Many times we have been asked if the Bosch AE115 and Bosch AE125 can accept pre heated water. The answer is no.
The reason lies in how the unit controls the temperature. These large KW whole house tankless water heaters are designed to take cool water and heat the water to shower temperature as the water passes through the heater. The heater turns on as flow is detected and turns off when the water flow ceases.
There is a temperature sensor on the inlet water pipe (cold) and a sensor on the outlet pipe (hot). The switch that turns the heater on is a solid state style switch or SCR switch. This type of switch has no moving parts and is very reliable. However, SCR or solid state switches generate heat as they switch electrical current. To cool these switches, they have been located on the inlet (cold) water inlet. So the copper pipes act as a heat sink for the SCR switches. If hot water is passed through the pipes and the heater turns on the SCR switches could over heat and FAIL.
Call us for any applications involving the need to have pre-heated water through a tankless heater. We can provided gas and electric heaters for floor heating or other applications needing to circulate warm or hot water.