The Mixergy Usable Hot Water Capacity Test

The Mixergy Usable Hot Water Capacity Test

We have owned our Mixergy cylinder since February 2022 and initially ran it via direct electric and solar diversion using our myenergi Eddi.  Before adding the 5kW Vaillant heat pump in October 2022.

I have been a fan of the cylinder to this point and have written quite a lot of blog posts surrounding it and it’s ability to give me facts, figures and statistics as well as great performance.

Despite this, there has always been an undercurrent of negative comments, especially from a certain section of installers.  Comments that it’s a “suboptimal” solution when connected to a heat pump, which I’ve never really understood.

I agree that on the face of it can seem not as simple a setup as having a conventional cylinder with a coil.  But as we’ll explain in this article, there are some unique features that potentially elevate it about a conventional cylinder.

One of the features of the Mixergy cylinder is that you get an app.  You can change a whole host of settings in there including schedules, choosing different sources (gas, electric, heat pump), select target temperatures as well as interactions with time of use tariffs etc.

The best thing I like about the app is that it allows you to see exactly how empty or full the cylinder is in percentage terms.

As you can see in this screen grab from the app where I’m at 56% capacity.

Another unique selling point of a Mixergy cylinder is the claim that the entire capacity of your cylinder is available at a usable hot temperature.

So in my case, I have a 250 litre cylinder, so I should be able to get all 250L as hot usable water.

I have a “direct” model of Mixergy with no coil inside. It just has an immersion element in there and has an externally mounted plate heat exchanger which is used when heating the water via the heat pump.

The external heat pump plate as shown on the diagram from the Mixergy website.

The image is taken from a document where they highlight the differences between a Mixergy and a Conventional cylinder when using a heat pump.

https://www.mixergy.co.uk/blog/how-mixergy-cylinders-work-with-a-heat-pump/

The Mixergy setup is different to a conventional cylinder because the conventional cylinder has a coil inside.

The space taken up inside the cylinder by coil can reduce the available volume.  Unless the cylinder is made physically slightly bigger to make up for capacity loss.

According to the specs of the Heat Geek Super Cylinder (model 250LA4),  the coil takes up 13L of the 250L capacity. So 237L remaining. 

https://newarkcylinders.co.uk/wp-content/uploads/2025/06/Heat-Geek-Super-Cylinder-V3.1-Specification-Installation-Booklet-Nov-2024.pdf

Another downside of having a coil is that depending how high (or low) the coil sits inside the cylinder, depends how much cold water can sit below the coil.  

This can lead to unusable hot water as cold water below the coil cannot be heated (as heat rises).  As suggested in this diagram.

So can you lose some usable capacity there too.

In this technical specification document from the Hot Water Association, the HWA requirements state that at least 70% of the badged capacity in a conventional cylinder needs to be usable and able to deliver usable hot water.

https://hotwater.org.uk/uploads/5EEB25D1ED859.pdf

So in a 250L cylinder that would be 175L that needs to be hot and usable.

In reverse then, this document suggests that some cylinders with coils, could be missing out on as much as 75L of usable water? 250L – 175L = 75L.

Mixergy themselves claim that their cylinders gain 20% more usable hot water than a conventional cylinder. 

So it could be either of:

• 20% of 250L  (50L extra)
• 20% more than a usable 175L is 210L (35L extra)

Either way, that’s a lot more usable hot water being claimed.

But there are some heat pump cylinders like the Heat Geek Super Cylinder by Newark mentioned earlier, that are putting the coil right at the bottom of the cylinder to counter the “cold at the bottom” problem.

You can see this in their technical diagram.

As many of the better conventional cylinders out there are now putting their coils close to the bottom, perhaps that diagram above from Mixergy (and in their article) with the coil bang in the middle of the cylinder is a little disingenuous these days?

What I’d really love to see, is some fellow geek with a conventional cylinder do a full draw down test.

Heat the whole of a 250L cylinder to 45C and draw off the hot water until it drops below 40C whilst measuring the exact number of litres used.

Just so we can see some real world facts and figures.

Having had our Mixergy since Feb 2022, we’ve anecdotally seen the capacity reach 5% and sometimes lower in the app, but never worried about a lack of hot water.

The purpose of this article was to see whether I do indeed get all 250L usable hot water with no drop off in temperature.  Testing out those advertised claims with real world data.

Let’s crack on.

Cold and Empty

Our story and investigations need to start with a cold and cylinder.  

This hot water run took my 250L Mixergy from empty (0%) up to full (100%) in one reheat, targeting a whole cylinder temperature of 45C.

Note: this complete reheat was done just a few hours after the “empty” test we do later in the article.

Direct link to the whole run is here:

https://emoncms.org/energystatsuk?mode=power&start=1751543670&end=1751549060

As you can see from this Open Energy Monitor trace, the return temperature started at 20.4C, finishing at 48C with 7.925 kWh of heat output required to complete the run.

The 20C starting point is around the temperature of the incoming mains for us in July.

Note: the return temperature needs to go upto 48C to heat all the water in the Mixergy cylinder to 45C because there are a few degrees of temperature loss across the plate heat exchanger that is bolted to the outside of the cylinder.

This reheat was on my 5kW Arotherm in Sweet Spot Mode.  If you want to read more about Sweet Spot it’s detailed in the following article.

Vaillant Arotherm Sweet Spot Hot Water Mode

This alternative graph also shows that same reheat as above but with different data points

• Mixergy Capacity (0% to 100%) in Yellow
• Mixergy Hot Water Temperature (this is internal to the cylinder) in Blue

You can see that we start in the bottom left at 0% capacity and 22C hot water temperature.

