Generally speaking, under sink water heater problems are similar to the those of the larger closet-type heaters. The most common issues include pipe or valve leaking, clogged pipes or drains and electrical system failure or danger. It is important to check the tank area and its components fairly often to make sure none of these problems are causing damage to the unit or are a potential fire hazard.
Leaks can occur at valves and pipe joints, or from a pipe itself if it becomes cracked or punctured. Stress to the joints and PVC plastic piping from frequent bumping or jostling of the tank by activity under the sink can loosen fittings and cause leaking. Sometimes, however, what appears to be a leak is really only runoff from heavy condensation. Adding cold water to a hot tank produces condensation. Smaller water heater tanks that experience heavy use must be refilled often, and therefore condensation will occur more frequently than it would with a larger capacity tank. Restricted airflow in smaller enclosed spaces also aids condensation build-up. If this is the problem, increased ventilation under the sink may help. A drip pan may also be helpful to collect the water, save the floor and reduce the chance for mold or mildew build-up. It can also limit fire potential from water dripping off the heater and onto electrical components.
Under sink water heater units are small, by virtue of the space they must fit in. The shorter the water pipe, and the more turns and joints, the more potential it has for becoming clogged. However, using a plumber's snake, or liquid or powder drain cleaner chemicals, will usually do the trick. Areas with hard or soft water problems are particularly susceptible to water flow blockage caused by mineral deposit build-up. Using water softeners or other treatments can help avoid this problem. Regular maintenance to keep water heater pipes and valves free-flowing will extend the life of the unit and minimize the need for costly repairs.
Any time water is near electricity, the potential exists for dangerous problems. It is important to make sure all electrical circuitry is insulated or covered, and to keep wires and outlets as far away from the water supply valves and pipes and the bottom of the water heater tank as possible. This is so that condensation or overflow seepage does not drip onto the system and cause a fire.
Most under sink water heaters are electric, but if the one in question uses gas, make sure no flammable cleaning supplies or other chemicals are stored under the sink. If combustible vapors came in contact with the pilot light or the spark from an electric igniter it could definitely cause an explosion.
If the unit has problems that are not easily solved by the tips above, it is best to call an experienced plumber to take a look at the system. The fee for a service call will be less expensive than the price paid for cleaning up flooded floors or fire damage.