I can help with the line sizing and regulator. Most regulators provided by your lp supplier will handle the increase in the btu load of the Rinnai, but if it didn't, they would provide a higher volume regulator for your needs.
The line size is a different story and thats why having your propane supplier provide the Rinnai and installation of that product is probably the most cost effective, because they can do the whole job. Still get a quote from your supplier. The line size calculation isn't as complicated as Jmen24 leads you to believe. The NFPA code books provide that info or most technicians have a handy pocket sized book with the info. The Rinnai recommends the minimum of 3/4" IPS (iron pipe size) for propane supply, but that may have to be increased if it is a long distance from the point the gas supply enters your home.
The other questions regarding line size was standard gas H2O heaters and gas ranges. "Typical" gas ranges are @ 60,000 btus and "typical" H2O heaters are @ 40,000 btus. The standard Rinnais are @ 200.000 btus and this is why it requires a larger line.
The on-demand water heaters are not really for larger homes (more than 2 bathrooms), but are really ideal for smaller second homes. That is based upon some of the information that was provided by Jmen24.
