Heat exchangers don't seem to care if the H2O coming in is hotter than the H2O it can spit out. It will process it all the same. Also, it doesn't matter if the input H2O is at 15 degrees or has been preheated to 165, the same energy is consumed to get it up to 500 degrees.
Mechanics for heat exchangers/boilers(and reheaters mod) need some work if this is going to work well. If I had to say, how the heat pipes diffuse heat from the reactor is a good example of how this should work. Heat exchanger checks if internal temp is higher than H2O pipe network, then diffuses some heat into the H2O if it is, else do nothing. Don't even need an input and output for the H2O, just a single connection point for the pipe network should work.
