One of the biggest differences between Japanese apartments and houses and their American counterparts is the water heating system. There is no big water heater tank that stores hot water. There is no "always on" broiler to keep the water ready to go.
Japanese houses and apartments use an "inline" water heating system. What I mean by this is that the water passes through fire to heat it. I am not actually sure what is more Eco or efficient, they are both just a little different.
The most important thing to remember is to turn these things "On" and "Off"!! As an American I don't know how many times I have gone into the shower without turning on the water heater here in Japan. My reaction is usually "*#&$()*@, I forgot to turn on the water heater!!"
If you watch the Jackie Chan movie Karate Kid you will see the American actress wondering why her shower doesn't have hot water....That's exactly how a lot of first time foreigners in Japan react....LOL
Here is the external unit. This is where the "FIRE" is! I will try to get a photo or video at night when it is turned on, and post it later.