We recently had a small flood in our apartment. I found that the metal-sheathed rubber hose connecting the cold water supply to the blue cold water expansion vessel had a wide split in it. The water supply had already been turned off by someone else; when I turned the water back on gently, water gushes out of the split.

Can/should I replace this hose myself? I have no plumbing qualifications, although I do have a solid grasp of physics and engineering, an eagerness to learn and I know how to turn off the water supply to my flat...

The tank is a Zilmet Ultra Pro 8 litre. Note that the water connection of the expansion vessel is at the bottom; the air valve is at the top - other similar products I've seen online seem to have them the other way around.

My concerns:

  1. I unscrewed the hose at the mains water end, a little water dribbled out. As I started to unscrew the hose at the vessel end, it gurgled a bit as if air was getting in and started to leak water (exactly as you'd get if you slowly unscrewed the cap on an upturned water bottle). I started to worry that I was letting air in to the water side of the expansion tank and that this might be bad - eventually I decided that the air pressure must be too low as I'm assuming that when the tank is pressurised correctly but there is no water pressure, the diaphragm will be pushed against the water inlet, so to re-expel that air I was letting in all I ought to do was to repressurize the tank. Is that correct? I attached a bike pump to the valve and the pressure was zero, worryingly. As I pumped it up to the factory pre-charge level (1.5 bar = 22psi), loads more water was expelled. Is this normal? It sounds like the expansion tank has been running depressurized for some time.
  2. The water coming out of the tank, especially towards the end, was a bit smelly, and had a few black bits (rubber?) swimming in it. This doesn't seem great - I think this feeds the rest of the taps in our house including the drinking water! The tank claims it has a (replaceable) 'food grade membrane' and is advertised as being for potable water.
  3. The rubber hose does not appear to have perished, nor the metal sheath on it corroded - I have no explanation as to why it split. I'm concerned it burst due to some pressure surge (would the expansion tank have absorbed this, had it actually had some air in it?), and that other parts of the system may be damaged.

2 Answers 2


I'd try to eliminate the flexible hose if at all possible. You can get a threaded piece of copper pipe and solder it into your plumbing system if you have copper plumbing.

In a well designed system, all of the hoses, valves, etc should exceed the threshold for the TPR valve on the hot water heater. So maybe the hose you had in there wasn't rated for enough pressure, or your TPR valve isn't functioning (very dangerous, you should test it).

For the tank itself, don't worry about water coming out of it. It will work just fine without water in it (in fact, it will likely work better since you've increased the volume of air absorbing pressure fluctuations). To pressurize it, you have to remove all water pressure first (shutoff the water supply and open a faucet) and then pressurize it with a pump up to your water pressure PSI level. As long as the tank holds pressure with the water turned off, the bladder is still good, and you can reuse the same tank.

If you're worried about dirt in the water side of the tank, fill and drain it a few times to see if you can clean it out. Chunks of rubber will likely get stuck in an aerator or shower head which would need to be cleaned out, so better to do this now.

In terms of mounting, they can be mounted any direction. If mounted vertically on a fixed pipe (doesn't matter which end is up) then you don't even need support brackets.

  • 1
    How should I test the TPR valve? (curiosity now - I'm getting a plumber in). I'm not worried about air being in the tank so much as air being in the 'wrong side' of the membrane - the memrane is there for a reason I guess? (why? So it can be mounted water side up is my guess?) How can I fill the tank and drain it? There's no valve immediately after the tank but before everything else - I think I'd have to attach something temporary to do this.
    – bacar
    Dec 8, 2011 at 9:44
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    TPR valve has a test lever on top. Put a bucket under the drain tube and pull the lever to test. The membrane helps with non air-side-up installs and air could dissolve in the water over time. Fill the tank by unhooking it and using a bucket of water and funnel to fill and then flip the tank over to drain (outside or in a sink).
    – BMitch
    Dec 8, 2011 at 12:34
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    OK, plumber showed me - the valves I have have a twist-to-test red cap. Re 'unhooking' it - it's in a very awkward location behind the boiler and bolted in place!
    – bacar
    Dec 8, 2011 at 17:01

My own guess of what happened, based on understanding I've gained from this question, and eventually speaking to a plumber, is that the expansion tank air pressure was zero and consequently full of water - so not acting as an expansion tank. When the water in the calorifier (which is connected to this expansion tank) heated, there was nowhere for the water to go and the rubber hose ruptured under the pressure.

The plumber said the rupture was probably caused by some corrosion to the metal sheath, exposing sharp metal strands which could have pierced the hose. This may have been there (leaking slowly for some time) before the hose burst. This sounds like a plausible reason for the hose to burst rather than TPR valves firing - he tested the valves and they work.

The black rubber bits were probably just from the hose. We haven't had any yucky bits come out of the taps (yet!).

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