If you own a home in St. Louis with a radon mitigation system, you probably do not think about it much. The fan hums, the manometer shows a steady reading, and you forget it is there. But that silence can be broken by something as simple as a radon pipe leak. A small crack, a loose joint, or a gap left by settling ground can turn an effective system into a waste of electricity and, worse, a health risk.
I have spent years working with these systems, and I have seen what happens when a radon pipe leak goes unnoticed. The symptoms are subtle at first. The manometer reading drops. The radon fan sounds different, maybe a little louder or more strained. The owner assumes the system is still working because they can hear it running. But hearing it and measuring it are two different things. A radon mitigation system relies on negative pressure to pull soil gas from beneath the slab and vent it safely above the roofline. When a radon pipe leak develops, that negative pressure weakens. The system stops pulling as it should, and radon can begin to seep back into the living space.
How a Radon Pipe Leak Develops
A radon pipe leak can happen in several ways. The most common cause I see is movement in the ground. St. Louis has heavy clay soils that expand and contract with moisture. Over time, that movement can shift the PVC pipe that runs from the suction point under the slab to the radon fan and up through the radon stack. If the pipe is not supported properly, the joints can separate or crack. Another cause is UV exposure. If the radon vent pipe runs along an exterior wall and the PVC is not UV-rated, the sun can make it brittle. A brittle pipe is one small impact away from a leak.
I once worked on a house in the Central West End where the homeowners had a Jersey-style radon system installed by a previous owner. The system had a long horizontal run that passed through an unfinished part of the basement. The pipe was not strapped to the ceiling, and it sagged over time. The sag created a low spot where condensation collected, and that condensate eventually froze in a cold snap. The ice expanded and split the pipe at a coupling. That radon pipe leak was small, maybe a quarter-inch gap, but it was enough to drop the system pressure from 0.5 inches of water column to nearly zero. The manometer was showing a reading that looked normal until I checked it with a digital gauge. The homeowner had been living with a broken system for months.
Why a Radon Pipe Leak Matters for Health
Radon is a radioactive gas that comes from the natural decay of uranium in the soil. The EPA considers it the second leading cause of lung cancer after smoking. The goal of a sub-slab depressurization system is to keep that gas from entering the home. When a radon pipe leak occurs, the system loses its ability to create negative pressure under the slab. Instead of pulling soil gas up and out, the leak can allow radon to escape into the crawlspace or basement before it ever reaches the fan. In some cases, the leak can even pull indoor air into the pipe, which dilutes the vacuum and makes the system less effective.
The only way to know if your system is working is to test it. A radon test kit is cheap and easy to use. I recommend testing every two years, or any time you notice a change in the system. If the test shows elevated levels, a radon pipe leak is one of the first things to check. Do not assume the system is fine because the fan is running. I have seen fans running happily on a completely disconnected pipe.
Diagnosing a Radon Pipe Leak
Diagnosing a radon pipe leak takes a little patience. Start by looking at the manometer. If the fluid levels are not stable, or if the reading is lower than it was when the system was installed, something is wrong. Next, walk the entire pipe run from the suction point to the radon stack. Look for cracks, separated joints, or pipes that have pulled away from their supports. Pay special attention to areas where the pipe passes through a wall or floor, because those are common spots for movement.
If you cannot see a leak but the pressure is off, you can use a smoke pencil or a piece of tissue paper to check for air movement around joints. Hold it near each connection. If the tissue flutters, you have found a leak. In some cases, the leak is not in the pipe at all but in the sealant around the suction point. The sealant that bonds the pipe to the slab can dry out and crack over time. That is still a radon pipe leak in the sense that it breaks the closed system, and it needs to be fixed the same way.
One tricky situation I run into involves condensate traps. A condensate trap is a U-shaped section of pipe that collects moisture and allows it to drain without letting air escape. If the trap is not installed correctly, or if it dries out, it can become a radon pipe leak itself. The trap needs to remain filled with water to seal the system. If it evaporates, air can move through it. I always check the trap first on systems that have a horizontal run, because that is where condensation builds up.
Fixing a Radon Pipe Leak
Fixing a radon pipe leak is usually straightforward, but it requires the right materials. For small cracks or separated joints, a good quality PVC cement and primer can seal them. For larger gaps, you may need to replace a section of pipe. I always keep a few extra couplings and a short piece of PVC pipe in my truck for these repairs. The key is to make sure the repair is airtight. A radon mitigation system operates under negative pressure, so even a tiny leak can reduce performance.
If the leak is at the suction point, you may need to reapply sealant around the pipe where it enters the slab. I use a polyurethane-based sealant that stays flexible and bonds well to concrete. Do not use silicone caulk for this, because it does not hold up to ground movement and temperature changes. I have seen silicone fail within a year. A proper sealant will last much longer.
Sometimes the radon fan itself is the source of the problem. If the fan housing has a crack or the gasket is worn, the system can lose pressure even if the pipe is sound. Fan replacement is a common service in St. Louis because the fans run continuously and eventually wear out. When I replace a fan, I always check the entire pipe run for leaks at the same time. It saves the homeowner a second trip and gives me peace of mind that the system is truly sealed.
Preventing a Radon Pipe Leak
Prevention is better than repair. When a radon mitigation system is installed, the pipe should be supported every four feet at minimum. The supports should allow for some movement without putting stress on the joints. If the pipe runs through an area that freezes, it should be insulated or routed differently. I also recommend using schedule 40 PVC rather than the thinner schedule 20, because it is more resistant to cracking.
Another thing to watch for is interference from other work. I have seen homeowners accidentally create a radon pipe leak by moving a pipe to make room for a new water heater or a French drain installation. The pipe gets bumped, a joint loosens, and nobody notices until the next radon test comes back high. If you are doing any work near the radon vent pipe, take a few minutes to check the manometer afterward. It is a small habit that can save you a lot of trouble.
In St. Louis, where basements are common and radon levels can be elevated, a properly functioning system is worth protecting. Companies like Air Sense Environmental see the same problems over and over: old systems that were never maintained, pipes that were never supported, and leaks that went unnoticed for years. A radon pipe leak is not a disaster, but it is a sign that something needs attention. Catching it early keeps the system working and keeps the air in your home safe.
The bottom line is simple. If you have a radon mitigation system, learn what your manometer looks like when it is working normally. Check it a few times a year. If the reading changes, or if you hear a change in the radon fan, do not ignore it. A radon pipe leak is one of the most common problems I see, and it is also one of the easiest to fix. With a little awareness and a few basic tools, you can keep your system running the way it was designed to run.