There are a couple of plumbing considerations to be made to be sure that you maximize the life and performance of your tankless water heater.

First, if you have hard water it’s a necessity with a tankless hot water heater that you add a water softener to your system.  It will protect your tankless water heater from scale build up in the heat exchanger and lines.  After time scale buildup will reduce energy and restrict water flow.

Second, I would recommend finding a plumber who can install a Manabloc system with your Tankless Water Heater.  The Manabloc system provides a cost effective distribution manifold for a plumbing system.   This system will allow hot water to reach all the faucets and showers in your house more quickly since each has its own line.  Think of this as a electric circuit breaker for your plumbing system.  You can also turn each individual line off at the manifold making repairs very easy.

ManaBloc and PEX Plumbing System

It won’t add cost to your project and in some cases with the right plumber it will be more cost effective than traditional PVC pluming.  Since the lines are flexible no joints are required between the source and the outlet so your chance of having leaks is much less likely.

This system uses flexible piping called PEX Plumbing and it is the future of the plumbing industry. PEX piping has been used in hot and cold water distribution systems and for hydronic radiant heating in Europe for decades.  PEX is much more resistant to freeze-breakage than copper or rigid plastic pipe.  Another benefit of plastic pipe is its small environmental impact compared to other materials, never hurts to be green.

Here are a few resources to do some more research on this product:

PEX Supply

PEX Information

PPFA