I was gonna call Rinnai to verify this, but I think you need to have it installed by one of their certified installers or you don't get a warranty.
Might check that first for other brands as well.
I installed a gen1 Bosch years ago, worst part was running the double wall vent pipe through the roof. Also, you should check your gas flow with a pressure gauge to make sure you've got enough supply.
They aren't maintenance free, your'e supposed to flush them once a year just like a tank style. The Bosch sucked - not sure if it was because it was a gen1 (15 years ago) or because it was a Bosch. Had another house with a Rinnai and it worked great.
i have a Rinnai and it has been great for about 10 years. I recently flushed it for the first time and didn't get nearly as much crude out of out as I thought. I didn't install it but I don't think it would be too difficult.
Make sure to check the gas piping to insure enough gas flow, and keep in mind if you have something downstream (like your furnace or a gas dryer). my furnace is downstream so I ended up having a larger pipe installed to the point of the water heater. There are charts online that show what size pipe you need for the distance and the BTU that the appliance uses. Those things need a healthy amount of gas.
My tanked WH was in a closet in the laundry room which had an exterior wall, right by the gas meter. I yanked the tank & put the tankless on the exterior wall, tore the old WH closet out and built new storage cabinets floor to ceiling.
As previously mentioned, tankless have an appetite for gas, so I (at the recommendation of a plumber friend) ran a new 3/4" gas line directly off the meter, capping the old 1/2" line inside.
Insulated all the water lines, and the location of the new tankless gave me the ability to put the remote thermostat near the washer/dryer, in the laundry room, which was convenient. The flush out kit gives you an added bonus- I hook a hose up to it, open the valve, and I have 130 degree water on tap to wash whatever I need (like paint brushes/trays, etc), on the outside of the house.
To do it over again, the additional run of water line causes you to draw so much more water off the tap until it gets hot, I question if I haven't robbed Peter to pay Paul.