Climate Friendly Retrofit Part 5: How to Decarbonise Hot Water

Josiah Lockhart
4 min readJul 16, 2021

It’s been a busy 6 weeks for our retrofit as we’ve started ticking off some of the big jobs like changing out hot water system and installing solar panels, car charger, and a smart energy management system. While there is loads to share, and lots of data coming in on how our house is performing, this blog, is all about the exciting world of hot water.

If you are not aware, heating hot water is one of the most energy intensive activities in a home. The two most common set ups in a UK household is either an insulated hot water tank fed by a gas boiler or a combi-boiler that heats on demand. Pre-retrofit, our home had the first set up with a D rated hot water tank fed by an 8-year-old gas boiler that had around an 80% efficiency rating (not an unusual set up). But with our plans to de-carbonise our home this was something we needed to tackle early in the process.

After some time researching LOTS of theories and both new and old technology for heating domestic water, Josh from AES introduced me to the Mixergy system. Relatively new to the market, the Mixergy is what a hot water tank would be if newly invented in 2021. It is a high efficiency tank that is leading the field in two ways beyond efficiency.

First, it uses volumetric heating. With a typical tank, you heat the whole tank all at once, but Mixergy’s volumetric heating you can heat a specific quantity of water rather than the whole thing which prevents wastage of heating.

Second, it uses both a smart controller and machine learning to learn your both habits and energy tariff to automatically adjust the tanks heating schedule to drive down energy usage and costs. To go one step further, if you are on a grid balancing tariff like octopus agile, it will automatically grab that data when published and update the schedule for the next 24 hours.

Volumetric Heating — From App

As I’ve said in previous blogs, the economics of decarbonizing and electrifying homes is of a real concern as it has a real risk of increasing costs and create a climate justice issue for those who cannot afford the switch. While I only have about 6 weeks data, it is already proving a surprising result. When we compared May and June’s energy consumption (the only pre-micro generation data we will have) I was not expecting the huge change in or usage. At the time we were on the Octopus Go electricity tariff that is .14p per kwh in the daytime and .5p overnight. Both of those are significantly higher than the .3p per kwh we pay for gas. But what emerged once we switched to the Mixergy was that although we increased our electricity usage by around 100 kwh, the more efficient use of the tank reduced our gas consumption by 600 kwh translating to a savings of £64 in June. For the full system including install, we paid £1,531. While that is only one month’s data, even without our switch to solar, early indications are that the mixergy would pay for itself in roughly 20 months.

The install wasn’t without issues (largely because it’s a young company and new tech), and we had a very hard time finding an installer in our part of Scotland who was willing to help us install the system. But in the end, it is already proving to be well worth the effort. And a big thanks specifically to Dan and Imogen at Mixergy who went above and beyond to help me with my system. If you are interested in more do check out the Fully Charged show’s episode on Mixergy and reach out to the Mixergy team to discuss your set up.

Fully Charged episode on the Mixery

The only last thing to say is to re-iterate what I’ve said many times, that every home is different and just because a system works well in one place doesn’t mean it will work in another. As you explore how to decarbonize your hot water I encourage to look at Mixergy, but also SunAmp, Caldera, Tepeo and others.

If you have missed the last 4 editions of this retro-fit blog, check them out here and stay tuned or the next one on our Micro Generation/Solar setup.

Use my referral code for £50 off switching to Octopus