Moving into the EcoBungalow 3 ½ years ago, I inherited an ancient gas boiler with a hot water cylinder. Having become used to gas combi boilers in my previous homes, I didn’t much like not having hot water on demand. It took a lot of experimentation to discover how much hot water I needed to heat and – even after months of refinement – I’d either run out or heat too much.
You can’t have direct hot water with an Air Source Heat Pump. You must have a hot water cylinder. So I was looking for a cylinder that would give me as much control as possible, to avoid the problem of running out of hot water or the wastefulness of heating too much. The Mixergy X tank recommended by the services engineer and used by the builder on his last project seemed ideal. It’s promoted on the company’s website as a smart cylinder where you can ‘heat what you need’. The idea is that the tank is heated from the top down so that hot water ‘floats’ on top of cooler water, using stratification.
It wasn’t until the tank was installed that we realised that – when connected to a Heat Pump – the feature of ‘heat what you need’ isn’t available. Instead, the tank acts as a conventional cylinder, heating the entire tank to a single temperature from the bottom up. This is because Heat Pumps heat water to lower temperatures than electricity or gas, so the stratification effect of hot floating on top of cold doesn’t work as the water isn’t hot enough.
How could a services engineer, heating engineer, and reasonably well-informed client all misunderstand this? I’ve reviewed Mixergy’s website, and concluded that – yes – the information on how the tank works with a Heat Pump IS there. However, it’s buried beneath the prominent messaging of ‘heat what you need’ and – as such - is highly misleading. After much googling, I found this helpful page tucked away on the company’s website which makes it clear how the tank works with a Heat Pump:
So far, so disappointing. Could the Mixergy App give me some of the controllability I was after?
The Mixergy Tank’s schedule is set via the Mixergy App. This has very enticing graphics of percentages of available hot water, and a highly sophisticated schedule that allows different percentages of the tank to be heated at different times of the day. Sounds great, but it doesn’t work like this with a Heat Pump.
As there’s no stratification, asking the tank to heat to 50% will result not in a layer of nice hot water ‘floating’ in the top half of the tank, but an entire tank 100% full of lukewarm water. So – as a Heat Pump user – the only option is to ask the tank to heat to 100%. You cannot partially heat the tank, despite this being available on the app for Heat Pumps.
There is a useful video from Heat Geek here which asks whether the Mixergy tank is the best option for Heat Pumps. In essence, Heat Geek recommends a high quality, correctly set up ‘dumb’ cylinder instead of a Mixergy cylinder when operating a Heat Pump. This is cheaper and simpler, with fewer parts to go wrong.
Interestingly, Mixergy have collaborated with Heat Geek to develop an update to the app to maximise the performance of the cylinder when operating with a Heat Pump. I’ve enabled ‘Heat Geek Mode’ and this allows me to lower the target temperature to 45ºC and control the anti-Legionella schedule. I’ve found that heating to 100% first thing in the morning is giving me enough hot water for a shower, along with some cleaning and washing up in the evening. Depending on the outside temperatures (an unseasonally cold June!), this is typically taking around 400-700Wh of power from the Heat Pump – averaging around about 15 pence per day. When I have visitors, I need to heat more. The temperature gradient on the Mixergy App is helping guide me as to how much more, but it's well below the level of sophistication and controllability I was hoping for.
I've e-mailed Mixergy with my feedback on the misleading website and the spurious sophistication of the app, with suggestions as to how the clarity of both could be improved in relation to Heat Pumps. They've responded positively, saying they'll use my feedback in looking at the evolution of the website and the app.
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