Unvented hot water cylinder - expansion vessel problem

[sleepyhead]

About half an hour ago there was the most almighty crash, which we eventually traced to the airing cupboard. A round white metal tank which was connected to the hot water tank by a flexible metal pipe had come off the wall. It weighed a ton.

After scrabbling through the pile of old manuals/receipts left by the previous owner and doing some emergency Google, I now know that we:

1. have an unvented hot water system
2. that white thing is called an expansion vessel which exists in order to give the water somewhere to go when it expands when heated (since the water in our tank is under pressure)
3. that expansion vessels falling off the wall and weighing a ton is a symptom of the vessel having "gone" and we probably need a new one.
4. that the trickling sound I heard at around 4am last night when up with ds2 was probably an early symptom of said vessel's "gone" status and was water escaping from the pressure overflow something or another.

We'll get a plumber round on Monday. What I want to know, if anyone can help, is whether we can safely have the hot water on in the meantime. I've switched it off just in case, but if this isn't necessary then it would be good to know.

The trickling sound is alarming (no sound currently as water is off, and I think it was only happening when the water was heating anyway) and I really, really don't want to flood anything - we're a 2nd floor flat and I'm not ready for my neighbours to hate us (and to pay huge bills). Ds2 is currently waking half a dozen times through the night so their patience is probably wearing thin and a leak wouldn't help. But I can't feel any damp so maybe (surely!) this overflow pipe goes outside? But it's a cupboard on an internal wall so I've no real idea.

it is likely that the pressure release pipe is squirting water down the outside wall of your house. It will soak the brickwork and will probably leave an unsightly stain of algae, followed by a limescale mark that will last for years, so the sooner you turn off the water supply, the better.

(the same thing happens with a combi boiler if its PRV blows)

You will have to do without hot water until it is fixed.

Unvented cylinders should be inspected regularly, each time you have your boiler serviced. They are festooned with safety devices, so unlikely to go seriously wrong, but can if neglected for long enough.

Thanks. I'll leave it off then. Is it likely to be pricey to replace do you know?

A pressure vessel recharge or replacement is not especially expensive, compared to some boiler repairs.

It is possible that it just needs pumping up, though if yours has fallen off the wall it may have been damaged so probably best to fit a new one. £100 or so maybe.