In the house where I live (two-storied building), we have a problem with water pressure. The pressure in the water mains is OK, it is ~ 4.3 bar, but the pipe that brings water to our house is simply too narrow. Therefore, when someone downstairs consumes water, the narrow pipe does not manage to supply enough water, the pressure in the system goes dramatically down, and someone upstairs has big trouble when taking a shower.

In an attempt to improve this situation, we obtained an expansion vessel.

It is located in the cellar. After it was first installed, we observed some improvement, but after a couple months, things got worse - almost same state as before. Since then, I have made several attempts to re-pressurize the air bag with various results.

I have devised the following methodology:

  1. Close the street water mains valve.
  2. Release all water from our water system in the house, including the expansion vessel. I do this by releasing the valve in the garden.
  3. Pressurize the air bag in the vessel to 1.5 bar (using a small electrical air compressor from Black&Decker).
  4. Release the street water mains valve and fill the system and the vessel with water again.

While doing step (4) I am always measuring how much water flows in by reading out the "before" and "after" value of the counter. I am doing a statistic of how much water flows in depending on the air pressure I put to the air bag.

My results look as follows:

  • 1.5 bar -> ~ 28 liters flow in
  • 1.0 bar -> ~ 34 liters flow in

In an empirical way, I am trying to figure out what the best pressure is to make best of the vessel. My findings are as follows:

  • If the air bag pressure is too high, little water is allowed into the vessel and so the vessel can supply us with water for only a very short amount of time.

  • If the air bag pressure is too low, a lot of water is allowed into the vessel but the air does not have the power to push it out of the vessel, so again not good.

Now the questions:

  1. I've always let out all water out of the system so I can measure the pressure of the air bag (1.5 - 1.0 bar). This is quite a significant waste of water. Is there any way to re-pressurize the expansion vessel while there's water inside?

  2. I am only trying to figure out the best air bag pressure empirically. I would prefer if I could calculate the values somehow. Could you point me in the right direction?

  3. Also, I seem to have to re-pressurize the vessel every 2 months or so. Is this a normal state, that the air from the air bag gets into the water so quickly?

  4. For a 100 liter vessel, what would be the best amount of water that can still be effectively used?

Thank you.


If you have to fiddle with it every two months, the air bag/bladder/diaphragm is leaking. If there is any sort of guarantee/warranty, you might want to use that. If not, you might want to select a different brand for the next one.

As for the pressure, think of it as the lower end of the pressure you'd like to see supplied in the house when several things are drawing. So if you set it to 1 bar, pressure will drop to 1 bar. If you have 4.3 bar supplied and would like to have it drop to no more than 3 bar, pressurize to 3 bar. You'll store less water, but at a greater usable pressure. If you need to store more water, you need a larger tank, or several small tanks working together.

While you cannot accurately set the air pressure without draining the system, you certainly can just pump air into the tank with pressure on the system, particularly if you are not going to replace the leaky bladder, since setting the tank pressure is not a thing that will stay set while the thing continues to leak.

  • I especially find interesting the following information: "As for the pressure, think of it as the lower end of the pressure you'd like to see supplied in the house when several things are drawing."
    – MartinCz
    Aug 6 '15 at 9:05

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.