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DHW Efficiency

 
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Domestic Hot Water & Cylinders
Last Post by Morgan 6 months ago
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Transparent
 Transparent
(@transparent)
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24/10/2025 5:11 pm  

Do you need the DHW tank as high as 50°C @egp01  ?

Have you tried running it at 45°C for a few days to see what effect it has on the COP?


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Mars
 Mars
(@editor)
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26/10/2025 9:43 am  

@transparent, that’s a bang on suggestion. 45C DHW tends to be the sweet efficiency spot for most heat pumps.


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Morgan
 Morgan
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26/10/2025 11:18 am  

Posted by: @editor
↑

@transparent, that’s a bang on suggestion. 45C DHW tends to be the sweet efficiency spot for most heat pumps.

@editor  is that correct? My installer set DHW to 50c and suggested it remain there.  I wonder if a 5c difference will leave me with sufficient hot water. (250L tank). I might have to give it a try because my COP barely gets up to even close to 3!

 


Retrofitted 11.2kw Mitsubishi Ecodan to new radiators commissioned November 2021.
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Transparent
 Transparent
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26/10/2025 11:38 am  

Yes, please do try it @morgan  and let us know what you find.

How long would you need to conduct such a trial in order to know the effect on COP with a reasonable level of accuracy?

Can you also say if you're running an anti-legionella cycle, and at what intervals?
That would also decrease efficiency of course, and make it unclear whether you setting the DHW down to 45°C is a 'fair test'.
There's a big difference running anti-legionella on a 250l tank and a 180l tank for example.

So many variables to consider...
... but please tell us what you can.


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Morgan
 Morgan
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26/10/2025 11:48 am  

@transparent I am running an anti-legionella cycle (60c), but I need to remember how to get to it again to see the intervals.  But that needs to remain, doesn't it?


Retrofitted 11.2kw Mitsubishi Ecodan to new radiators commissioned November 2021.
14 x 500w Monocrystalline solar panels.

2 ESS Smile G3 10.1 batteries.
ESS Smile G3 5kw inverter.


   
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Transparent
 Transparent
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26/10/2025 12:05 pm  

Well it depends on what we're going to consider a 'fair test'!

Does the phrase "my COP barely gets up to even close to 3!" include operating the anti-legionella cycle?

Assuming the AL cycle consists of

  1. the ASHP taking the DHW tank to 50°C
  2. a 3kW immersion heater switching full-on until the tank achieves 60°C
  3. the immersion heater cycling off/on to maintain the 60°C for 10-mins

then that could use an enormous amount of electricity for a 250-l tank.

Depending on how well insulated is the tank and surrounding pipework, it would also take about 90-mins to complete the cycle.

See my calculations in this topic about Heat Batteries.


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Morgan
 Morgan
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26/10/2025 12:15 pm  

@transparent Yes to questions 1 to 3.  Also, yes to the phrase "my COP barely gets up to even close to 3!". It does include the anti-legionella cycle.


Retrofitted 11.2kw Mitsubishi Ecodan to new radiators commissioned November 2021.
14 x 500w Monocrystalline solar panels.

2 ESS Smile G3 10.1 batteries.
ESS Smile G3 5kw inverter.


   
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