Tankless systems, in contrast, heat water on demand. Good for your heating bill, bad for the water bill. Since there isn't warm water waiting for you, there's a lag time between the moment you turn the shower on and when you start to feel warm water. That whole time, you're letting perfectly good (but very cold) water wash down the drain.
"'Distance plays a very large role in how fast hot water gets to the outlet,' says Joe Wiehagen, a senior research engineer at the National Association of Home Builders’ Research Center. The farther away your system is from your faucet (in my case on the other side of the house and on another floor) the more piping filled with cold water needs to empty before new, heated water can make its way to you."
So if you want to make sure your new tankless water heater is as thrifty with water as it is with heat power, make sure you have it installed nearest to where you'll be using it most.
Good thing those tankless units are tiny, right?