Am comtemplating installation of a gas 8.5 gallon per minute Rinnai unit for use in Master Bath. Current 50 gallon Water Heater does not supply enough hot water to fill tub. Is the pressure high enough to fill a tub pretty quickly? Also, does anyone know about the battery back-up package to use in case the electricty goes out. The installers in our area don't seem to know too much about it.

I have a 6.4 gpm Noritz that works fine as the only water heater in my house. It is rated for 3 fixtures and has no trouble supplying two showers at the same time. My Noritz has 3/4" inlet/outlet pipes pretty standard for residential water heaters. I would imagine a 8.5 gpm unit would also have 3/4" or maybe even 1" connections -- this is where flow is restricted.

These units have a microprocessor that controls water and gas flow which does not use much electrical current. In addition there is electrical ignition (no piolet light). There are two types of electric ignition, spark and glow tube. The glow tube does use some power, my guess is it would wear a battery down quickly. The spark probably would not. Find out what kind of iginition your unit uses.

If no one in your area knows about this stuff, call the manufacturer.