Review Rinnai or Navien web sites for more information.
These must be installed by a liscensed plumber and someone who has the expieranced technicians to perform yearly maintance.
Morris Park Realty
I don't know why you would want one, but one came with my house. I have spoken with HVAC people and plumbers over and over and so far, no one has given me a reason to go to a separate water heater. This system works just fine.