Although they're beginning to get up to speed, not all plumbers are familiar with the nuances of installing tankless water heaters. The way a tankless heater works can lead to an installation problem.
When all of the devices in the house that use hot water are off and not calling for hot water, the tankless water heater is idle. With an electric unit, turning on a hot water faucet generates a flow through the water heater. A mechanism in the heater senses the water flowing and trips an electrical switch that turns on the current to the heating elements.
Unlike a conventional water heater, which keeps water in a tank continuously heated until called for, the tankless water heater must sense the need for current to be supplied to heat the water. This means that there must be a sufficient flow through the water heater to a sensor in order to create the electrical current to heat the water. A water heater of this type has a screen filter like the one on a washing machine to trap impurities and sediment in the water stream flowing into the heater.
A plumber who is only familiar with conventional water heaters may not be aware of this and may connect the piping directly into the water heater, sealing or soldering in the plumbing (or "sweating a pipe joint," as plumbers say) so there is no access to this screen. Once the screen becomes clogged it reduces the flow of water through the unit and may prevent the sensor from tripping. This means that no matter how many faucets or appliances are on, with hot water being called for, the water heater will not generate hot water. The homeowner will think the water heater has failed and needs replacing, when actually the unit itself is in good order.
When a plumber sweats a copper pipe joint with a torch and solder, the pipe gets very hot. If this is done to a pipe connected directly to the tankless water heater, the heat can be conducted to the internal parts of the heater, damaging them and reducing the heater's life expectancy. The plumber should never use a torch on pipes directly connected to a tankless unit.
Whether the plumber connected the unit correctly in the first place, or if it had to be redone so that there is access to the cleaning screen, the water heater needs to be cleaned periodically, or "backflushed." When this is done and everything is reconnected, there can be air in the system. When the heating elements have air flowing through them instead of a solid stream of water, this can accelerate burning out the heating element. It won't necessarily happen all at once, but it can reduce life expectancy of the unit.
There is a solution. After disconnecting to clean the screen, or backflushing, make sure the power supply to the water heater is turned off. (Switching off the circuit breakers is a convenient way to do this.) With the power off, running water through the water heater until all of the air is out of the system will eliminate this problem. This can be done by opening a hot water faucet in the house and letting the water flow for a period of time until all the air is out. At this point, the power can be turned back on and the unit returned to service.
Poor water quality can affect the tankless water heater's life expectancy. Impurities in the water can be corrosive, and corrosion can, of course, reduce the life expectancy of the unit. Treating the water prior to its entering the unit may help.
Another factor that will influence life expectancy is the amount of usage. Naturally, heavy demand on a unit will very likely reduce its longevity.