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.
1 Like
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?
2 Likes
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?
1 Like
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. 
1 Like
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.
1 Like
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!
I use Tom McGuiness excellent Home Assistant integration.
Then I just look at % full before and after the run.
100% - 34% = 66%.
66% of 250L = 165L
You could also just use the standard Mixergy screens to get that… via their app or https://www.mixergy.io/
You can still tell the compressor to run flat out regardless of outside temp… you’re just gonna use less electric doing so.
I think it’s less about outside temp once it gets past 20C or so.
How full the tank is and the target temp seem to be bigger factors.
COP 3.99 heating approx 135L of my 250L to 51C at 26C outside.
So hotter outside, but lower COP cos the target temp was higher (51C versus 47C).
All good investigations!
Ah ok it’s a Mixenergy thing. Am I correct that it has lots of temperature sensors up the tank and that’s how you can know how much of it is being recharged?
Working out how much water you’ve heated is just maths.
You can use a simple calculator like this
As long as you know the started temp (cold water temp), finishing temp(target temp) and the energy used kWh (or watts) you can determine the volume (litres).
Stop me if i’m boring you now… another record.
COP 4.36 this afternoon.
Heating 200L of my 250L cylinder to 51C (20% to 100%) at 23C outside.
More proof that running the cylinder cold/emptier helps getting a better COP.
If I would have cut this run short at 46C return, COP would have been 4.9!!
So proof that under the right circumstances (empty tank, low target temp) we can get decent numbers from the Mixergy plate. But it still falls behind a coil based tank.
2 Likes
Had my first heat pump and Mixergy year serviced yesterday.
We found the strainer to be quite grotty, although the water was crystal clear and Ph, Conductivity, Hardness and Iron were all on point.
Once the strainer was cleaned there was an instant jump in max flow rate.
100% pump DHW before cleaning: 840 litres/hour
100% pump DHW after cleaning: 1190 litres/hour
We had done quite a lot of work on the system since install last year.
- Heat meter move x 3 and eventual swap out
- IMI valves removed from rads
- A couple of old rads removed / new installed
- bathroom revamp with new rad
Take away from this, if you can, check your strainer!! Perhaps more often than you think or if you’re experiencing lower flow rates than expected.
So perhaps over the next few weeks I might seen better (or at least different) performance behaviour.
3 Likes
Do all systems have a strainer? We have a magnetic filter on our system would there be a strainer somewhere as well?