I have a Rinnai which I installed in my catalac
some years ago when my paloma diaphram went and spares were no longer available. It is a better heater, and smaller. Mine provides hot water
for both sides of my cat, and for a shower
was easyas the vent piping was already installed, but it is very simple.
Decide where the vent pipe and heater are going to be. cut out the cowl position and line up the vent pipe, trimming to fit the contour, and for length for final position of the rinnai. Then offer up the rinnai, mark for 3 bolts, drill and secure. copper lpg line preferably from a manifold so you can isolate it from the other systems, connect up the hoses, and away you go.
Sorry that I cant report on ease of repair, cause nothings gone wrong on it yet!