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Mixergy Tank Failure

 
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Domestic Hot Water & Cylinders
Last Post by bontwoody 9 months ago
87 Posts
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bontwoody
 bontwoody
(@bontwoody)
Noble Member Contributor
Joined: 4 years ago
Posts: 1055
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 🙂

image

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
10kWh 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)
Honorable Member Member
Joined: 2 years ago
Posts: 486
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
(@bontwoody)
Noble Member Contributor
Joined: 4 years ago
Posts: 1055
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%. 


House-3 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
10kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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