Water Distribution Manifolds
Tyson sounds right on the money to me.  However keep in mind that these on-demand water heaters require regular service.  They must be acid washed regularly depending on how hard your water is to keep the calcium build-up at bay.  As the calcium starts to build-up on the inside of the heat exchanger tubing, it insulates the water from the wall of the tubing.  As this happens the sensors will have a more and more difficult time sensing actual water temperature.