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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)

Noble Member Contributor

Joined: 4 years ago

Posts: 1012

Topic starter 01/11/2025 4:45 pm

So after playing about with DHW settings for a while I think the optimum use of the coil is for about 30 mins. After that time the COP drops to a point where I wonder if the cost saving is worth the wear on the heat pump 🙂

The COP over this time is about 1.9 which nearly doubles the efficiency compared to the immersion. (So costs about 3.5p per unit compared to 7p overnight). I think that the 30 min slot provides about half of my daily DHW needs.

Im sure the situation could be improved if I got the destratification pump working when running but thats a job for another day. 🙂

House-3 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60

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Old_Scientist

(@old_scientist)

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Joined: 2 years ago

Posts: 414

02/11/2025 12:37 pm

@bontwoody I couldn't see from the graph, but what temp do you heat the DHW to, and how hot does it get to in 30mins?

Is it just not able to heat to the desired set point, or would just take too long to get there? Do you know what the instantaneous COP is at the end of the cycle (at it's worst point)? From your chart, it looks like it may be around 1.6, steadily falling from the 15min mark as flow temp increases.

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

(@bontwoody)

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Joined: 4 years ago

Posts: 1012

Topic starter 02/11/2025 1:09 pm

@old_scientist I dont monitor the tank temperature but inferring from the flow temp I think the ASHP can heat to around 50-60C. That water is probably sitting around the coil and thats what causes the COP to plumate after about 30 mins.

From the Mixergy app the tank gets to about 35% full and then.I use the immersion to take it to 45%.

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