V53DEN 120,000 BTU High Efficiency Outdoor Tankless Heater (Natural Gas)

I'm writing a point form review because I don't want to spend any more time on this very average water heater. We have two installed in different areas of our house and both behave identically, so it's not a "lemon" problem. 1. They both make a loud groaning noise when they fire up. I've tried everything – including increasing the gas pipe diameter and changing every setting I can – and still they groan. We may have to move one unit because it is disturbing a neighbor. 2. We wanted to use these as solar water backups. Once the inlet water falls below a certain temperature, they should kick in and maintain hot water. Nope; they don't fire up unless you increase the flow rate to a ridiculous level, eg full hot in the shower. Then you have to wait until they start and deliver hot water. Yes, flow rates during normal usage – eg, shower – are well within spec . This is the real deal-breaker. A fundamental design fault. Did anyone at Rinnai ever test these things? 3. They do NOT come with a controller. We knew this but figured we'd buy one, use it to set each unit up and then disconnect it. Nope; if you disconnect the controller, the unit reverts to non-controller settings. Dumb. 4. Totally agree that the manual is a joke. It's as if it was written only so that someone could tick a box and say, "Comes with a manual". (We also have two Rinnai FC810N vent free wall heaters and they also have inept manuals as well as having some crazy-stupid design faults such as a child proof lock that unlocks if the power goes out!) 5: Rinnai service is non-existent. They just say to contact a registered installer. There is ZERO support in any way for these things. 6. The mechanical build quality is quite high, very solid and good machine tolerances and finish. It's just a pity that the things don't work very well. In short, I would never buy another Rinnai product ever again. And this is after thinking that they were a good, solid Japanese brand I could depend on. Nope!