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I live in a very old house - build 1902 - in the 4th floor in the city of Karlsruhe (Germany). I have a shower and the gas-heater for the water is in it:

enter image description here

If I hold the shower head over some height (see image), the water pressure is suddenly reduced. Whats the reason for that?

(I guess the way the pump works might be the reason.)

enter image description here

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The problem seemed to be something within the gas-heater. After it was cleaned, it worked with good pressure. – Martin Thoma Aug 10 '14 at 1:40
up vote 3 down vote accepted

Your shower is fed by a water tower. The maximum pressure at your shower head is determined by the equation $P_{shower}=\rho g\Delta h$, where $\Delta h$ is the difference in high between your shower head and the water level in the water tower.

Because you live on the 4th floor, which is apparently close to the water line in your near by water tower, as you lift your shower head, $\Delta h$ decreases and so does your water pressure.

Other things like pipe lengths, pipe bends and water restrictors impact the water pressure, but those things are (mostly) constant and shouldn't depend on height.

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