Open outlet water heaters in Germany
I'm writing about these because a man in my apartment building didn't understand his and he ended up with a broken heater and water everywhere.
Most German buildings have a communal source of hot water and the usage of each apartment is metered and folks are charged accordingly. Normally, you then see a pair of pipes coming out of the wall and you buy a mixer tap with two pipes and just connect everything up.
My building does not have this. Instead, there is just a single pipe into the kitchen and a small electric heater for the water. The heater has input and output pipes and you might just think that you connect a T-piece to the cold water supply and then connect the cold tap and the heater input to the T-piece and the hot tap to the heater output.
That way, you will get a big expensive puddle in the kitchen.
The deal is that the heater cannot sustain pressure and so it must always be connected to an open pipe and the tap to control the flow must be on the input side of the heater!
What you need, is a tap with three pipes. Normally, this will be a mixer tap but even a hot water tap by itself would need three pipes. One pipe is the cold water input and that is connected to the domestic supply. When the hot tap is opened, water will flow in from this pipe and then out through the second pipe. This second pipe the goes to the input pipe for the heater. The output from the heater then goes to the third pipe on the tap which is connected, without obstruction, to the outlet.
You can see that when the tap is turned off, there is no pressure in the water inside the heater and, as the heater heats up and the water expands, it can drip freely out through the tap. When when the tap is on, there is little pressure inside the heater as the water is flowing freely out. Only when the tap is fully open and the flow is slightly impeded by having to go along a pipe and through the outlet will there be any small amount of pressure in the heater.
Get it wrong and the heater will always be under the pressure from the incoming water and will soon burst open.