Tankless Water Heater Leaking From the Bottom

The Short Answer: Water under a tankless heater usually comes from one of five places: the condensate drain on a condensing unit, the pressure relief valve, the inlet filter or service valves, a fitting split by freezing, or the heat exchanger itself. Note where the water appears, whether it is warm, and whether it drips only while the burner runs. Water inside the cabinet or on the controls needs a technician.

Is it condensate or a water leak?

Condensing tankless heaters (for example Navien NPE, Rinnai SENSEI, and Noritz NRC models) are designed to produce water. Noritz’s NRC98 owner’s guide (opens in a new tab) explains that the secondary heat exchanger pulls moisture out of the exhaust and discharges it through a drain pipe, up to about 2 gallons per hour on that model, and states plainly that this “is not a water leak.” The condensate is acidic (Noritz gives a pH of about 2 to 3), which is why many installations route it through a neutralizer before the drain.

Condensate is a problem only when it ends up somewhere other than the drain: a cracked or disconnected drain line, a neutralizer that has clogged, a line that is blocked or frozen, or a condensate pump that has stopped. When the drain cannot flow, the water backs up into the heater. Several brands detect this:

• Noritz condensing units show code 29 when the condensate drain line is clogged or frozen, according to the NRC111-DV installation manual (opens in a new tab).
• Rinnai lists code 10 as “Air Supply or Exhaust Blockage/Condensate Trap is Full” in the SENSEI manual, and code 25 for a tripped condensate safety switch or failed condensate pump in its diagnostic-code guide (opens in a new tab).

Non-condensing units can drip too. Navien’s NPN installation manual (opens in a new tab) notes that the non-condensing NPN may produce condensate in the vent system, drained through a condensate collector. Rinnai’s diagnostic guide describes a condensate drain trap at the vent connection on its non-condensing models. A drip that runs down from the vent collar is usually this, not the water side.

Where the water is and what it usually means

Rinnai’s diagnostic guide adds one useful clue for technicians: liquid coming out of the condensate drain while the heater is not operating is a common indicator of a leaking heat exchanger. Condensate is only made while the burner fires.

What you can check safely

• Note timing. Does it drip only while hot water is running, all the time, or only at certain times of day? Dry the area and watch it after one hot-water draw.
• Note temperature. Cool water near the drain points to condensate. Warm or hot water near the relief valve or inlet points to the water side.
• Look at the display. Write down any code before resetting anything.
• Check the condensate drain end. The visible end of the drain line should be open, not kinked, and not frozen. Do not pinch, cap, or tape the drain.
• Look at the filter cap and service valves. A drip from a cap or valve stem right after a flush or cleaning often means a cap not fully seated; tell whoever did the work.
• Close the cold-water inlet valve if water is spreading or reaching the controls, and turn the heater off.

Leave the front cover on. Behind it are gas, electrical, and combustion parts, and Navien’s NPE-2 owner manual says not to operate the heater with the front cover open.

Relief valve discharge

Every tankless installation needs a relief valve on the hot outlet. Navien specifies a pressure-only valve rated at a maximum of 150 psi for the NPN, and both Navien and Noritz manuals say that a valve discharging periodically may be caused by thermal expansion in a closed water system, and that it must not be plugged. Rinnai’s SENSEI manual says testing the relief valve should only be done by a trained and qualified professional because the discharge can scald. For a valve that keeps dripping, see Rinnai tankless relief valve leaking; the causes are not brand-specific.

Freeze damage

Freeze protection depends on the heater having power, and on some models gas. Rinnai’s SENSEI manual rates its units to -22°F indoors and -4°F outdoors when powered, fueled, and out of the wind, and warns that losing freeze protection can burst the heat exchanger or water lines. Noritz likewise notes that its freeze-prevention heaters do not protect plumbing outside the unit, and Navien, Rinnai, and Noritz all exclude freeze damage from warranty coverage. A leak that starts on the first thaw after an outage is the typical pattern; see tankless water heater leaking after a freeze.

What the technician will check

With power, gas, and water isolated, the technician opens the cabinet and traces the water: the internal condensate trap and hoses, water fittings and O-rings, the heat exchanger for scale trails or corrosion, the vent connection, and the relief valve. If the relief valve is discharging, they also measure static water pressure and look for expansion control.

Repair or replace

Condensate lines, neutralizer media, condensate pumps, filter-cap O-rings, service-valve packing, and relief valves are routine parts. An internal water fitting is usually a moderate repair. A leaking heat exchanger is the expensive outcome; depending on the model and its age, the choice is a replacement exchanger (sometimes under the manufacturer’s heat-exchanger warranty; Navien’s, for example, excludes freezing and scale damage) or a new heater. Ask the technician to check warranty status using your serial number before you decide.

Model notes

Codes and parts differ between product lines, so confirm against the manual for the model on your rating plate.

Frequently Asked Questions

Q: How much condensate is normal from a tankless heater? A: It varies with the model and the load; Noritz gives up to about 2 gallons per hour for the NRC98. What matters is that it reaches the drain or neutralizer and does not pool or back up into the heater.

Q: My non-condensing tankless is dripping from the vent area. Is that a leak? A: Often not. Navien and Rinnai both describe condensate forming in the vents of their non-condensing units, handled by a collector or trap. If the drip is warm, comes from a water fitting, or appears inside the cabinet, it is a water leak instead.

Q: Can I keep using the heater while it drips? A: If it is only condensate escaping from a loose or cracked external drain line, and the cabinet and controls are dry, it can usually run until the line is fixed. If water is inside the cabinet, comes from the heat exchanger, or appeared after a freeze, turn it off and close the cold-water inlet until it is inspected.