What a Radon Pipe Leak Means for Your Mitigation System

Understanding the Hidden Problem in Your Radon Mitigation System

If you own a home in St. Louis with a radon mitigation system, you probably know the basics: the radon fan pulls soil gas from beneath your slab and vents it above the roofline. What you might not realize is that the entire setup depends on a sealed network of PVC pipe. A single failure anywhere in that network - what I call a radon pipe leak - can compromise the whole system. I have seen it happen more often than most homeowners expect, and the fix is rarely obvious.

The typical radon mitigation system in this area uses sub-slab depressurization. A suction point is cut through the concrete floor, a pipe is run to a radon fan mounted in the attic or on the exterior, and the discharge goes through a radon vent pipe to the outdoors. The fan creates negative pressure under the slab, and that negative pressure pulls soil gas up and out. If the pipe network loses its seal anywhere, the negative pressure drops, and the system stops working as designed.

Where Radon Pipe Leaks Show Up

I have worked on dozens of systems that looked fine from the outside but had a radon pipe leak hiding in plain sight. The most common location is at the joint between the suction point and the first length of PVC pipe. If that seal is poor, or if the pipe shifts after the concrete cures, you get a slow leak that never fully develops but never goes away. Another frequent spot is where the pipe exits the slab. That transition is vulnerable because the pipe can rub against the concrete edge, especially in older homes with some settlement.

A less obvious place is the horizontal run in the crawl space or basement. Many systems have a long horizontal run before the pipe turns up to the radon fan. That horizontal section lies on the ground or is suspended from joists, and it can be bumped, stepped on, or crushed by stored items. I once found a radon pipe leak caused by a box of holiday decorations resting directly on the pipe. The weight had deformed the PVC enough to crack the coupling.

Condensate Traps and Jersey-Style Systems

St. Louis homes often have a Jersey-style radon system, which uses a condensate trap to handle moisture that builds up inside the pipe. That trap is a T-shaped fitting with a cleanout cap at the bottom. If the cap is not sealed properly, or if the trap is installed backward, you get a radon pipe leak that lets soil gas escape into the living space. I have also seen condensate traps that were never primed with water, which means the trap itself becomes a path for gas to flow back into the house instead of out the radon stack.

Another issue I run into is the radon stack itself. The stack is the vertical pipe that carries gas above the roof line. If the joints are not glued properly, or if the stack is not supported correctly and sways in the wind, the connections can loosen over time. That creates a radon pipe leak at the top of the system, which is hard to detect because the leak is outside the building envelope. But the effect is the same: the radon fan has to work harder, and the negative pressure under the slab drops.

Why a Small Leak Matters

A tiny radon pipe leak might not sound serious, but consider this: the radon mitigation system operates under very low pressure. A typical manometer reading for a properly sealed system is around 0.5 to 1.5 inches of water column. That is not much pressure. A small leak can drop the manometer reading to near zero, and at that point the system is barely moving soil gas. The radon fan itself might still be running, but the system is not effectively removing gas from under the slab.

I have tested homes where the radon fan sounded normal, the manometer showed a small reading, but the radon test kit came back high. In every case, there was a leak somewhere in the pipe network. The fan was moving air, but it was pulling air from the leak instead of from under the slab. That is why a manometer is not just a nice-to-have; it is the only way to confirm that the system is actually depressurizing the sub-slab space. If you have a manometer installed, check it regularly. If the reading changes without explanation, you have a radon pipe leak.

What Causes Leaks Over Time

PVC pipe is durable, but it is not indestructible. In St. Louis, the freeze-thaw cycle can cause the ground to shift, which puts stress on buried or exposed pipe. I have seen a radon vent pipe that was run through a French drain area, and the gravel eventually settled and pulled the pipe downward, snapping a coupling. The homeowner did not notice because the system still hummed, but the radon levels in the basement had crept back up.

Another cause is poor workmanship from the original install. Some systems are assembled with too much primer or not enough glue. The joints might hold for a year or two, then start leaking as the pipe expands and contracts. I have also seen systems where the installer used the wrong type of sealant around the suction point. A caulk that is not rated for underground use can crack and shrink, opening a gap for soil gas to bypass the pipe entirely. That is not a radon pipe leak in the strictest sense, but it has the same effect on system performance.

When a Leak Is Not the Pipe

Sometimes what looks like a radon pipe leak is actually a problem at the radon fan itself. The fan housing can develop cracks, especially in older units exposed to weather. The gasket where the pipe meets the fan inlet can degrade. If the fan is mounted on the exterior, the sun can weaken the plastic over time. In those cases, the pipe itself is fine, but the system still needs repair. That is where fan replacement becomes necessary. If your system is more than ten years old and you are seeing high radon readings, it might be time to replace the fan rather than chase a phantom leak.

I have also found leaks at the point where the pipe enters the radon stack. That joint is often hidden inside a wall cavity or a chase, and it is easy to miss during a visual inspection. If you suspect a radon pipe leak but cannot find it, a smoke test can help. A technician introduces a small amount of smoke into the pipe while the fan is running. If smoke escapes anywhere, you have found the leak. That is a simple diagnostic tool that saves a lot of guesswork.

What to Do If You Suspect a Leak

If your radon test kit shows levels above 4.0 pCi/L, and your system seems to be running, do not assume the problem is the fan. Check the manometer first. If the reading is lower than it was when the system was installed, you likely have a radon pipe leak. Then inspect all visible pipe joints, especially around the suction point, the condensate trap, and the radon fan. Look for cracks, gaps, or signs of moisture. Moisture around a joint often indicates a leak because the escaping air condenses on the pipe surface.

For joints that are accessible, you can sometimes apply a fresh layer of sealant or PVC cement. But be careful: if the joint is under tension or the pipe is misaligned, a patch might not hold. In those cases, cutting out the damaged section and installing a new coupling is the better fix. That is a job for someone who knows how to work with PVC pipe and understands the pressure requirements of a radon mitigation system. A general handyman might not know that the pipe has to be absolutely airtight, not just watertight.

Keeping the System Reliable

Regular maintenance can prevent most radon pipe leaks. If you have a system in St. Louis, check the manometer once a month. Look at the radon fan for signs of wear. Make sure nothing is leaning against the radon stack or the horizontal run. If you have a condensate trap, make sure it is primed. If you have a Jersey-style system, check the cleanout cap every season. A little attention goes a long way.

When I work on these systems, I always tell homeowners that the radon mitigation system is like a chimney: it only works if the flue is clear and the joints are sealed. A radon pipe leak is the equivalent of a hole in the chimney wall. The fire (or the fan) might still burn, but the smoke (or the radon) is not going where it should. If you catch the leak early, the repair is usually simple. If you ignore it, the radon levels will keep rising, and you will end up paying for a full diagnostic visit anyway.

If you are not sure whether your system has a leak, call a company that specializes in radon system repair. Air Sense Environmental has been doing this work in St. Louis for years. They know the local soil conditions, the common installation mistakes, and the best ways to find and fix a radon pipe leak. They can also tell you if the problem is the fan, the pipe, or the sealant. A trained eye makes the difference between a quick repair and a recurring headache.