OOh thats nasty.   Unless your installer overpressurised it during installation, which is only really possible if you have very high pressure mains >>6bar, then its almost certainly a manufacturing fault, a fault with the heating element causing it to overpressure or a fault with the inlet PRV.   Mixergy have clearly decided that one of these is the case as they have accepted responsibility.

Given the size of the bulge (50-100mm if I interpret the picture correctly?) Id be seriously considering living without running hot water for a few days, shutting off the supply to the cylinder and reducing the pressure to atmospheric by switching the taps on until they stop running.  I'm surprised Mixergy didn't tell you to do that TBH.

Maybe I'm being over sensitive, but remember that this is a pressure vessel containing hot water, so bursting is not clever (hence the mandatory tundish arrangement).  Clearly its failing.

I agree with @jamespa.

If you are not using the tank following advice from Mixergy, then I'd drain it as there's no point leaving it pressurised with the potential for a catastrophic failure.

I assume that bulge has got significantly worse since the slight bulging at installation?

What is your normal water pressure? Is it very high and/or do you experience surges in pressure?

Firstly if your not g3 qualified then I wouldn’t touch it.

Mixergy can’t tell you what pressure to set the expansion vessel at as they don’t know the incoming pressure.

the valve will open due to expansion failure, high pressure, high temperature.

its needs to be investigating why it happens.

for expansion valve to fail it’s either not serviced annually and incorrect charge or the diaphragm has failed.

for pressure to increase you could have a fault combi valve.

you need to check what the pressure reducing valve is set too, what you have before it and then what you have after it and if it’s working correctly.

is the expansion positioned correctly.

personally I would isolate and drain the cylidner with it in that state or get your local plumber to get out,

I’ve not seen this on a pressurised cylidner before in all honesty it maybe a weld that’s failing it would be interesting to see. 

I’d just contact Mixergy again on Monday and ask for an update, they are pretty good with issues from what I’ve found with them, albeit these have only ever been electrical. 

@old_scientist @jamespa 

Yes. I'm not heating the cylinder at the moment which is a pain as it's our only source of hot water. I haven't drained it and I dont think it has a seperate isolation valve so may not be able to. I'll check when I get home.

I dont think mains pressure  is abnormally high in our area either.

@damon thanks. I will do as you suggest. Hopefully I will be able to get some information on Monday.

So after asking the technical department for an update on Friday and hearing nothing, I used the enquires@ email address this morning with a forwarded copy of the technicians assessment sent on Wednesday:

"Sorry for the trouble. This would be replaced under warranty, and it is not safe to use. I will raise a ticket first thing in the morning and will report it to the quality manager. Sorry, I didn't see any bulge pic in your last email. Could you give us your original installer's information so they can provide a quote for a like-for-like replacement? Alternatively, we can arrange our own approved installer if they are not available."

Even after stressing that we have no hot water, I again have not heard anything today. I beginning to think that they have changed their mind and are not going to replace the cylinder, but if that is the case they should at least tell me so I can make other arrangements. 

I amazed at such a poor response. @editor do you have any contacts at Mixergy?

Cheers Mark

Posted by: @bontwoody

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I amazed at such a poor response. @editor do you have any contacts at Mixergy?

Sadly I don’t, but let me see what I can do.

@editor thanks Mars. I've managed to find a phone number for them tonight, so I will give that a try tomorrow. Weirdly my emails to them are not triggering an autoresponder "we will respond asap" message as they usually do.

@toodles If this all comes to naught I may pick your brain about the Sunamp thermino. They have come down in price whilst cylinders seem to have gone up! 

@bontwoody Fine! 😊 Toodles.

@bontwoody, I’ve made contact with @mixergy on your behalf and they have informed me they’re on the case and you’ll receive a reply soon, and they will also post a reply here.

@editor  Wow Mars! I was just building up to phoning them. Thank you.