And by the top right we see 100% capacity and 45C hot water temperature being recorded.

Next, let’s head over to my favourite water heating calculator and input what we’ve just seen and corroborate the story.

https://gettopics.com/en/calc/water-heating-calculators

So we can see the maths suggests that 7.99 kWh of heat energy is required to heat 250L from 20C to 48C just as we showed in the Open Energy Monitor trace.

For me, this is proof that we have indeed heated all 250L of water in my cylinder.  

Reminder, the Open Energy Monitor trace showed we used 7.925 kWh of energy, so as near as damn it.

So is all 250L in my Mixergy usable then?

This next graph adds another data point

• Mixergy Capacity (0% to 100%) in Yellow
• Mixergy Hot Temperature (this is internal to the cylinder) in Blue
• Hot Water Temp (inside the coal shed) in Red.

The “hot_water_coal_shed” reading is a ds18b20 temperature probe strapped to the hot water pipework before it splits off to the kitchen and up to the bathroom.

It’s not a coal shed anymore, it’s an accessible hatch at the side of the house where some of the primary pipework lives.

So this “coal shed” reading is as close to “at the taps” as we are going to get.

Firstly on the graph, you can see the overnight (1AM) re-heat from around 40% capacity (in yellow and on the right hand scale) to 100% full.

Then at around 6:45AM you can see the start of the family morning showers with the red spikes being the water coming into the house (coal shed probe), with Mixergy capacity dropping accordingly (in Yellow).

Note: both mixergy_hot_water and hot_water_coal_shed readings are via the left hand scale.

You can also see a long red spike around 10AM when I start to fully drain down the whole cylinder.  You can see the “Mixergy Capacity” drop again as I do this.

What is curious is that straight after the 1AM reheat until the very first hot water draw off, around 6:45AM, the internal recorded hot water temperature within the cylinder stays over 45C (target temperature).

But as soon as our morning showers start, the internal hot water temperature reading drops to around 42.5C and the red spikes at the outlet are recorded around 41.5C.

The internally recorded temperature then continues to drop and settles at around 42C.

42C is still more than hot enough for showers, but it’s not quite the 45C target I chose.

Now let’s zoom into the section where I start to fully empty the cylinder.

Do we still get fully usable hot water even down at 1% capacity?

At around 9:55AM I fully cranked open the hot water tap and started the process of draining off the remaining hot water. The Mixergy capacity was showing 54% at this point.

As you can see, the hot water temperatures (both internal and at the tap) stay rock solid at 42C and 41.5C respectively for a good 25 minutes.

When the Mixergy reached around 10% capacity remaining I closed down the hot water tap a little bit, so I could elongate the whole drawdown process and record more data points as capacity fell towards 0%, and eventually an empty cylinder.

As you can’t interactively play with the graph above, here are the data points in table format.

So as you can see, from 42% capacity right down to around 10%, we get the same 42C / 41.5C temperatures recorded internally and at the tap.

Only then, from around 9% capacity do we start to see a drop off, albeit a very small one, just 0.1C.

At 10:26 the capacity of the cylinder is down to just 2%, yet we are still getting 41.6C / 40.4C out of it.

This article from Kesselmann Plumbers suggests the ideal showering temperature is between 37C and 40C.

https://kesselmann.co.uk/guides/plumbing/what-shower-temperature-is-best

So even down to 1% capacity at 10:33 we are still seeing usable shower water (40.2C / 39.8C).

At 10:36 the Mixergy reports we are at 0% capacity yet is still delivering 36.3C to the taps.

At 10:40 I closed the tap off as we finally got below 30C water temperature both internally and recorded at the tap.

Summary and Conclusions

So I think the answer to the question “So is all 250L in my Mixergy usable?”, is a pretty resounding yes.

We proved my heat pump indeed heated 250L of water to 45C (0% to 100%)

And the data showed us that as we drew off water, that even at 1% capacity we were still getting almost 40C water at the taps.

There was the slight annoyance that despite choosing a 45C target temperature that you only really got 42C-43C out of the cylinder and to the taps.

But on the flip side, if the claims of gaining 20% usable water with a Mixergy over a conventional cylinder are correct, then I could be gaining anywhere between 35L to 50L more usable hot water?

So I’m happy with that trade off. Especially as a family of four (with two teenagers) that go through a lot of water.

Maybe so not “suboptimal” after all?

Obviously I have no way of corroborating the claims of exactly how many extra litres are really gained, other than pointing you to the Hot Water Association specification document that demands at least 70% of the badge capacity in a conventional cylinder should be usable.  You’d hope that many cylinders with their coil now placed lower down, like the Heat Geek Super Cylinder would do way better than the minimum demanded by the HWA.

So as I asked earlier in the article, if you are a nerd with a 250L cylinder and you want to undertake a drawdown experiment, I’d be happy to tag the results onto an update of this investigation.

Or please point me in the direction of somewhere that has already done it.

Big shout out as always to Open Energy Monitor for their amazing monitoring package.  I couldn’t have created this article without it.

And also kudos to Mixergy for recording and holding all datapoints as well as exposing this information via an API so we can run Tomas McGuiness tremendous Home Assistant integration.

Available at: https://github.com/tomasmcguinness/homeassistant-mixergy

Hope you’ve found this article useful.

Below are some other recent articles you may be interested in

The Heat Pump Hot Water Golden Rules

Third Winter with our Vaillant Heat Pump

The Problem with Eco Hot Water Mode

Whole House Running Costs and Stats 2024

Hot Water Cylinder Sizing and Mixing

Mixergy Heat Pump Performance