Troubleshooting Gas Heaters
Gas heaters can suffer from several problems. The most common involve the ignition system for the gas flame that heats the water. Most tankless heaters use a pezio-electric igniter to ignite the natural gas that heats the water. In older systems, a manual mechanical push-rod activates the pezio-electric ignition system. If this manual system is not working, check the alignment of the electrodes that ignite the gas and make sure that they are in the path of the gas.
Newer tankless heaters use an automated system that senses the flow of water, turns on the gas, and activates an automatic pezio-electric system that ignites the natural gas heat source. Although over time, alignment of the pezio-electric electrodes can also shift, if this ignition system is not working it is usually battery related. Replacing the ignition battery, usually a D-cell battery, solves ignition problems in newer gas heaters.
As tankless hot water heaters age, problems can occur with corrosion of the gas jets that heat the water. Depending on the location of the heater, whether indoors or outdoors, and the climate, jets could need to be replaced periodically.
Troubleshooting Electric Heaters
Electric tankless hot water heaters are actually more reliable than natural gas. In most cases, if an electric tankless heater fails to heat, it is due to a circuit breaker, either a safety interlock inside the heater or an external power supply circuit breaker. Simply re-setting the circuit breaker will usually solve the problem. If a circuit breaker trips repeatedly, it may be due to a ground fault. In this case, safety could be an issue. Call a qualified electrician to troubleshoot the problem.
Heat Transfer Mechanism Failure
In areas with very hard water or areas with impurities in the water, the pipes that carry the water through the heat transfer fins can clog. The first indication of clogged pipes would be a reduction in water flow from the heater. Problems like this usually don't occur until very late in the life of a tankless heater. Clogged pipes can affect gas or electric tankless heaters equally. In most cases, this is an indication that the entire tankless hot water heater will need to be replaced.
Modern Digital Tankless Heaters
Some current model tankless hot water heaters offer error messages that tell you where a potential problem has occurred. For these high end hot water heaters, simply refer to your model's documentation and follow the directions to fix the fault.