Tankless Water Heater Leaking: Causes & Fixes

1. Loose or Improperly Sealed Pipe Connections

Over time, vibration and thermal expansion can loosen the threaded or push-fit connections where the cold water supply and hot water outlet meet the unit. This is the easiest and most common source of drips to fix.

1. Turn off the cold water supply valve to the heater.
2. Turn off the gas or electricity to the unit.
3. Dry the area around the connections with a towel and locate the exact drip point.
4. Use an adjustable wrench to gently snug up any threaded fittings — do not overtighten.
5. If threads feel stripped or the fitting still weeps, wrap the threads with fresh plumber’s tape (PTFE tape) after removing and cleaning the fitting.
6. Restore power and water, then monitor for 10–15 minutes.

2. Faulty or Weeping Pressure Relief Valve (T&P Valve)

The temperature and pressure relief valve is a safety device designed to open if pressure inside the unit becomes too high. A worn or scale-encrusted valve can weep or drip continuously even under normal operating conditions.

1. Locate the T&P valve — it is typically on the side of the unit with a small discharge pipe leading downward or to a drain.
2. Place a bucket under the discharge pipe.
3. Carefully lift the test lever for 2–3 seconds, then release it. You should hear water rush and then stop cleanly.
4. If the valve continues to drip after releasing the lever, the valve seat is likely fouled or worn and the valve must be replaced.
5. Shut off the cold water supply and relieve pressure before unscrewing the old valve.
6. Wrap the new valve’s threads with PTFE tape, thread it in hand-tight, then snug with a wrench. Do not exceed the torque rating in your unit’s manual.
7. Restore water and power, then check for leaks.

3. Corroded or Damaged Inlet/Outlet Fittings

The brass or copper fittings at the water inlet and outlet ports are exposed to constant moisture and can corrode or crack, especially in areas with hard or acidic water. Corrosion typically appears as green or white mineral buildup around the leak point.

1. Shut off the cold water supply and power to the unit.
2. Inspect each fitting closely for green corrosion (copper), white crust (mineral buildup), or visible cracks.
3. If corrosion is superficial, clean the area with a wire brush and white vinegar, then dry thoroughly and check for underlying pitting.
4. If the fitting is cracked or deeply pitted, it must be replaced. Use a pipe wrench to remove it.
5. Take the old fitting to a hardware store to match the size and thread type exactly.
6. Apply PTFE tape to new fitting threads, install, and snug firmly.
7. Restore water and power and check for leaks.

4. Condensation Mistaken for a Leak

Tankless water heaters, especially high-efficiency condensing models, naturally produce condensation on the cold water inlet pipe and sometimes on the unit’s exterior casing. This moisture is often mistaken for a plumbing leak.

1. Dry all external surfaces of the unit and its pipes completely with a towel.
2. Wrap a dry paper towel around the suspect area and leave it for 30 minutes while the heater runs.
3. If the paper towel is uniformly damp but not soaked, and there are no drip marks below, you are likely seeing condensation rather than a leak.
4. Ensure the utility space is adequately ventilated to reduce ambient humidity.
5. Consider insulating the cold water supply pipe with foam pipe insulation to reduce condensation on that line.
6. If the paper towel is wet and there are drip marks, continue diagnosing other causes in this guide.

5. Failing Internal Seals or O-Rings

Inside the unit, rubber O-rings and seals keep connections between internal components water-tight. These degrade over years of heat cycling and may cause water to seep from the unit’s casing or from where service caps are seated.

1. Shut off water and power to the unit and allow it to cool fully (at least 30 minutes).
2. Remove the front cover of the unit — typically secured by 2–4 screws.
3. Inspect the interior for moisture, corrosion tracks, or mineral deposits that indicate where water has been traveling.
4. Locate service caps, unions, or manifold connections and carefully remove them.
5. Inspect O-rings for cracking, flattening, or brittleness. Replace any that are damaged.
6. Apply a small amount of silicone grease to the new O-ring before seating it to ensure a proper seal.
7. Reassemble, restore water and power, and check for continued leaks.

6. Cracked Heat Exchanger — Tankless Water Heater Leaking Internally

A cracked heat exchanger is the most serious cause and typically results from freeze damage, severe scale buildup, or manufacturing defects. This repair is complex and usually requires a certified technician or full unit replacement.

1. Shut off water and power immediately if you suspect an internal crack — continued operation risks electrical hazards and water damage.
2. Remove the front cover and look for water pooling at the bottom interior of the unit or corrosion streaks running down from the heat exchanger block.
3. If your unit is still under warranty, stop here and contact the manufacturer directly — self-repair may void the warranty. Refer to the U.S. Department of Energy’s guidance on tankless water heaters for useful background on how these units work and what to expect from servicing.
4. If out of warranty, consult a licensed plumber or certified technician, as replacing a heat exchanger typically costs nearly as much as a new unit.
5. Ask the technician to also flush and descale the system, as scale buildup is a primary cause of heat exchanger failure.

Frequently asked questions

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