1. What Is a Manometer?
A manometer is a simple pressure gauge installed on your radon mitigation system's pipe. It is typically a small clear U-shaped tube filled with colored liquid (usually red or blue oil) that is mounted on the PVC pipe in your basement, garage, or utility room.
The manometer measures the pressure difference between the inside of the pipe (where the fan is creating suction) and the surrounding air. This pressure difference tells you whether the fan is running and creating the vacuum that draws radon away from your foundation.
Think of it as a dashboard indicator for your radon system. Just as the check engine light tells you something about your car's health at a glance, the manometer tells you whether your radon system is functioning properly.
Manometers are required by industry standards on all active radon mitigation systems. If your system does not have one, contact your installer. It should have been included with the original installation. For a broader overview of what to check, see our radon system maintenance guide.
2. How to Read Your Manometer
Reading a manometer is simple. You are looking at a U-shaped tube with liquid in it. The liquid responds to the pressure difference created by the fan.
Find the manometer
It is mounted directly on the PVC pipe, usually in the basement, garage, or utility area where the pipe is visible. It is a small clear tube in a U shape, about 6 to 8 inches tall, with colored liquid inside.
Look at the liquid levels
The tube has two sides. One side connects to the inside of the pipe (the suction side). The other is open to the room air. If the fan is working, the suction pulls the liquid down on one side and up on the other.
Check for a difference
The key question is: are the two sides at different levels? If yes, the fan is creating suction and your system is working. The specific amount of difference is less important than the fact that a difference exists.
The Simple Rule
Uneven liquid levels = system is working. Equal liquid levels = something may be wrong. That is really all you need to know for a basic check. If the levels are uneven, your system is doing its job.
3. What Normal Looks Like
A normal manometer reading shows one side of the liquid higher than the other. The amount of difference depends on your specific system, fan model, and sub-slab conditions. Some systems show a small difference (0.5 inches or less), while others show a larger difference (1 to 2 inches or more).
Your radon professional should have marked the initial reading on the manometer when the system was installed. This mark shows you what normal looks like for your specific system. Some installers use a line drawn on the tube or a sticker indicating the expected reading.
If there is no initial marking, note the current reading yourself. Write the date and the liquid levels on a piece of tape and attach it near the manometer. This gives you a reference point for future checks.
It is normal for the reading to fluctuate slightly over time. Wind, barometric pressure changes, and seasonal temperature variations can cause minor changes. What you are watching for is a significant change from the established normal, or the levels becoming equal.
4. Signs of a Problem
Several manometer readings indicate that something needs attention.
Equal Levels
Both sides of the liquid are at the same height. This means the fan is not creating suction. Check the fan power first. If the fan has power and is running, there may be a pipe blockage, disconnection, or fan failure. Contact your radon professional.
Significantly Lower Than Normal
If the difference between sides has decreased noticeably from your normal reading, the fan may be losing efficiency. This can happen as bearings wear or if a partial blockage is developing. Monitor it and call for radon mitigation services if it continues to decline.
No Liquid Visible
If the liquid has evaporated or drained out, the manometer cannot give you useful information. It needs to be refilled. This does not mean the system is not working. It just means you cannot verify it visually until the gauge is refilled.
Reversed Reading
If the liquid is higher on the pipe side (the opposite of normal), it could indicate a pressurization issue. This is unusual and should be evaluated by a professional. It might mean the pipe connections are reversed or there is a significant system problem.
5. Common Reading Changes
Some manometer changes are normal and do not indicate a problem. Understanding these helps you avoid unnecessary worry.
Wind can affect the reading temporarily. Strong wind blowing across the exhaust cap can create additional suction or back-pressure, causing the manometer reading to fluctuate. This is normal and resolves when the wind dies down.
Barometric pressure changes can also affect the reading slightly. A dropping barometer (before a storm) may increase the reading slightly, while a rising barometer may decrease it. These are minor fluctuations and not a concern. Learn more about how radon enters your home and why these pressure dynamics matter.
Seasonal changes in soil moisture can affect sub-slab conditions. During heavy rain periods, saturated soil can reduce the permeability beneath your slab, which may slightly change the manometer reading. This is normal seasonal variation.
If you open a basement window or door near the system pipe, the change in air pressure may briefly affect the reading. This normalizes once the opening is closed.
6. Manometer Maintenance
Manometers require very little maintenance, but a few things are worth knowing.
The liquid can evaporate over time, especially in warm, dry environments. If the liquid level has dropped to the point where you cannot see a meaningful difference between the two sides, it is time to refill. Most manometers use red gauge oil, which is available at plumbing supply stores, or you can use water with a few drops of food coloring.
Keep the manometer tube clean. Dust and debris can accumulate on the outside, making it hard to read. Wiping it with a damp cloth occasionally keeps it legible.
If the manometer becomes dislodged or falls off the pipe, reattach it with the zip ties or brackets it came with. Make sure the tubing connections to the pipe are intact. If you are unsure about reattaching it properly, ask your radon professional at the next service visit.
Do Not Ignore It
The manometer is the easiest early warning system for radon system problems. A 5-second glance once a month can tell you whether your family's protection is intact. If you notice a change from normal, act on it. Do not wait and hope it resolves itself. For automated alerts, consider an electronic radon system monitor.
