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Mixergy Tank Failure

bontwoody · 2025-09-13T05:40:14Z

Hi. I had my Mixergy tank fitted nearly three years ago and on the whole I've been very happy with it. A week or two ago we noticed a small leak on the floor beneath the piping. Initially I thought it was a leakage compression joint which I nipped up. This week I was woken by the sound of water and saw that the tank was discharging water via the tundish when heating. The most likely reason seemed to be a low charge in the expansion vessel, so I removed this and checked it. It was at 50psi. I've contacted mixergy and they suggested I reduce this to 2.5 bar and monitor the situation. It was at this point I noticed that the side of the tank seemed bulged out. When I had the tank installed the plumber pointed out to me a slight bulge in the outer case which I put down to over zealous insulating foam insertion at the time. I recontacted Mixergy and they told me to stop using the tank and they would replace it under warranty. That was Wednesday and I have heard nothing since. What I would like to know is what might have caused the problem and if it lies with the tank or the installation by the G3 plumber or the failure of a part, presumably either a valve or the expansion vessel which seems fine. Images attached. Any comments welcome. Thanks

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Domestic Hot Water & Cylinders

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bontwoody

(@bontwoody)

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Topic starter 10/10/2025 12:31 pm

@editor Ive just emailed the CEO, Mars, so lets see what happens.

House-3 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
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Mixergy

(@mixergy)

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10/10/2025 2:14 pm

Thanks for raising this. We’re really sorry we haven’t been active in this thread, our priority has been to work directly with the customer and our technical team to get this resolved as quickly as possible.

This issue remains a top priority for us. Our engineers are continuing to investigate and implement a fix, and we’re working closely with our service partners at Robert Heath to make sure it’s resolved as soon as possible.

We know how frustrating this has been and genuinely appreciate your patience while we get it sorted.

🤔

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bontwoody

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Topic starter 10/10/2025 6:00 pm

@editor lol if I was the priority I would hate to be lower down the list.

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Old_Scientist

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10/10/2025 7:56 pm

Posted by: @bontwoody

↑

@editor lol if I was the priority I would hate to be lower down the list.

You'd think, given the public profile of this thread and the presence of a representative from @mixergy that they would be pulling out all the stops to get this resolved TODAY.

Unfortunately this just reads as a shocking indictment of extremely poor customer service and warranty. It's not difficult to get a replacement tank and engineer on site to replace a faulty part.

Not acceptable to be left without hot water for nearly a month (will be a month on Monday).

This post was modified 7 months ago by Old_Scientist

Samsung 12kW gen6 ASHP with 50L volumiser and all new large radiators. 7.2kWp solar (south facing), Tesla PW3 (13.5kW)
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Mars

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10/10/2025 9:12 pm

@bontwoody, I completely agree.

I have to say, the level of support here has been really disappointing especially from a brand as established and reputable as Mixergy. I genuinely can’t understand why a replacement unit wasn’t dispatched and installed within days, and why you have a working cylinder that has been abandoned in your hall. This should have been treated as a priority, not something that drags on for weeks.

I hope you’ve requested compensation, because this level of service is frankly unacceptable. You paid for a premium product, and it’s only fair to expect a premium level of support. As part of our own retrofix project next year, Mixergy was a product we were seriously considering, but this experience has given me real pause for thought, and I think we'll take a different route.

You mentioned you’re using the instantaneous water heater for showers... does that mean you’ve been left without hot water in the kitchen and the rest of the house as well?

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bontwoody

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Topic starter 10/10/2025 11:38 pm

@editor fortunately we have a boiling water tap in the kitchen. 😁

I had a very courteous and supportive reply from the CEO so let's see what happens next week.

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bontwoody

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Topic starter 17/10/2025 6:38 pm

Just an update. The cylinder has now been replaced and is up and running again. Once it was fitted, the technical department were very helpful in talking me through a commissioning problem (my bad!)

Im waiting for them to pick up the old cylinder and then hopefully we shall be wiser as to what went wrong!

This post was modified 7 months ago by bontwoody

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Toodles

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17/10/2025 6:42 pm

@bontwoody Thank goodness you are ‘back on-line’ so to speak - that must be a great relief! Do keep us posted on what the old cylinder was suffering from please! Regards, Toodles.

Toodles, heats his home with cold draughts and cooks food with magnets.

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bontwoody

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Topic starter 20/10/2025 9:55 am

So following on from my new Mixergy cylinder replacement, they kindly sent me an indirect cylinder instead of the direct one I was previously using.

It seems likely that I might be able to use the coil to at least preheat the cylinder and maybe top it off with the immersion. However Im having some difficulty in getting it to work. I dont have the Mixergy Heat Pump control box but am just trying to use a schedule to preheat the cylinder.

The schedule seems to start as predicted but after a few bursts just stops, as in the graph below:

My FSV values are:

3011 Use(Hysteresis)

3021 50C

3022 0

3023 5C

3024 5C

3025 95 mins

3026 3 hours

3031 Use

3032 95 mins

3033 0

3041 Not Use

Any ideas anyone? Thanks

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JamesPa

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20/10/2025 11:20 am

I think the mixergy coil is quite small, they sell an add on circulator and phe for heat pumps. I don't know how Samsung's algorithm works but is it possible coil is near top, once it's heated the water immediately around that's it, given a relatively low ft from the hp?

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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bontwoody

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Topic starter 20/10/2025 11:47 am

@jamespa That was one of my thoughts too, but looking at the flow and return temperatures that doesn't seem to be the case. The coil is about midway down the tank.

I also thought that perhaps because I dont have the mixergy heat pump control unit that the desertification pump probably isn't on.

Surely if these were the issue though the flow and return temperature would be close and why does it stop after just 3 attempts?

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JamesPa

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20/10/2025 12:04 pm

What are the actual flow temps, the scale seems to be hidden.

Close flow and return indicates insufficient heat transfer.

Do you have the destrat pump. If so a good idea to run it somehow!

This post was modified 7 months ago 2 times by JamesPa

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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