I have an on demand water heater in my bus and I'm well satisfied with it. However, there are three conditions that need to be met for an on demand to work properly. First, the exhaust from the heater will reach dangerously high temps and needs to be vented outside to avoid fire hazard. Second, design a recirculating pathway for the system or you will waste a lot of water during the heating cycle. Third and most importantly, add a temp controlled switch to the outlet side of the heater. These heaters are designed to add a set amount of heat to the incoming water. Mine happily adds 40 degrees F to the incoming water temp but couldn't care less if the absolute water temp goes to boiling. Your system will withstand somewhat higher water temps as it is pex but mine being pvc began swelling and leaking when the water temp hit 140 degrees F--hence the outlet temp control.