A tankless water heater leak most often comes from a loose or worn fitting, a failed condensate line or a corroded heat exchanger. Shut off the water and the gas to the unit, dry the area and watch where new drops form. A fitting or a condensate leak is usually a short repair, while a leaking heat exchanger usually means a new unit.
Shut off first, diagnose second
- Turn off the power or the gas valve to the unit.
- Close the cold water isolation valve and the hot water valve.
- If you have a flush valve kit, close both flush valves.
- Dry the unit and the floor with towels.
- Open one hot tap in the house to relieve pressure, then check where drops return.
A tankless unit is small and wall-mounted, so water that runs down the casing can come from a pipe above it. Look up the line before you blame the heater. A drip from a flexible connector, a union or a valve above the unit is a plumbing leak, not a heater leak.
The usual leak sources
| Source | What you see | Fix |
|---|---|---|
| Water connections | Drip at the cold or hot fitting | Tighten or replace the fitting or the washer |
| Isolation valves | Weeping at a valve stem | Replace the valve |
| Condensate line | Water from the drain tube or trap | Clear the line or replace the neutralizer |
| Pressure relief valve | Discharge at the relief pipe | Check pressure and the expansion control |
| Internal fitting or heat exchanger | Water from inside the casing | Replace the part or the unit |
Condensate leaks come from high-efficiency units. They cool the exhaust enough that water vapor condenses, and the condensate is acidic. A blocked or frozen condensate line can overflow into the casing.
Scale, corrosion and age
Hard water leaves scale on the heat exchanger, and in time that can cause hot spots and cracks. Acidic water can also corrode components from the inside. A yearly flush with a descaling solution reduces both problems.
If the heat exchanger has failed, the unit may still be under a manufacturer warranty, which usually covers the exchanger for a longer period than other parts. Check the label for the model and serial number and the warranty documents. A plumber can help you file a claim if the warranty applies.
Error codes with leaks
Many units display an error code when they sense water in the wrong place, or flow without ignition. The code, plus the leak location, tells a plumber what to test. Photograph the display before you reset anything.
Repair or replace
| Condition | Likely verdict |
|---|---|
| Drip at a fitting, unit in good condition | Repair |
| Condensate line blocked | Repair, clean and flush |
| Failed pressure relief valve | Repair |
| Cracked heat exchanger, unit under 10 years | Check the warranty, then repair or replace |
| Cracked heat exchanger, older unit | Replace |
The tankless page explains installation and maintenance. If the unit fails and you need hot water tonight, a tank heater may be the fastest stopgap, and some households keep the tankless and add a tank as backup.
Preventing the next leak
- Flush the unit each year
- Keep the condensate line clear and protected from freezing
- Install isolation valves if you do not have them
- Check connections after any service
- Test your water for hardness
What a plumber checks on a leak call
The plumber starts by finding whether the water is clean or has color or smell. Clean water points to a supply or fitting leak. Rusty or discolored water suggests a corroded part inside the unit.
- Check the water connections and the isolation valves for weeping.
- Inspect the condensate line, trap and neutralizer.
- Test the relief valve and the system pressure.
- Look for white scale or rust trails on the heat exchanger.
- Read the error history on the display.
Photograph the unit before the visit, including the model label and any error codes. It saves time and helps the plumber bring the right part. If the unit is old and the part is hard to find, ask for a price on replacement next to the price on repair so you can compare.
Do not run the unit with a known leak. Water can reach the controls and the burner, and a repair that was cheap becomes a replacement.
Protect the floor while you wait
Put a tray or a stack of towels under the unit and check them every hour. Move anything stored below it, and keep electrical cords and chargers off the floor. If the leak is steady, shut the water to the unit and use a stovetop kettle for hot water until the plumber arrives.
Finally, keep the receipt and the installer's name handy. If the unit is still under warranty, the installer can often file the claim and tell you whether the manufacturer will send a replacement part or a full unit.
Frequently asked questions
Common causes are a loose water fitting, an isolation valve, a blocked condensate line or a failed heat exchanger.
Water near electrical parts and gas piping is a risk. Shut off power and gas and call a plumber.
Many manufacturers cover the heat exchanger for a longer period. Check your warranty documents and the model label.
Keep the line clear, use a neutralizer if required and protect it from freezing.
The utility's 2023 water quality report does not list hardness. Test your tap or ask the utility.