My Eddi has been installed for about two months now. It has been a great addition to my setup quietly mopping up mountains of surplus PV and heating hot water.
I installed it myself and documented it over on my YouTube channel, so if you’re interested in that, check it out:
But it hasn’t all been roses!
The first very sunny day after I installed it saw the temperature of the water in my Mixergy tank hit 80 degrees Celsius!!! With small children in my house, it’s downright dangerous!
“At 60°C, it takes one second for hot water to cause third-degree burns.”
https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/burns-and-scalds-children
Once I was made aware of the problem, I reached out to Mixergy and they explained how to limit the temperature using the safety cut-off of the heating element. I’ve made a video showing how I went about it, and you can watch that here – https://youtu.be/tIxomiHLhI0
To go off topic a little, another solution to this dangerously hot water is the installation of a thermostatic mixing valve. These are the same type of valve that control the temperature of the water coming out of your shower (assume it’s not electric!). It will blend the water coming out of the tank with cold water to ensure it’s at a set temperature (45 or 50). If you had solar thermal for example, this would be an absolute must!
I digress.
Having initially dropped the maximum temperature down, I started to wonder. Having a higher cut-off temperature would mean I could divert more solar into the tank. This would increase the amount of solar PV I “consumed”, which I’m always reading is better than exporting.
Is diverting all the surplus the right approach?
In order to understand more about the Eddi and Mixergy’s behaviour, I pushed the cut-off temperature back up to 75. This would give me an idea of how it worked when I was diverting *all* the surplus into the tank.
This is an example of my tank’s behaviour on a good sunny day:
You can see around 11:30 the orange line starts going up. This is the percentage charge. Mixergy works this percentage using the temperature gradient across the tank and compares it to the desired water temperature. My desired water temp is 50 degrees. As the top and bottom of the tank hit around 50 after an hour or so, the charge reaches 100%.
As the sun was still shining, the Eddi kept diverting. Note the red line. This is the water temperature at the top of the tank. This continues to rise to around 75 at which point the element’s cut-off kicks in and the Eddi stops diverting. For the rest of the afternoon, it turns on and off a dozen times as the tank’s water naturally loses heat. As the temperature is still well above 50, the tank registers 100% charge.
It’s not until 10pm that we see something happen. The temperature suddenly drops. I’m not sure why this isn’t a nice gradient, but that doesn’t matter. You’ll see the percentage charge starts to drop as the water cools.
This cooling continues into the next day, hitting 83% before the first shower. It’s interesting to note the bottom temperature drops like a stone as cold water comes into the bottom of the tank.
We haven’t had to heat any water this morning as there was enough in the tank to cover two showers.
However, we’ve pumped in several kWh of electricity, only to lose it overnight in the form of heat loss. That seems wasteful. Not only is 75 degree water dangerous, it’s also kinda pointless. We’re not paid a great deal for exported energy (4p per kWh), but it’s better than heating the air in the garage.
Is there a better balance?
I think there is a better balance to be found. I want to combine the percentage charge data I get from the Mixergy with the Eddi, so that the Eddi will stop diverting once the Mixergy reports 100% charge. This will mean we’re not constantly diverting and having a lower temperature in the tank will also slow the rate of heat loss.
How?
For my smart home, I use Home Assistant. I have written a plug-in that let’s me access the information from my Mixergy tank. So that’s one piece of the puzzle in place.
For the Eddi, it’s a little more involved. Myenergi do offer an API for their Eddi, but the control is limited to On/Off. For my needs, I want to be able to tell the Eddi to pause diverting. Fortunately, the Eddi has an add-on board which does just that!
When I purchased my Eddi, I added the relay board to my order as I had visions of heating my bathroom’s dual-fuel radiator.
The plan is this: Install the relay board into the Eddi and configure it so that it’s linked to the immersion element. Home Assistant will then trigger a relay when the Mixergy reports 100% change, which will tell the Eddi to stop diverting. When the charge drops below 100%, Home Assistant will trigger the relay again, causing the Eddi to resume diverting!
If this automation fails for any reason, the hardwired cut-off will still be present to stop the water getting too hot.
I’ll document this and do another blog post & video with the results.
@tomasmcguinness 