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A tankless water heater leaking onto your garage floor or utility closet wall is alarming, and it confuses a lot of homeowners. After all, these units do not store 40 or 50 gallons of water like a traditional tank. So where is the water coming from? The truth is that a tankless unit still moves a lot of water through a compact heat exchanger, valves, and connections, and any one of those points can fail. Because the unit is small and often mounted on a wall, even a slow drip can soak drywall, framing, and flooring before you notice it.
In this guide we explain how tankless heaters work, where a tankless water heater leak usually starts, the six most common causes, and the safe steps to take the moment you spot water. If you are in Lynnwood, WA and need help finding the source, our water heater leak detection service pinpoints the problem without guesswork. Call (425) 380-0028 for availability.
How a Tankless Water Heater Works (and Why Leaks Are Different)
A tankless, or on-demand, water heater has no storage tank. When you open a hot water tap, cold water flows through a flow sensor that fires a gas burner or electric heating elements. The water passes through a heat exchanger, a maze of copper or stainless steel tubing, and exits hot within seconds. When the tap closes, the unit shuts down.
Because there is no tank to rust through, tankless leaks behave differently from tank leaks. Instead of a catastrophic tank rupture, you usually see:
- Drips from pipe connections at the top or bottom of the unit
- Water weeping from inside the cabinet, often from the heat exchanger
- Moisture around the pressure relief valve discharge pipe
- Water pooling beneath a wall-mounted unit, running down the wall
The compact design means leaks hide inside the cabinet. By the time water appears on the floor, it may have been dripping internally for weeks, which is why a moisture intrusion inspection is smart if you see staining on the wall behind the unit.
Common Tankless Water Heater Leak Locations
Start your investigation at these five points, working from the outside in:
1. Hot and cold water connections
The threaded connections where water lines meet the unit are the most common drip point. Vibration, thermal cycling, and aging thread sealant loosen them over time.
2. The heat exchanger
This is the heart of the unit. Mineral scale buildup creates hot spots that stress the metal, eventually causing pinhole leaks or cracks. In areas with hard water, this is the number one internal failure.
3. Pressure relief valve
Tankless units have a temperature and pressure relief valve, just like tank heaters. If system pressure runs high, it will weep or discharge water through its drain pipe.
4. Condensate drain line
High-efficiency condensing tankless units produce acidic condensate that drains through a small line. A clogged or disconnected condensate line looks exactly like a leak.
5. Internal gaskets and O-rings
Rubber seals inside the unit harden with heat and age. When they fail, water seeps from seams in the cabinet.
6 Reasons Your Tankless Water Heater Is Leaking
- Loose or corroded connections. The simplest cause. Pipe fittings work loose over years of heating cycles, and dielectric corrosion eats at mixed-metal joints.
- Hard water scale. Mineral deposits insulate the heat exchanger, forcing it to run hotter. The resulting thermal stress cracks the metal. Annual descaling prevents this.
- Excessive water pressure. Municipal pressure above 80 PSI strains every seal and valve in the unit. A failing pressure reducing valve on your main line is often the root cause.
- Failed internal seals. Heat ages rubber O-rings and gaskets. After 8 to 12 years, internal seals are a common drip source.
- Condensate blockage. In condensing models, algae or debris clogs the small condensate trap, and water backs up into the cabinet.
- Freeze damage. In an unheated garage, residual water in the heat exchanger can freeze during a cold snap, cracking the tubing. Most units have freeze protection, but it only works when the unit has power.
A tankless water heater mixes water, gas or high-voltage electricity, and combustion in one compact box. If you smell gas or see scorching along with the leak, leave the area and call for help immediately. Do not try to open the cabinet yourself.
Is It a Leak or Just Condensation?
Not every drop of water means failure. Condensing tankless units legitimately produce water as a byproduct of efficient combustion, sometimes a gallon or more per day, which should exit through the condensate drain. Normal condensation appears as a steady drip from the condensate line into a drain. A real leak, by contrast, comes from connections, the cabinet seams, or the relief valve pipe, and it often leaves mineral stains or rust marks.
| Leak location | Most likely cause | Urgency |
|---|---|---|
| Pipe connections | Loose fittings, worn thread sealant | Medium: fix soon |
| Inside the cabinet | Heat exchanger crack or failed gasket | High: shut water off |
| Relief valve pipe | High system pressure or bad valve | High: check pressure |
| Condensate line | Clog or disconnected hose | Medium: clear the line |
| Vent pipe joints | Improper slope or failed seal | Medium: inspect venting |
What to Do Right Now: 5 Safe Steps
- Shut off the water. Close the isolation valves on the hot and cold lines at the unit. If you cannot find them, shut the main water valve to the house.
- Cut the power or gas. For electric units, switch off the breaker. For gas units, turn the gas valve to the off position if you know how.
- Contain the water. Mop up standing water and set a bucket under the drip. Move anything valuable away from the wall below a wall-mounted unit.
- Document the leak. Take photos of where water is coming from. This helps a technician diagnose the problem faster.
- Call a professional. Internal tankless repairs involve gas, high voltage, and pressurized water. A leak detection specialist can confirm the exact source before any repair begins.
Repair or Replace? How to Decide
Use the age of the unit as your guide. Tankless heaters typically last 15 to 20 years with maintenance. If your unit is under 10 years old, most leaks, including connection drips, valve failures, and condensate issues, are worth repairing. A cracked heat exchanger on an older unit is a different story: the part is expensive, and labor is intensive, so replacement often makes more sense once a unit passes the 12 to 15 year mark. Also weigh efficiency. A 15-year-old non-condensing unit replaced with a modern condensing model can cut water heating energy use noticeably.
Preventing Future Tankless Leaks
- Descale annually. Flush the heat exchanger with a descaling solution every 12 months, more often with hard water.
- Check water pressure. Keep household pressure between 40 and 60 PSI with a working pressure reducing valve.
- Inspect yearly. Look at connections, the relief valve discharge pipe, and the condensate line each fall before heating season.
- Keep freeze protection powered. Never unplug the unit in winter, even if you are away. The internal heaters need electricity to protect the heat exchanger.
- Install a leak detector. A small water alarm under the unit alerts you to the first drip, long before damage spreads.
A tankless water heater leaking is not something to watch and wait on. Small drips become soaked framing, mold, and ruined drywall faster than most homeowners expect. If you have spotted water around your unit in Lynnwood, Alderwood, or anywhere in Snohomish County, call ProLine Leak Fix Pros at (425) 380-0028. We will find the exact source of the leak with non-invasive detection methods and explain your options clearly.
