Is Carnot % the best way to compare heat pump hot water runs?

As detailed in my own hot water adventure thread.

I’ve become somewhat obsessed trying to get the best out of my own Vaillant and Mixergy setup and how that compares to other systems on

And constantly moaning that my Mixergy plate based setup isn’t as good as the coil based cylinders many of you own. :joy:

When comparing a single hot water run on my system to that of other folk it’s almost impossible to do meaningful comparisons due to different outside temperatures, hot water target temps, Eco modes, flow rates, plates/coils etc all leading to different COP for the single runs.

So i’m wondering if using ‘simulate heat out using carnot COP’ is a better way of comparing systems and hot water runs?


Here’s a chart of data I pulled together from multiple random hot water runs from some of the top of the 30 day charts from

Carnot %, 30 day SCOP, COP when running

  • Glyn: Carnot 51% (30 day SCOP 4.0, running COP 4.6)
  • Trystan: Carnot 50% (30 day SCOP 3.7, running 4.7)
  • Heat Geek: Carnot 49% (30 day SCOP 4.5, running 5.5)
  • Custom Renewables (Oxford): Carnot 50% (30 day SCOP 3.5, running 3.8)
  • Custom Renewables (Stratford): Carnot 45% (30 day SCOP 4.2, running 4.2)
  • My Mixergy: Carnot 41% (30 day SCOP 3.1, running 3.4)

So there is some correlation there?
Obviously COP can be different due to higher target temp.

Custom Renewables Oxford likes 65C water for example, hence lower COP, but it is ‘still’ 3.6 cos his Carnot efficiency is 52% on this one.

Is 50% Carnot what everyone “should” be achieving / striving for on hot water runs?
If they can’t then it’s some sort of physical or setup problem/limitation with their system?

I’m curious what others think. What’s your hot water carnot % efficiency look like?
And would you plan to improve things?

Finally, when looking at the top performers again

  • Glyn: Carnot 51% (30 day SCOP 4.0, running COP 4.6)
  • Trystan: Carnot 50% (30 day SCOP 3.7, running 4.7)
  • Heat Geek: Carnot 49% (30 day SCOP 4.5, running 5.5)
  • Custom Renewables (Oxford): Carnot 50% (30 day SCOP 3.5, running 3.8)

Is the fancy dual coil on Adam Heat Geek cylinder really making all the difference in performance here? Compared to ‘standard’ coils in the other 3?

Details of his design in his video from 7:53 onwards.

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Interesting questions @Zarch !

It’s interesting how little difference there is between Glyn’s when running COP and mine, I have a twin coil 3m2 in parallel at the bottom of the tank (similar to Adams) and Glyn has 1m2 apparently…

Here’s a run from mine earlier:

and from Glyns:

Why so little difference i wonder? carnots are almost identical

I cant quite believe Glyn’s is 1m2… I wonder if the spec is listed wrong?

Another possibility is that my twin coils are not balanced perfectly and so Im getting a lower effective area and should try plumbing them reverse return rather than equidistant (i think that’s the term…?)


This is my tank temperature:


is that mid temp correct @glyn.hudson ? seems quite a bit lower than the target?

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Interesting, looks like we both have about a 10C difference between max FlowT and final tankT. I was expecting you to have less of a difference since you’ve got a larger coil?

That’s correct, in ‘Eco’ mode Samsung heats the tank about 5C flower than the targetT, I compnsate for this by setting the targetT about 5C more than I need! I use Eco Mode to avoid it kicking on the immersion if the cycles takes longer than 30min which is the default behaviour in ‘normal’ mode.


That’s what the Vaillant datasheet says, It’s a Vaillant 150L slimline uniSTOR. I’ve just downloaded the latest brochure, and it still says 1m^2

It’s a small slim tank, I wonder if this is improving the stratification compared to winder conventional cylinders?


Can you explain me exactlly what is the meaning of this heatpump factor

as it changes the simulated COP alot?

This is the window for my last DHW cycle from Is this mean that my heatpump is 55,8% Carnot efficient?

And this is thq same DHW cycle but from my local HP monitor? This shows Carnot effciciency of 40,7? Which is the right one? The same values for condensing temperature 4 and evaporation temperature -6 are used?


Between the Samsung and Ecodan 5kW what is the evaporator coil temp at the same ambient temp?
Is there a difference were the lower evaporator temp system uses extra energy to compress?
Is the different flow rate giving a different result? Better condensing in one or the other?

Is there a good explanation of ‘Carnot efficiency’ please. And what is a good benchmark

Hi, Welcome to the forum.

This page explains the Basics — OpenEnergyMonitor 0.0.1 documentation

Heat pump factor is typically between 40% and 55%, median around 48%.
Here’s a graph from