Rob wrote:

This morning I noticed a fairly significant bulge on the top surface of
my hot water cylinder (standard gravity fed system, gas heated with a
backup immersion that we never use). The insulating material is cracked

Sounds like it is on its last legs. Worrying that the bulge is at the
top. It would be interesting to know if the bulge is just damp
inuslating material or an actual bulge in the copper.

around the immersion heater and the pipe that goes in at the top (which
I guess feeds the coil from the boiler?). There's no sign of any leak,

The pipe "in" at the top is actually where the hot water comes out to
feed the taps. The should be another three pipes. One at the bottom from
the header tank in the loft - that is the supply of cold water in. Plus
another two for connections to the boiler. These are normally located
roghly in the middle of one side and at the bottom again.

It's obvious that I probably need a new cylinder but I have some
questions:

- What might've caused it? I thought the system was vented, so no high
pressure was involved?

The pipe that comes out of the very top of the cylinder should fork -
one bit going back into the loft and (usually) terminating over the top
of the main tank. The other will feed the taps. This vent can get
blocked with scale in hard water areas, although there ought to still be
an expansion path available by pushing water back into the cold cistern
via its feed pipe.

- Is it safe to carry on heating the water or should I leave it
switched off? I guess the latter, but my wife is nonplussed about that
idea.

She would be less keen on 100L+ of hot water cascading down through the
house I would have thought!

- How much is a new cylinder likely to cost?

Depends on size - a 900 x 450 indirect part L compliant one would
usually be in the range of £100 to £140:

http://www.screwfix.com/app/sfd/cat/...10423&ts=86904

You will need to add the cost of some suitable fittings to connect to
your pipework (you may be able to remove and reuse the ones on the
existing cylinder). It might be worth replacing the immersion heater at
the same time. These are cheap enough:

http://www.screwfix.com/app/sfd/cat/...87042&id=63692

Don't forget an immersion spanner and a couple of reels of PTFE tape
(you need plenty on tank connectors)



--
Cheers,

John.

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