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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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bontwoody

(@bontwoody)

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Topic starter 20/10/2025 11:46 pm

I'll leave it tonight and post the graph tomorrow to see it anything starts to make sense !

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IaAck

(@iaack)

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Joined: 3 years ago

Posts: 59

21/10/2025 10:32 am

@bontwoody "but after a minute the 3 way valved closed."

I might be wrong but could it be a 3 port, 2 way valve with mid-position?

Screenshot 2025 10 21 09 44 33 87 40deb401b9ffe8e1df2f1cc5ba480b12

Operation by the following voltages

Screenshot 2025 10 21 09 43 24 90 40deb401b9ffe8e1df2f1cc5ba480b12

My question would be is it a new valve or the original, has it been plumbed correctly, has the wiring to it been disconnected and reconnected correctly?

Was the new tank fully recommissioned and signed off accordingly as per Mixenergy installation manual? It kind of sounds like to me it hasn't been😗 Is that a fair assessment?

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bontwoody

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Topic starter 21/10/2025 11:05 am

@jamespa @old_scientist

@iaack Hi, thanks for the input.

So the situation is that I had a direct mixergy cylinder fitted 3 years ago but plumbed my heat pump ready so that I could add the Mixergy heat exchanger at a later date. The original cylinder failed and the replacement I was sent is an indirect cylinder.

Getting the plate heat exchanger isnt really financially viable at the moment so I thought I would try partially heating it from the coil.

The plumber who installed the new cylinder wasnt very confident with the wiring around my PV diverter so I said I would commision it as he was nice enough to plumb the coil into my system for me.

I dont think I have a mid port valve (I hope!) as I made that mistake in my last house and knew the issue Picture of my valve is below.

This mornings DHW run shows the same pattern. The flow rate is shown in red.

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bontwoody

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Topic starter 22/10/2025 12:31 pm

I may have found the problem. There is a valve on the return side to stop the water when it was originally plumbed in with nowhere to connect to. It was closed! I've opened it and will try again tonight 😁

👍

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Old_Scientist

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22/10/2025 6:05 pm

Fingers crossed @bontwoody

Samsung 12kW gen6 ASHP with 50L volumiser and all new large radiators. 7.2kWp solar (south facing), Tesla PW3 (13.5kW)
Solar generation completely offsets ASHP usage annually. We no longer burn ~1600L of kerosene annually.

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bontwoody

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Topic starter 22/10/2025 7:11 pm

@old_scientist Well it does seem pretty evident that it wouldnt work with it closed 🙂 but its odd OEM was showing flow, so lets see!

👍

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bontwoody

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Topic starter 23/10/2025 8:39 am

@old_scientist Yip Yip! it worked.

So the DT was small, probably because the stratification pump is off and the coil isnt big, but it got to a 40% charge on its own before it was stopped and the immersion kicked in. COP was pretty dire though averaging about 1.4. Ill try it in eco mode now which should be slower and give the water more time to conduct the heat away.

👍

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JamesPa

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23/10/2025 8:45 am

Posted by: @bontwoody

↑

So the DT was small, probably because the stratification pump is off and the coil isnt big, but it got to a 40% charge on its own before it was stopped and the immersion kicked in. COP was pretty dire though averaging about 1.4. Ill try it in eco mode now which should be slower and give the water more time to conduct the heat away.

If you can turn on the destrat pump COP should improve because the water in contact with the coil will be colder. You will also heat more of the tank likely to a higher temp.

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 23/10/2025 9:00 am

@jamespa absolutely. But I think I might have to get the Mixergy Control unit to achieve that. Ill keep my eyes open on ebay. In the mean time I will also drop the desired tank temperature and maybe heat in short batches with a longish gap.

👍

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Old_Scientist

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23/10/2025 11:55 am

@bontwoody so pleased it's finally working.

Posted by: @bontwoody

↑

@old_scientist Well it does seem pretty evident that it wouldnt work with it closed 🙂 but its odd OEM was showing flow, so lets see!

Yes, this is what threw me when I read your post yesterday - how were you recording flow for 1min with a closed valve, and if there were no flow, why did the controller not immediately raise an E911 error code. Anyway, kinda irrelevant now it's working.

I hope you are able to improve the COP a little. I use Quiet Mode to limit the output when heating my DHW, which hopefully makes it a little more efficient (lower and slower). My LWT normally sits around 8C above the DHW tank temp during the DHW heating cycle. Without Quiet mode enabled, it's a little higher.

Don't forget to turn Quiet Mode OFF when going back to space heating as it does strange things in space heating, as noted by Glyn Hudson here

👍

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bontwoody

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Topic starter 23/10/2025 1:07 pm

@old_scientist yes. No E911 error. Very odd. Thanks for the link, very interesting.

👍

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bontwoody

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Topic starter 24/10/2025 3:45 pm

@old_scientist @jamespa

So this was last nights DHW run using economic mode.

Im a bit unsure as to what the actual COP was as it states 3.00 for the whole window, but the instanteous values are much lower. Either way its better than last night.

Ive knocked the target temperature back 5C and scheduled two 30 minute runs with a break for tonight. 🙂

👍

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