How to Read a Radon Manometer Reading for Your Mitigation System

If you own a home in St. Louis with a radon mitigation system, you have likely glanced at the device mounted on a pipe in your basement or crawlspace and wondered what it actually tells you. That small gauge, often a U-tube manometer or a digital manometer, is one of the most important tools for confirming your system is pulling radon out from under your foundation. A radon manometer reading is not just a number. It is a snapshot of how well your sub-slab depressurization system is working, and it can alert you to problems long before a radon test kit would show elevated levels.

I have spent years inspecting and repairing these systems across the St. Louis area, and I can tell you that many homeowners ignore the manometer until something goes wrong. By then, the radon fan may have failed silently, or a suction point may have become blocked. Learning to check a radon manometer reading takes about thirty seconds once you know what to look for. This article will walk you through the basics of manometer installation, how to interpret the reading, and what to do if the numbers look off.

What a Manometer Measures

A manometer measures pressure difference. In a typical radon mitigation system, the radon vent pipe creates negative pressure under the slab, drawing soil gases up and out through the radon vent pipe. The manometer, whether it is a simple U-tube model from Dwyer Instruments or a digital manometer, shows the difference between the pressure inside the pipe and the air pressure in your basement. That difference is measured in inches of water column, a standard unit for low-pressure systems.

A normal radon manometer reading for a functioning system usually falls between 0.5 and 2.0 inches of water column. The exact number depends on the soil type, the number of suction points, and the strength of the radon mitigation fan. What matters more than the absolute number is consistency. If your manometer reading changes suddenly or drops to zero, something is wrong.

Types of Manometers You Might See

Most older systems in St. Louis use a U-tube manometer. It is a clear plastic tube shaped like a U, partly filled with manometer fluid, often a red-dyed oil or water. One end connects to the radon vent pipe; the other is open to the room air. When the system is running, the fluid rises higher on the side connected to the pipe, and the difference between the two fluid levels is your reading. These are reliable and cheap, but they can be hard to read if the fluid has evaporated or if the tube has gotten dirty.

Newer systems often use a digital manometer from Dwyer Instruments or another manufacturer. A digital manometer gives a precise numeric readout and often includes a pressure alarm. It is easier to read at a glance, but it runs on batteries. If the batteries die, you lose the display, and you may not notice until the radon system has been off for weeks. For that reason, I always recommend checking a digital manometer at least once a month during a radon system inspection.

How to Take a Radon Manometer Reading

Taking a radon manometer reading is straightforward. For a U-tube manometer, locate the two fluid columns. The fluid should be at the same level on both sides when the system is off. With the system running, look at the side connected to the pipe. The fluid will be higher than the open side. Read the number at the top of the fluid column on the pipe side. That number, minus the level on the open side (usually zero), is your inches of water column. For a digital manometer, simply press the power button if it is not already on, and read the displayed value. Make sure the unit is set to inches of water column, not pascals or millibars, unless you know how to convert.

A stable radon manometer reading that stays within the range your installer documented means the radon mitigation fan is running, the suction point is clear, and the sub-slab depressurization is active. If the reading is lower than normal, the fan may be failing, or there could be a blockage in the radon vent pipe. If the reading is zero, the fan is off or the pipe is completely blocked. If the reading is negative pressure on the open side, the manometer may be installed backwards, or the system could be pulling air from inside the house, which is not good.

Common Problems Revealed by the Manometer

Over the years, I have seen the same issues show up again and again during fan replacement and radon system inspection calls. A sudden drop in the manometer reading often points to a failed radon fan. Radon fans are built to run continuously, but they do wear out, especially if they were undersized for the house. A gradual decline in the reading can mean the fan is losing efficiency, or that soil is settling around the suction point and reducing airflow.

Another common problem is a manometer that shows no change at all when the fan is running. This usually means the manometer installation is faulty. I have seen U-tube manometers where the fluid has evaporated completely, leaving only a dry tube. I have also seen digital manometers with dead batteries that still show a last reading from months ago. If you suspect your manometer is not working, replace the batteries or refill the U-tube with the correct manometer fluid, which is usually a colored oil available from Dwyer Instruments or your local radon contractor.

When to Call a Professional

Not every change in a radon manometer reading means you need a service call. Seasonal changes in soil moisture can cause small fluctuations. But if the reading drops by more than half of what it was when the system was new, or if it stays at zero for more than a day, it is time to call a professional. In St. Louis, radon levels can vary widely even between neighboring houses, so relying on a radon test kit alone is not enough. A test kit gives you a snapshot over a few days, but the manometer gives you continuous feedback. If you see a concerning reading, schedule a radon system inspection with a qualified contractor like Air Sense Environmental to check the entire system, including the radon vent pipe, suction point, and fan.

I have seen homeowners try to fix their own systems after misreading the manometer. They assume a low reading means the fan is too weak, so they replace it with a larger unit. Sometimes that works, but often the real issue is a crushed pipe or a blocked suction point. Oversizing the radon mitigation fan can actually cause problems by pulling too hard and creating negative pressure that draws in moisture or radon from other pathways. A professional can diagnose the root cause and recommend the right fix, whether that is a simple fan replacement or a more involved repair to the sub-slab depressurization system.

Maintaining Your Manometer

To keep your manometer reading accurate, check it monthly and clean the tube if it gets cloudy. For U-tube manometers, make sure the fluid level is topped off and that there are no air bubbles in the line. For digital manometers from Dwyer Instruments or other brands, replace the batteries once a year even if the display still looks fine. Also, verify that the manometer is mounted level. A tilted manometer can give a false reading, especially on a U-tube model. If you have a digital manometer, check the zero calibration occasionally by disconnecting it from the pipe and seeing if it reads zero in open air.

One more thing: if your system has multiple suction points, there should be a manometer at each one. A single manometer at the main radon vent pipe tells you the overall system pressure, but it does not tell you if one branch is blocked. During a radon system inspection, a technician should check each manometer individually to ensure all suction points are drawing properly.

Final Thoughts

A radon manometer reading is your early warning system. It costs nothing to look at, and it can save you from living with high radon levels for weeks or months before you notice a problem. In the St. Louis area, where radon is a known issue due to the geology, every homeowner with a mitigation system should know how to read their manometer and what a normal reading looks like for their house. If you ever have doubts, call a local expert like Air Sense Environmental to perform a radon system inspection. They can verify your radon manometer reading, check the radon mitigation fan, and ensure your sub-slab depressurization system is working as designed. A few minutes of attention each month can keep your family safe and your system running for years.