Hi folks, I am planning on getting a Mixergy hot water cylinder (Indirect Unvented) to replace my ancient indirect vented cylinder, but I am struggling to decide between the Solar X (that has the built-in PV Diverter) and the standard Mixergy X that doesn’t, and wonder if anyone has any experience that could help me?
Background: We have 3kWp of Solar PV, a 12kWh Powervault 4 battery, a Pod-Point Solo EV charger and I’ve just replaced the gas boiler with a Tepeo ZEB. Due to our central heating demands, during the winter there will be many days when we will exceed the 40kWh of stored heat in the ZEB, so I am now on Octopus Cosy which provides 8 hours of cheap-ish electricity which should be enough to cover the space heating demand. But for this reason I don’t plan to use the ZEB to heat the Hot Water and will instead use the immersion heater.
I want to install a Mixergy so that I can better control the volume of hot water that needs heating - currently I am using a programmable immersion timer switch with my old cylinder but I could be heating more water than I need, and although it is a 7 day timer, it’s not exactly easy to update when I know our needs might change, especally if it’s just for one day (e.g. heat the water earlier, or later for one day). The Mixergy will give me the ‘visibility’ of how much hot water we need, and the Home Assistant integration should make it easier to adjust when needed.
Now my dilemma: The Mixergy Solar X has a built in PV Diverter that would (in theory) mean it could monitor the solar and soak up spare energy until the required temperature has been reached, and the Mixergy video says they can work with solar batteries. I have asked them to provide more details. But assuming it does work as claimed, it adds complexity to my whole energy management as I have to adjust how I charge the Powervault, and take into account how much surplus energy the Mixergy will grab first, and then I have to keep an eye on both systems and make sure they are working correctly. Not impossible, but it’s more development and maintenance.
Alternatively I think I could just get a regular Mixergy X, which does not have the Solar PV diverter (so I expect will be a little cheaper) and just let the Powervault grab all the excess Solar energy, and then when the Mixergy needs to heat via the immersion, it would either draw power from the Powervault or Grid or combination of PV and Powervault depending on the time of day and sunshine. This is simpler because only the Powervault is trying to monitor the surplus solar and use it for charging, but, the round trip loss of storing that energy in the Powervault and discharging it again is around 20%, compared to just letting the Mixergy Solar X convert that energy directly into hot water in the cylinder. But then we don’t use much hot water during the day, so the Mixergy would suffer some heat loss overnight until we have showers the following morning.
I’m not trying to calculate the exactly right answer based published heat losses of the Mixergy tank, I’m looking for any experience of either option, or suggestions of anything obvious that I am missing?
Oh, I am planning on upgrading the PV (which was installed in 2006) and maybe even more battery if I still have any funds left over.
Thanks. Ian