OK, for some reason you are staying in Germany, but you did not have the financial means to check into a ****** hotel. Now even if Germany can be mistaken for being a civilized country, a large number of their houses are less comfortable to live in than a Mercedes or a BMW. Since these cars do not come with bathrooms as part of their standard equipment, you must resort to the facilities found in the house where you are staying. If you are out of luck, your bathroom is eqipped with a thing like the one shown in the picture above, the dreaded "Durchlauferhitzer".
Step 1: Leaps and Screams
You strip naked, grab your shower soap, and jump into the shower cabin. Then you turn on the water and curse. It's friggin cold. Well, maybe it takes some time for the heater to get going, or for the water to reach your shower from the central heating.
No. The water remains as cold as a cod fart. Since you're child of your time, you guess that the machine on the wall must be programmed in some way. Before you reach it, you have time to get worried about your smartphone: Was it quad-band or only tri-band? Will it be able to interact with the heater?
Wrong. As you can see in the picture above, there is ample reason to despair. The dumb thing does not even have a power on button. (No, not even on the sides that are facing away from the camera.) Well, "one dot" and "two dots" are at least labels that are gender neutral and perfectly incomprehensible regardless of cultural background.
You give up and call one of the natives. After some battle tank gearbox noises in your ear, you have learned that the thing is fully automatic. You just have to open the tap some more, then the heater is turned on. The native explains that a "Durchlauferhitzer" heats the water on the fly (swim?), as it passes through the thing. There is no reservoir with hot water.
With new hope you step into the shower and open the tap, some more, then some. When the flow rate rivals that of the river Rhine, the box on the wall finally goes "clack". Within seconds, the water goes from "cardiac arrest" to "cerebral haemorrhage", and you slam the tap shut in self defence.
After experimenting for twenty minutes, your performance remains erratic. The machine goes "clack" or "thunk", that is, turns the heat on and off, respectively, virtually at random. Most of the time, the water actually gets colder as you open the hot water tap more, but sometimes it can suddenly become much, much hotter. What's the deal? Who invented this piece of <censored by the instructable team>? Is he still alive? If yes, where does he live?
Stay cool. I'll explain everything in the following steps. Above all, don't scream and throw the thing out the window, your neighbours will call the police. (...not because you vandalised the bathroom of your flat, or because you threw the "Durchlauferhitzer" on the street, the Germans do that all the time. No, they'd call the police because you screamed.)