Wierd bulge appeared on hot water cylinder

[Rob]

Hi,

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 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, although some silicone-like substance has oozed out around the immersion heater.

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?

- 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.

- How much is a new cylinder likely to cost?

Thanks in advance,

Rob

[Andrew Mawson]

cracked

Expansion pipe blocked maybe ?

AWEM

[Nick]

When you turn the hot tap on, does the pressure seem to be higher than usual? There does need to be an expansion vent, which in my country (NZ) is a pipe going straight up and is open at the top, or else it's a small horizontal pipe going outside with an expansion valve which may have a flushing lever on it to clear foreign matter from the valve seat.

[Fred]

I would drain the system down, to relieve any stress and impending disaster, and remove the insulation from around these areas. Hopefully it'll just the insulation which has swelled up due to this leak.

[Rob]

The pressure seems normal, as does the temp. The only vented pipe appears to run from the top of the central heating pump and is closed off by a thumbscrew which looks to have a small hole in the top. Loosening this thumbscrew produced a very small amount of air and some warm (clean) water. The only other unsealed pipes are the overflows for the cold tank and the central heating header tank.

If the tank was under enough pressure to change its shape, wouldn't the hot water just have backed up into the cold storage tank? The cold water is normal temperature and was first thing this morning (when the system was still on).

[Adrian C]

The alien's eggs have hatched....

RUN.

[Aidan]

Is it a cylinder with foamed on polyurethane insulation, or a separate insulation jacket? If the former, is the bulge in the copper cylinder, or just in the insulation?

I replaced an immersion heater on a hot cylinder once and a small amount of water spilt from the immersion heater boss. There was an alarming crackling sound (it sounded like a bonfire) and the insulation around the bottom of the cylinder started bulging and splitting. It appeared to be that the water was evaporating and blowing the insulation off, or that it was some chemical reaction between the water and the polyurethane insulation. The cylinder wasn't hot enough to boil the water.

If it's just the insulation bulging, it might be just a leak from the immersion heater causing the insulation to separate from the cylinder. remove insulation and check.

[Rob]

Sadly it doesn't seem to be just the insulation - there's no sign at all of any leak and I can see that the coupling at the top is raised at one side where the tank's blown.

Will drain the system as you suggest and get a plumber out tomorrow - this isn't something I'll be allowed to tackle on my own :-(.

[John Rumm]

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.

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.

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.

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

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

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:

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

[Rob]

Now you mention it, the in-laws came to stay a few weeks ago. I always had my suspicions...

[meow2222]

It sounds truly forked.

To the OP, is it copper or insulation bulging? Running a knife through it would tell you. If its the copper bulging, you have a serious problem, either involving an unsuitable tank being under mains pressure, or a badly blocked system boiling. Either would be unsafe and imminently destructive, so if the copper is bulging I'd remove both heat and water supply from it.

If its just an insulation bugle, thats trivial.

If its the copper bulging, replacing the cylinder will not solve the problem.

NT

[Edward W. Thompson]

The bulge is due to either over pressure or a local weakness in the cylinder. Over pressure seems to be unlikely as for that to occur would require both the vent pipe at the top of the cylinder and the cold feed pipe from the cylinder, both from the header tank in the loft, to be blocked at the same time.

A local weakness in the cylinder appears to be equally unlikely.

Both possible causes are not impossible however.

What to do? The cylinder should not be used and replaced. The cost will be about £120 if you do it yourself. I just replaced a leaking cylinder, the cost was £104 from Wickes. I also took the opportunity to replace the immersion heater about £24 if I remember correctly. You can use all the existing fittings on the cylinder. Time to drain down, remove the cylinder and refit, about 4 hours provided there is reasonable access..

Clearly while doing the job you must prove the connection to the header tank clear.

Good luck!

[Bob Eager]

We had exactly this kind of failure last year. The cylinder (about 20 years old) bulged and failed (luckily just a fast weep) at the top. No other blockages or problems.

[Chris Bacon]

There should be a pipe coming from the top of the cylinder, tee'd to taps, that is the vent.

Hopefully. Cut out a section of lagging to see what's up.

[Chris Bacon]

Don't *forget* to notify the council's "building control" department, and pay the relevant fee.

[Roger Mills]

Did *you* do that?

[Chris Bacon]

Bother, I forgot... oh, er, actuelly, I ain't dun one since the Part P regulationes comed into farce, honest, guv.

[Roger Mills]

I don't think that Part P has anything to do with HW cylinders!

[Tim S]

Indeed - it's Part L. So even less reason to bother telling anyone IMHO

Cheers

Tim

[EricP]

Ah, so that's the one that turned "daft" into "total farce". Thanks, I had forgotten the letter for it. :))