Tankless Water Heater Leaking? 5 Sources and How Urgent Each One Is

1. The service valves and their caps

The hot and cold isolation valves under the unit have stems and caps, and they get handled during every flush. A cap that was not seated properly, or a stem that has started to weep, drips slowly onto whatever is below. Wipe it dry and watch it for a minute. A weeping stem usually means the valve is replaced, which is a plumber's job. Urgency: low, but fix it before the drip reaches the unit.

2. The pipe connections under the unit

The fittings where the house pipes meet the unit can loosen or corrode. Look for wet joints, green crust or stained pipe. A steady drip at a fitting needs a plumber to reseat or replace it. One owner whose old copper line was already corroded saw it start to seep after a vinegar descale, so check the connections after every flush. Urgency: moderate.

3. The relief valve

Many units have a pressure relief valve. When it drips or runs, either the valve is worn or the pressure behind it is too high. Never cap or plug it. A plumber should find out which. Urgency: moderate, and higher if the flow is steady.

4. The condensate line

Condensing units vent in PVC and make acidic condensate that needs a drain, often through a neutralizer. A little water at that drain while the unit runs is normal. A disconnected, blocked or cracked line is not, and neither is a brass fitting that has corroded, which is why one plumber's advice is to swap a brass adapter for PVC. A blocked condensate drain can also shut the unit down with a code. Urgency: moderate, because acidic water on a floor or wall does damage over time.

5. The heat exchanger or its gasket

Water coming from inside the cabinet, or from the body of the unit, can mean the heat exchanger or a gasket around it has failed. Water quality is the usual cause: an owner with two failed exchangers in four years was told exactly that. One HVAC technician has also reported heat exchanger gasket failures that leaked water and carbon monoxide into the cabinet, and a cracked exchanger can let combustion gases into places they should not be.

Treat this one as urgent. Switch the unit off, close the cold valve and call. If a carbon monoxide alarm sounds, leave the house and call emergency services. Urgency: high.

How to tell which one you have

Water on the floor beside the unit with a dry cabinet is usually a plumbing leak, from a valve, a connection or the relief valve. Water inside the cabinet, or on the circuit board side, is a unit leak. Drips that only show while the unit runs point at the condensate line. A leak that goes on for weeks does far more damage than one caught on day one, so do not wait to see whether it stops.

What a leak repair costs

A valve or connection fix sits inside the estimated $150 to $800 range for part repairs, and a diagnosis visit is estimated at $100 to $250. A heat exchanger replacement is estimated at $1,000 to $2,500. Gas tankless units commonly last 20 years or more with maintenance, so a leak on a younger unit is normally worth repairing. On an older unit with an exchanger leak, the five repair-or-replace questions help you decide. A regular flush is the best protection for the exchanger.