The bladder — the rubber “bag” inside a pressure tank — ages and eventually splits. The tank still looks fine, but the pump starts every time a tap is opened. The good news is that most tanks have a replaceable bladder, and changing it takes about an hour.

What the diagram shows

1 — the new bladder, 2 — the removed flange with bolts around it, 3 — a pressure gauge to check the air charge, 4 — the flange bolts.

Signs the bladder has burst

  • Briefly pressing the air valve core lets out water instead of air.
  • The pump starts as soon as a tap opens and stops as soon as it closes.
  • The air charge in the empty tank does not hold: you pump it up and a day later it is back to zero.
  • The tank is much heavier than it should be because it is completely full of water.

First rule out the simple cause — a low air charge. See “How to check and top up the air in a pressure tank”. If no water comes out of the valve and the pressure holds, the bladder is intact.

Which tanks can be repaired

A replaceable bladder is fitted in tanks with a flange — a round bolted metal plate through which the water connects. Some tanks have a fixed diaphragm bonded or crimped into the shell: these are not repaired but replaced. Choose a new bladder by tank volume and flange diameter, ideally from the same maker.

Step-by-step replacement

  1. Switch off the pump, close the valve to the tank and open the nearest tap to release the pressure.
  2. Let all the air out through the valve, then disconnect the tank from the pipe and drain the water.
  3. Undo the flange bolts in a criss-cross pattern and remove the flange. Pull out the old bladder — in large tanks its neck is also held by a pin on the opposite side.
  4. Rinse the tank and inspect the inside: heavy rust means water has been sitting on the metal for a long time, and the tank is better replaced.
  5. Insert the new bladder, spread it out and bring its neck through the flange opening. Refit the flange and tighten the bolts gradually in a criss-cross pattern so the rubber seats evenly.
  6. Pump air into the empty tank to 0.2–0.3 bar below the pump cut-in pressure, and only then reconnect the water.
It is worth testing the bladder before fitting it: put a little air in, dip it in water and look for bubbles. Defects are rare, but they are awkward to find once everything is assembled.
Do not tighten one flange bolt all the way first: the rubber will skew and the tank will leak around the flange.

Bladders and flanges are in pressure tank accessories. If it is simpler to replace the tank, see “How to choose a pressure tank”.