Tankless owners get to skip one classic anxiety — there's no forty-gallon tank waiting to rust through. But "no tank" doesn't mean "no leaks," and when a tankless unit starts dripping, the hunt looks different from a tank heater's. The water is coming from somewhere; it's just a shorter list of somewheres. Let's walk it.
Suspect one: the condensate line
High-efficiency condensing units pull so much heat from exhaust gases that water vapor condenses inside — that's the "condensing" part, and it's by design. That acidic condensate drains away through a small line. If the line clogs, cracks, or wasn't given proper slope, water backs up into the cabinet and drips out wherever it finds a path. A surprising number of "mystery leaks" on tankless units turn out to be condensate drainage, not plumbing at all.
Suspect two: isolation and service valves
Tankless units have hot and cold isolation valves with little service ports, installed so the unit can be flushed and serviced. Like any valve, their packing and seals can weep with age — especially the service port caps, which get opened during maintenance and don't always reseal perfectly. These drips are usually small and steady, and they show up right at the valve bodies under the unit.
Suspect three: heat-exchanger seals and connections
The heat exchanger is where the magic happens — water flashing through superheated coils — and its seals and connections live a hard life of rapid heating and cooling, thousands of cycles a year. Over time, seals harden and weep, and connections work loose. These leaks often appear only when the unit is actively heating, which makes them intermittent and tricky: dry when you look, damp when you don't.
Why tankless leaks deserve prompt attention
Here's what makes tankless leaks a bit more urgent than their tank cousins: the cabinet also houses the unit's electronics, ignition components, and gas valve. Water and circuit boards are a bad combination, and a small leak left alone can graduate from a plumbing repair to a plumbing-plus-electronics repair. If you see water in or under the cabinet, shut the unit down if you safely can and don't wait weeks to have it looked at.
Different machine, same approach
Diagnosis is the same discipline we bring to tanks: trace the water to its true source instead of guessing, fix the actual failed part with components made for your model, and run a full operational check afterward. Tankless or tank, a leak is a leak — it just takes knowing the machine. Seeing water around your unit? Call (833) 713-2101 and tell us what you've spotted.