Here’s some interesting detail from last nights hot water run.
- Red is electric input (watts) from my Vaillant controller (which drives the external mixergy plate pump)
- Yellow is the electric input (watts) from the Mixergy (which drives to pump on the bottom right, the normal churner pump) - same scale as the controller (scale on left)
- Blue is the heat output (watts) from the heat meter (scale on right)
So the PHE pump drives like mad (40 odd watts) for the full duration of the DHW run.
It looks like the other Mixy pump doesn’t really do anything major until 15 mins after the DHW finishes (green circle).
Although it’s hard to tell whether the external pump kicks in on a low setting as there is only a few watts increase.
Although this snapshot without the PHE plate shows something is happening? Maybe the CT clamp can’t pick it up properly at such low watts?
Interesting, as it seems to show the PHE only circulating water at the bottom of the tank. If this is the case, there shouldn’t be large amounts of mixing. (Unless the pump running at a high speed stirs things to some extent).
The second (non-PHE) pump AFAIU only runs when using the immersion heater or top coil. This is because it’s only required to spray cold water into the top of the tank when heating using one of these two methods.
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Interesting to see how Trystan’s system (with coils) performs compared to my system (heat exchange) when the temperature of the tank is warmer than the starting temperature of the ASHP loop.
With my system, the return temperature is actually higher than the flow temperature for the first few minutes, as ASHP circuit steals (borrows) heat from the cylinder until the compressor takes over.
The overall COP (2.8) is lower than if the bottom of the tank starts colder (3.6) (excluding standby).
- July 27th, 20°C ambient, water heated to 52.5 °C, 0.9 kWh electricity => 2.5 kWh heat
- July 28th, 20°C ambient, water heated to 47.5 °C, 1.4 kWh electricity => 5.0 kWh heat
This phenomenon is discussed in the thread Improve efficiency: Heat the hot water when it’s cold.
I might tweak my system so that it doesn’t try heating DHW at all if the bottom of the tank is still warm…
Do you see the same thing happening with Mixergy?
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Seems to occasionally, but I can’t see much COP difference between when it does and doesn’t. Seems of little significance on runs here?
Your mixergy_cold_temp is starting at a low temperature though. Do you have examples where it starts at, say 30C? Or does that not happen with Mixergy?
Only when the tank is above say 80% full is the cold warm.
As you use the full tank, the cold must rush in and the separation they’ve got going on takes place?
So for the most part, my cold part of the tank is usually cold for the start of DHW runs.
But that’s still not really helping COP for me compared to the Coil Brigade.
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3.4 COP last night, outside temp 14C.
This was from 35% to 100% at 50C target of my 250L cylinder.
Mixergy pump on setting III (max)
Arotherm flow rate on 100% max.
The Mixergy certainly seems to perform better when the tank is heading towards empty.
- 3.63 today at 20C outside.
- 26% full to 100% full (so around 185L of my 250L tank)… so it spent more time doing ‘cold’ work?
- Target hot water was 51C (51C is my legionella target temp in the app, so this means I don’t have to due any other cleansing runs)
- Mixergy pump on setting III
- Arotherm flow rate auto (around around 690 l/h)
1.8kWh electric in for 6.6kWh of heat out.
I think this is about as good as it get for my Mixergy plate?
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Posted this on Twitter earlier. Posting here so can compare against the previous run on ‘normal’.
his time we are in Eco mode on the Vaillant controller (compressor limited to 50%) but the pump speed at 100% (800l/hr).
- This run is around 140L of my 250L cylinder heated to 51C.
- 17C outside and a COP of 3.7
- 1.4 kWh of electric used
- 5.3 kWh of heat output
Seeing better performance imposing these limits, but it comes with a time cost, so heating takes much longer.
Heat output it limited to around 4kW in Eco compared to 7kW you can get in ‘normal’ mode on my 5kW Arotherm. So close to twice as long. 1hr 20mins is gonna be too long to have the heating off in winter I feel.
And despite imposing these limits, I still can’t get my plate setup to match the performance of the coil based cylinders listed on the heatmonitor list.
Coils for the win?
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Looking at the data tables, page 36, you can see that often there’s not much of a difference in COP at 4kW vs 7kW output. This changes depending on the flow temp.
And if you would be to compare your plate with a coil at the same exact temps and delta you would find a smaller difference in COP.
But overall who(and how) is checking if the actual HP, does what it says on paper?
Maybe some are not charged properly, or all the product line underperforms. Etc
@Zarch thanks for sharing these results, in the above example what was the COP getting the cylinder up to the 42C cylinder water temp point, if you cut it off on the graph a bit early?
4.37 to 43C return on 26th Aug
Then 3.19 for the rest of the run (to 53C return)
On 24th Aug (the run in post 44)
Then 4.27 (to 43 return) and 3.11 for the rest of the run
So quite good I suppose upto that 45 ish mark?
Interesting COP figures if I cut short the graphs (on the 24th Aug run) at the following return temps
- 40C - 4.42
- 41C - 4.35
- 42C - 4.31
- 43C - 4.26
- 44C - 4.20
- 45C - 4.18
- 46C - 4.08
- 47C - 4.03
- 48C - 3.95
- 49C - 3.87
- 50C - 3.82
- 51C - 3.77
- 52C - 3.71
- 53C - 3.65
I’m still about 0.5 COP behind some of the coils to 45C at the same outside temp (14C) for that run. 
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It’s almost like the tipping point in Vaillant ‘balance’ mode picks itself (45C return). 
Balance runs full compressor to 45C (normal mode) and then switches to Eco (50% compressor) for the rest of the DHW run.
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One thing that intrigues me is why no one blends the return (with the flow) so it is at the optimum temperature (i.e. optimum delta) all the time.
The run was 1:20 mins @ 800l/hr the water in the tank has cycled roughly 4 times. So the top of the tank is your target temp and the rest of the tank is at the delta at that time.
If you only want 140l of water, only heat 140l to the target temp.
If you blend the return so you reach the flow target temp sooner, you only then dump that water into the tank.
Perhaps the in-built controls won’t like it .
On your differing COP figures at different temps, could that be static losses?
Just wondering…
Stop the Press!! New COP record overnight for the Mixergy!!
COP 4.18 heating 165L (of my 250L) to 47C at 20C outside.
I definitely seem to get better results the emptier the cylinder is at the start. More cold swishing around / low temps in play?
This is good, better than immersion obviously… but COP still not as good as the coil based systems. 
@Zarch you’ve probably explained this already elsewhere but, how do you know how much of the tank you are heating?
Good stuff Mick! Though that 20C outside temp would have given the compressor an easy ride…:). Streets ahead of my Sunamp though!