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DHW Efficiency

Lakey · 2025-03-02T11:54:58Z

I'm currently looking at two installer options. Eon and IMS heat pumps. Eon are proposing either a Vaillant Aurotherm plus 7kW on brackets (not a fan) or Ideal Logic Air 8kW floor mounted. On both options they have quoted a DHW efficiency of 1.75, can anyone help understand what the actual efficiency would be? 55 degree store temp for a 200L. IMS are proposing a Samsung gen 7, 7kW, with Adia. Their efficiency for DHW is reportedly 3.41(which seems impressive). Again, 55 degree store temp for 200L. I know DHW is only one section of the puzzle here, especially with Adia having zero rad changes (for now), but something that might help steer my decision on the installer quality. Thanks in advance

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Transparent

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24/10/2025 4:11 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

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26/10/2025 8:43 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

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26/10/2025 10:18 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

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26/10/2025 10:38 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

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26/10/2025 10:48 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

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26/10/2025 11:05 am

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

the ASHP taking the DHW tank to 50°C
a 3kW immersion heater switching full-on until the tank achieves 60°C
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

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26/10/2025 11:15 am

@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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