This shows two options for tankless heating alongside reverse osmosis(R/O). Preheating the water before R/O makes the process more efficient. If the system is set up to heat water post purification, the water heater will need internal teflon coating. Outside water should be strained before entering either the heater or the purification systems. Expansion tanks can have several optional positions, but they will need a high temperature bladder if installed after the water heater.