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Arotherm Plus DHW Modes

Over the past few days there’s been some back and forth on Twitter / X trying to explain the Arotherm Plus DHW Modes (DHW being Direct Hot Water).

But as we’ve found out, there’s just not enough characters to fully explain things properly.

So here is a summary of the available options

  • Normal
  • Balance
  • Eco

You can choose which mode you want to run in via the VWZ AI Heat Pump Controller via the Installer Menu.

Once into the Installer section, look for DHW Mode

You can find full instructions for the VWZ AI via the Vaillant Website.

The default code to get into the installer menu is 17.  But be very careful, the options behind this menu should be for installers only.

If you don’t know what you are doing, stay out of here.

DISCLAIMER: I accept no responsibility for any issues you may encounter by poking about with settings.

With great power comes great responsibility.

My Heat Pump Overview

Remember, you can look at the full details of my 5kW Arotherm Plus here:

My 5kW Vaillant Arotherm Heat Pump

And how it performed across the first winter in this article:

First winter with my air source heat pump

My cylinder is a 250L Mixergy model which you can read about in the dedicated plant room and Mixergy article here:

Plant Room Project

If you want to look at day to day stats and performance of my heat pump you can grab that here via my own private Open Energy Monitor link:

My heat pump is part of an ever growing community of heat pumps being monitored and tracked by the initiative.

Summary of Modes

  • Normal: Max. compressor speed 120 rps possible.
  • Eco: The max. compressor output is reduced to 50 rps (S+M)/40 rps (L). The speed limit is lifted at air inlet temperatures below -7 °C.
  • Balance: If return temperature in the cylinder charging circuit, is equal to or below 45 °C, the full max. compressor output (Normal) is enabled, while at temperatures above that, the reduced max. compressor output is enabled (Normal).

In summary

  • Normal: no limit on compressor
  • Eco: limits max compressor to 50%
  • Balanced: limits max compressor to 50% for finish-heating

Note: Balance is just a combination of Normal and Eco.

I will post up some graphs below from my Open Energy Monitor full heat meter package to explain the different modes.

As the saying goes, a picture paints a thousand words.

Normal Mode

This is my preferred mode, just run the heat pump at full beans for the entirety of the hot water run.

Here is the data from my OpenEnergyMonitor full monitoring rig during a ‘Normal’ run.

26% full to 100% full (so around 185L of my 250L Mixergy cylinder), target 51C at 195.C outside temperature.

Which took around 60 minutes to run, peaking at over 7kW heat output (for max 2.5kW electrical input).

ECO Mode

On a fresh installation this is the default option.

As the summary at the beginning of the article describes, Eco reduces the compressor to either 40% or 50% depending on the heat pump model size.

I’m assuming S+M means Small and Medium (3.5kW, 5kW and 7kw) and Large (10kW and 12kW) .

And Eco does exactly what it says on the tin.  It keeps electric input low and as such it keeps heat output low too.

Take a look at this example from my 5kW Arotherm Plus.

This run is around 140L of my 250L cylinder heated to 51C at 17C outside.

This run took around 80 minutes because the heat ouput was capped at around 4kW (compared to 7kW of a ‘normal’ run).  But electrical input only reached a maximum of 1.35kW (compared to 2.5kW of ‘normal’).

I see this option perfect if you have solar PV and you want to try and you want the heat pump to only try and use solar.

But this comes at the expensive of running much longer.  Would you want the heating off for almost an hour and a half in the depths of winter?

Remember, just like a combi boiler, a heat pump can only do heating or hot water at any one time.

Balance Mode

Balance mode is a combination of Normal and Eco modes.

The hot water run will start off in Normal mode running full compressor and continue until it gets to around 45C flow temperature.  Once past there it drops the compressor speed and falls into Eco mode for the remaining amount of the run.

Obviously, this could either be a short time if your target is only just past 45C or a long time if you’re looking for a very hot run.

Here’s a snapshot of a Balance mode run on my system.

205L of my 250L Mixergy to 51C target temp at 18C outside.

You can see the distinctive drop off halfway through when it falls out of Normal mode and into Eco.


I’ve played with all these modes across the first year of owning my Arotherm and poured over the graphs.

On my Mixergy setup I have seen very little efficiency to be gained between the different Modes.

Note: Don’t read too much into the single runs shown in the article.  Many different things contribute to good or bad COP in hot water runs.  Outside temperature, incoming cold water temperature and how empty the cylinder is when the run starts all make a difference.

So I’ve not found any great advantage going ‘Eco’. And it has the big downside of making hot water runs way longer.  That is perhaps fine in the summer or when you only have a small hot water demand.

But in my house, with a family of four, I need the hot water topped up and raring to go.  I especially want fast hot water runs in the winter as I want the heating off as little as possible.

Now i’ve only got a 5kW Arotherm, so logic dictates I can only get 7kW out of that on hot water runs, hence the longer recharge times.  If you had a 12kW model and you could get 15kW out of heat output, then the story could be different.

I also have a 250L cylinder, so that’s a lot of water to reheat.  If you had 120L you’d only need half the time.

I will repost below the recharge times table I created and used in other articles as I think it is useful for this article too.

Time in minutes to heat water using various power output sizes (10C to 50C)

3kW 5kW 7kW 10kW 12kW 14kW 16kW
50L 47 28 20 14 12 10 9
100L 94 56 40 28 24 20 18
150L 140 84 60 42 36 30 27
200L 187 112 80 56 48 40 36
250L 233 140 100 70 60 50 45
300L 280 168 120 84 72 60 54

So for me, I’m sticking with ‘Normal’ mode and full beans hot water.

Your circumstances, heat pump size and cylinder capacity may be different, so you may choose a different approach.

But hopefully this article has helped you understand the main differences between the three Arotherm Plus DHW Modes.

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