A tankless unit trades the big steel tank for a compact heat exchanger — which removes the classic rusted-shell failure and replaces it with a larger cast of smaller characters. Leaks become fitting-scale rather than tank-scale, but there are more fittings to watch.
The diagnostic approach stays the same — find the water’s author — but the suspect lineup changes. Here is the tankless anatomy, part by part.
No tank, different anatomy
The major organs: the heat exchanger (copper or stainless), the cold inlet and hot outlet fittings, the condensate drain on condensing models, a pressure relief valve, and the service valves installed for flushing. Smaller water volume, but the same pressure rules — 40 to 60 psi normal, 80-plus high.
Because there is no tank to fail at once, tankless leaks tend to be weeps and drips rather than floods. Small is good — provided someone notices them.
Heat exchanger leaks
The exchanger is the hardest-working part: thin metal, intense heat, constant thermal cycling. Scale buildup insulates spots, those spots overheat, and thermal stress eventually breaches the metal — especially where hard water never got its scheduled descaling.
Hard water is the tankless unit’s sediment. Regular descaling flushes are the exchanger’s anode rod — the maintenance that decides whether it lives its full life or dies young.
Condensate line drips
Condensing tankless units wring acidic condensate from exhaust, and that water has to drain somewhere. A clogged, kinked, or disconnected condensate line drips exactly like a plumbing leak — but it is a drain problem wearing a leak’s costume.
Trace the drip to its port: water from the condensate outlet is a maintenance issue, not a pressure failure. Clear the line, check the neutralizer, and the “leak” retires.
Fittings, unions, and vibration
A tankless unit has far more joints than a tank heater — service valves, unions, the inlet filter housing — and each one is a future weep. Years of vibration and heat cycling slowly loosen what was once tight, and the first sign is usually a mineral crust at a union.
Annual inspection catches these while they are still crusts, not drips. The fitting chapter is short on every tankless unit — read it yearly. The inlet screen and service valves get their check at the same visit.
The pressure relief valve — same story, smaller tank
Tankless units carry a pressure relief valve too, and it weeps for the same two reasons: a worn seat, or real pressure events it is faithfully reporting. The rule travels with the part: never cap it, and read the discharge pattern.
Constant weep means replace the valve; discharge after heating means check system pressure and the expansion setup. Same whistle, smaller choir.
When repair beats replacement
A leaking fitting on a healthy mid-life unit is a clear repair; a scaled-through heat exchanger on an old, never-descaled unit usually is not. The decision weighs age, condition, and water history — not the size of the drip.
We lay out both paths with the unit’s actual condition in front of you. Small anatomy, same honest triage as the big tanks.
In short
Tankless leaks are smaller-scale but more numerous — fittings, condensate, the exchanger — and descaling is their version of anode care. Annual service keeps every one of them small. Call (833) 713-2101.