor can drift over time. If your reading seems off and you have ruled out the other causes, replacing the manometer is a relatively inexpensive way to get back to accurate monitoring.
When to Call for a Fan Replacement
If your radon manometer reading drops to zero and stays there even after you have checked for blockages and confirmed the manometer is working, the radon mitigation fan is almost certainly the culprit. Fan replacement is a job that requires matching the correct fan to your system's pressure and flow requirements. An undersized fan will not pull enough negative pressure, and an oversized fan can cause the problems I mentioned earlier.
Air Sense Environmental handles fan replacements for existing systems throughout the St. Louis area. The process usually takes a couple of hours and involves disconnecting the old fan, installing the new one, and verifying the manometer reading returns to the correct range. After the replacement, a follow-up radon test is a good idea to confirm the system is back to full effectiveness.
The bottom line is that your manometer reading is your system's way of telling you whether it is doing its job. Learn what it says when the system is working, check it regularly, and take action when it changes. A small investment in monitoring can save you from a much larger problem down the road.