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With a gravity water feed system, a header tank is in the attic. Every 1 metre drop from the water tank typically equates to around 0.1 Bar in pressure.

There is a 500l cold water tank in the attic, the tank size is approximately 1.7m3. In the bathroom directly below the attic, the water pressure is low, around 0.15 bar; because those taps are less than 2metres below the tank.

I can gain an extra 0.1 bar by raising the existing tank 1m in the attic.

From what I understand about Newton gravity law the pressure would be measured from the surface of the water, so as the tank empties, the pressure falls.

Does the size and shape of the tank make a difference?

Pressure would increase when using a tall and narrow tank (presuming it is full). So instead of a 1,7m3 tank sitting at level 1m, or at level 2m, there would be a 2m tall cylindrical tank with 28cm radius, installed at level 1m.

The tank holds the same volume; the water pressure from the cylindrical tank would initially be higher, but decrease quicker as the water level fell. Would the pressure from a narrower tank be significantly different to a 1m high tank? Of course if the tank is almost empty, then having the base of the tank higher up would always be the best option.

I am wondering why water tank cisterns are not typically narrow and tall? They are easier to transport (up stairs and through the loft hatch) and it results in higher water pressure when full.

Discuss.

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  • $\begingroup$ As sooon as the water level drops the pressure will reduce, so a narrow tank is the worst solution. You need a tank that doesn't change its level during the course of a shower. $\endgroup$
    – JMLCarter
    Commented Sep 16, 2017 at 20:39
  • $\begingroup$ Thanks for the input. But with a cylindrical tank radius of 28cm, draining 25litres will displace ~10 cms of water, won't it? Will that effect the water pressure during a 5 minute shower? Based on (3.14*28*28)*10 = 24.6l $\endgroup$
    – yesmaybe
    Commented Sep 16, 2017 at 23:14
  • $\begingroup$ Showers are more like 15l/min, so 75L for 5min. This is height change of 30cm. Actually many people shower for longer, 10-12min or even 20min. So you can see a height change upto about 1m starting to occur. If other water is being used in the house, this makes the situation worse. However there is also a feeder that fills the tank. $\endgroup$
    – JMLCarter
    Commented Sep 17, 2017 at 2:28

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For most uses, some minimum pressure is required all the time, not just when the tank is totally full. The usual solution is to place the tank at some elevated position. As JMLCarter commented, using a narrow and tall tank would cause pressure to change a lot depending on the amount of water in the tank. That's the reason for water towers to be a tank on top of a tower instead of being a narrow tank as tall as a tower.

However, when building tanks there are other constraints, as ease of transport -as is pointed in the question- or available space which may lead to narrow and tall tanks. In such cases, either even pressure is not a requirement or the tank is placed high enough that its dimensions are negligible compared to its position.

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  • $\begingroup$ Thanks for the input. A plumber explained why the top bathroom water pressure was so low; there is only 30cms from the shower head to the water tank outlet; resolve this by raising the tank. I understood that the pressure comes from the surface of the water (10m depth in a pool or the ocean is the same pressure), so wondered rather than building a raised platform, install a taller tank, so the water surface of a full tank was higher, while the outlet pipes remains. If the tall tank remains always half full, it is better than a low tank being half empty.. $\endgroup$
    – yesmaybe
    Commented Sep 17, 2017 at 14:23
  • $\begingroup$ Furthermore, if you installed a taller tank with the same are, you will have a larger tank, which would be more expensive and would add a lot of weight. $\endgroup$
    – Pere
    Commented Sep 17, 2017 at 14:39

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