Why Is My Tankless Water Heater Draining So Much?
On this page (8 sections)
The Short Answer: A tankless heater has no tank to drain, so ongoing water usually has one of four sources: condensate from a condensing (high-efficiency) unit, which is normal if it flows into a drain; the pressure relief valve opening on high pressure or temperature; a freeze-protection drain valve that opened and did not close; or a leak inside the unit. Identify which pipe or fitting the water comes from first.
Which outlet is the water coming from?
| Where it comes from | What it is | Normal? |
|---|---|---|
| Small plastic drain line or condensate trap, only while the unit is heating | Condensate from a condensing unit | Normal, if it reaches a drain or neutralizer |
| Condensate overflowing the trap, pooling under the unit, or backing into the cabinet | Blocked or poorly sloped condensate drain, failed condensate pump | Not normal; fix promptly |
| Relief-valve outlet (brass valve on the hot outlet or service valves) | High pressure or overheating | Not normal |
| A drain valve or freeze-protection valve, often after a cold night | Freeze protection releasing water, or a valve stuck open | Expected during freezing conditions if equipped; should close afterward |
| Service-valve purge ports | Purge port caps loose or valves partly open | Not normal; easy fix |
| Inside the cabinet, from the heat exchanger or water fittings | Internal leak | Not normal; turn the unit off |
| At the vent after rain or wind | Water entering through the vent | Not normal; see tankless not working after rain |
Condensate is supposed to be there
Condensing tankless units extract extra heat from the exhaust, which turns water vapor into liquid. That water is mildly acidic and must run through a trap (and usually a neutralizer) to a drain. It flows only while the unit is firing. Problems arise when the drain line is blocked, kinked, runs uphill, freezes outdoors, or depends on a condensate pump that has failed. Water then backs up into the unit or spills onto the floor. Brand-specific help: Navien condensate drain leaking.
Relief valve discharge
Most tankless installations include a pressure relief valve. It opens when water pressure or temperature exceeds its rating. Common causes are high house pressure, thermal expansion in a home with a check valve or pressure-reducing valve and no expansion tank, and a valve that did not reseat after being disturbed. Never cap the valve. See Rinnai tankless pressure relief valve leaking.
Freeze protection
Many tankless units protect themselves from freezing with built-in heaters that need power. Some installations also have drain-down valves that open to empty the unit if power fails in freezing weather. These are supposed to release water, but they should close once conditions return to normal. A valve stuck open will keep draining. If a unit was exposed to freezing temperatures without power, it may have cracked; see tankless leaking after a freeze.
What you can check safely
- Dry each pipe and fitting under the unit with paper towels and run hot water for a few minutes to see which one gets wet.
- Follow the condensate line to its end and make sure it is not kinked, crushed, frozen, or submerged in a full drain.
- Check that the caps are on the service-valve purge ports and the purge valves are closed.
- Read your house water pressure with a hose-bib gauge.
- If water is coming from inside the cabinet, turn off the unit’s power and close the service valves.
Do not open the cabinet or disconnect the condensate trap inside the unit.
What the technician will check
They will inspect the internal condensate trap and drain path, check the relief valve, house pressure and expansion control, test freeze-protection components, and look for leaks at the heat exchanger and internal water connections. They will also check service history, since scale buildup and overheating codes can accompany some of these problems.
Repair or replace
Condensate drain fixes, purge-port caps, a relief valve, an expansion tank, or a freeze valve are repairs. A leaking heat exchanger is a major repair; on an older unit, replacement may be worth considering. For leaks at the bottom of the unit in general, see tankless water heater leaking from the bottom.
Frequently Asked Questions
Q: How much condensate is normal? A: It depends on the unit, how hard it is working, and incoming water temperature. Condensate should flow only while the unit is heating and should stop soon after. Water that keeps flowing with the unit idle is not condensate.
Q: Does a non-condensing tankless produce condensate? A: Not normally in its drain system. Water dripping from a non-condensing unit is more likely a relief valve, a fitting, or vent condensation, and should be checked.
Q: Can I keep using the heater while it drips? A: If it is a properly draining condensate line, yes. If the water is coming from inside the cabinet or the relief valve is discharging steadily, turn it off and arrange service.
Manuals and supporting references
These links distinguish general safety information from model-specific documentation. A manual directory helps you locate your unit’s instructions; it does not establish that every model uses the same parts, codes, or test values.
- A. O. Smith technical bulletin 15: condensation (opens in a new tab)Why combustion condensate can resemble a tank leak; not a diagnosis of another manufacturer’s model.
- A. O. Smith technical bulletin 45: thermal expansion (opens in a new tab)Background on pressure rise in a closed water system. Pressure testing and repairs are professional work.
- Rinnai: official document library (opens in a new tab)Select your exact model and owner manual before interpreting a fault or reset procedure.
- Navien: manuals and product documents (opens in a new tab)Model-specific owner information; service manuals are intended for qualified professionals.
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