The electric tankless water heaters, Powerstar models AE115 and AE125 are high kilowatt electric tankless heaters that heat water on demand as the water passes through them. The modules inside consist of coiled elements that heat the water. The heating elements produce extreme heat inside the modules to heat the water on demand.
The heat that is produced by the heating elements in the modules sometimes counteracts with the mineral deposits in the water. If there is a high mineral content due to a hard water area then these mineral deposits known as "scale deposits" will over time attach themselves to the heating coils inside the modules.
All Bosch Powerstar electric tankless water heaters should never be descaled with any type of solution such as vinegar, muriatic acid, etc. The use of descalants flushed through the heater does not have much effect on the mineral deposits attached to the elements. Another point of trying to flush descalant through an electric tankless heater is that it will loosen some of the mineral deposits in the heater and will have a damaging effect on the flow transducer and other parts inside the tankless heater that detects the flow of water and will interrupt the signal to the circuit board itself.
The best protection against mineral deposits (scale) forming on the coils in the electric tankless heater is to check the quality of the water in your area before installing and if the water has a high mineral content then it is recommended to install a water softener or scale stopper system. The water softener or scale stopper will filter out almost all of the minerals and treat the water so the electric tankless water heater will have a longer life with less minerals building up inside.