Just a homeowner who does not want to have problems going forward. Thank you in advance for your help.

Old tank is leaking. Going to replace. Plumber says go Rinnai. I say, lets talk. We chat and he says that he needs to upgrade the gas line to 3/4" from the 1/2". Makes sense, no sweat. Here is the kicker.

Propane tank has 1/2" copper into the ground. It runs for 55' underground.
House has 3/4" black pipe coming out of the ground 8".
8" line expanded to 1" line for 3"
90 degree bend into house with 1"
1" line in attic

House was built in 75. Tank is surrounding on all 4 sides with concrete. Appears to have 1" line running along and then they tapped off with 1/2" to feed each propane appliance. House has 2 gas LP water heaters, LP range/oven, and LP furnace.

So plumber says he will tie into the 1" line with 3/4" and all should be fine? That scares me a little since 1/2" at the start seems to be the bottle neck. Now maybe the 1" allows for enough volume since it is exapnded and there is nearly 50' of 1" black pipe in the attic but I still am feeling uneasy about this. There is only one regulator and that is at the house.

Since the tankless will only be supplying 2 bathrooms and the wife takes long showers I thought that it would be a nice touch. Propane ain't cheap either and if I save 10% a year on my propane bill then that would be $200. It would take 4.5 years to break even on the upcharge.

Regardless, I think I want this if the plumbers idea works. My question to you guys is this, will it work? I called Rinnai and they said the unit I was told to use needed 8.5"-13.5" water column pressure (?) to run properly. It's odd but I think it will but don't want to spend the money only to fail.