Bottom Leaks: The Two Very Different Causes
When water appears at the bottom of a wall-mounted tankless water heater, it almost always comes down to one of two things, and they could not be more different in seriousness.
The harmless cause is condensate. High-efficiency condensing tankless units intentionally cool their exhaust to extract extra heat, and that creates acidic water that drains out the bottom through a dedicated tube. If that tube is blocked, disconnected, or its neutralizer cartridge is full, the condensate backs up and drips from the base — looking alarming but doing no harm to the heater itself.
The serious cause is a cracked or corroded heat exchanger. The heat exchanger is the sealed core where the burner heats your water. When scale, age, or freeze damage causes a crack, water seeps out and collects inside the cabinet before dripping from the bottom seam. This is the leak that usually points toward a major repair or unit replacement.
How to Tell Condensate From a Heat Exchanger Leak
Trace the water. Condensate comes from a specific plastic drain fitting or tube, usually clear, and tends to appear during or shortly after the unit runs. If you follow the moisture up and it originates from the condensate line or neutralizer box, that is your answer — and it is the better answer.
A heat exchanger leak behaves differently. The water seems to emerge from the bottom seam of the metal cabinet itself, not from any tube. It may be present even when external fittings are bone dry, and you may also notice rumbling or popping sounds during heating (scale on the exchanger), reduced hot water output, or error codes related to flow or temperature. Some units will show a fault code and refuse to fire once internal water reaches a sensor.
If you cannot clearly source the water to the condensate line, assume it could be internal and treat it as urgent.
Immediate Steps to Limit Damage
Turn off power to the unit at the breaker or switch. A bottom leak that reaches the burner compartment or control board can damage electronics and create a hazard.
Close the cold-water isolation valve (commonly the blue-handled service valve) to stop water feeding the unit. If you cannot locate it, shut off the main water supply to the home. Place a towel or shallow tray to catch drips and keep the area around the heater dry.
Do not remove the front cover and start probing inside. The cabinet houses gas, high-voltage components, and the sealed exchanger. Note any error code on the display, take a photo, and have it ready for the technician — it often speeds diagnosis considerably.
Repair, Replace, and Who to Call
If the source is condensate, the fix is usually straightforward: clearing the drain line, replacing a clogged neutralizer cartridge, or correcting the tube's slope. This is an affordable repair, and a quote in the lower range is typical — call 604-359-1081 for an exact quote based on your unit.
If the heat exchanger has cracked, the math changes. On a unit still under its heat exchanger warranty (many carry 10–15 years), the part may be covered, though labour usually is not. On an older or out-of-warranty unit, replacing the exchanger can approach the cost of a new heater, so replacement is often the smarter long-term choice. Costs vary widely by brand and warranty status, so we provide a clear range only after seeing the unit.
Tankless water heaters are serviced by our parent company, CanroHeat. Call 604-359-1081 and we will diagnose a bottom leak quickly, tell you honestly whether it is condensate or the exchanger, and lay out your repair-versus-replace options without pressure.