Quick Answer: Most “tankless water heater is leaking” calls turn out to be one of three fixable things: a loose or corroded fitting at the inlet/outlet connections, a pressure relief valve that’s started dripping, or — on a condensing unit — normal acidic condensate from a clogged or disconnected drain line that only looks like a leak. An actual leak from inside the heat exchanger is real but comparatively rare, and it usually traces back to freeze damage or years of scale buildup rather than a random failure. Cutting power to the unit and finding exactly where the water is coming from takes about five minutes and tells you which of those you’re dealing with.
Water where it shouldn’t be is alarming, but tankless units hold only a few ounces at any given moment (unlike a 40-80 gallon tank), so a leak is rarely the flooding emergency it would be with a storage tank heater. Here’s how to find the source and what each one means.
First: cut power, then find where the water is actually coming from
Shut off power to the unit at the breaker before touching anything — gas units still have an electrical ignition/control board, and electric units are obviously live. Then look for exactly where water is appearing:
- Pooling at the bottom of the unit, near the pipe connections — almost always a fitting issue, not the unit itself.
- Dripping from a small brass valve with a lever, usually on the cold inlet side — that’s the pressure relief valve, and it’s designed to drip under specific conditions.
- A steady trickle from a plastic tube running to a floor drain or nearby pump, only on a condensing (PVC-vented) unit — check the condensate line before assuming a leak at all.
- Moisture appearing to come from inside the cabinet itself, with no clear external source — this is the one case worth calling a technician for rather than troubleshooting yourself.
The four real causes, ranked by how often they actually happen
| Cause | How to tell | Typical fix |
|---|---|---|
| Loose or corroded fitting | Water at the base, near union nuts or threaded connections | Tighten by hand + quarter-turn with a wrench; replace if corroded |
| Pressure relief valve dripping | Drip from the lever-style brass valve on the cold inlet | Replace the valve — see our relief valve guide |
| Condensate line clogged/disconnected (condensing units only) | Trickle from a plastic drain tube, not from fittings | Clear or reseat the line — see our condensate drain guide |
| Cracked heat exchanger (freeze damage or age) | Water inside the cabinet, no external source found | Call a technician; often points to replacement |
The one that isn’t actually a leak: condensate
If your unit vents through white PVC pipe instead of stainless steel, it’s a condensing model, and it’s designed to produce 5-8 gallons of mildly acidic condensate per day during active use, per our condensate drain guide. That liquid comes from cooling exhaust gas down to around 100-140°F to reclaim extra efficiency — the same reason a cold glass sweats on a humid day. As long as it’s draining through the dedicated condensate line into a neutralizer or pump, that’s the system working correctly, not a leak. The confusion happens when the line clogs, freezes, or pops loose and the condensate ends up pooling under the cabinet instead — same water, but now it genuinely needs a fix.
When it’s a fitting: what to check first
Loose or corroded inlet/outlet fittings
- If you installed one of our recommended isolation valve kits (Webstone, Watts, Eastman), check the union nuts on the cold and hot sides first — these are threaded connections that can back off slightly over time, especially after the first few heat-cycle expansions post-install.
- Hard water accelerates corrosion at brass and copper fittings; if your leak keeps recurring at the same joint, check our hard water guide and descaling schedule rather than just re-tightening repeatedly.
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Catch it early: a leak sensor under the unit
Govee WiFi Water Leak Detector (2-Pack)
- Placed directly under the unit or drain pan, a small puck-style sensor catches a fitting or condensate leak within minutes instead of days — the difference between a $20 part and water damage to a finished wall or ceiling behind the unit.
- Especially worth it for closet, attic, or upper-floor installs where our drain pan guide already flags a leak as more consequential than a garage or basement install.
When to stop troubleshooting and call a technician
If you’ve ruled out fittings, the relief valve, and (on a condensing unit) the condensate line, and water is still appearing inside the cabinet, that points to the heat exchanger itself — usually from freeze damage or, less commonly, pinhole corrosion after years of scale buildup. Water expands roughly 9% in volume as it freezes, and a heat exchanger’s thin copper or stainless waterways have very little room to absorb that before something gives; our freeze protection guide covers how to prevent it going forward. A cracked heat exchanger is not a DIY fix, and on a unit already well into the 15-20 year range from our lifespan guide, replacement is usually the more cost-effective call than repair — check your warranty coverage before paying for either.
The bottom line
A leaking tankless water heater is worth investigating immediately, but it’s rarely the emergency a leaking tank heater is, since there’s no 40-80 gallon reservoir behind it. Five minutes of looking — fittings first, then the pressure relief valve, then the condensate line on a condensing unit — resolves the large majority of cases for the cost of a small part. Water actually coming from inside the cabinet with no external source is the one scenario that means calling a technician rather than reaching for a wrench.