Arotherm Plus Hot Water Modes

Arotherm Plus Hot Water Modes

A common question is what are the DHW modes on the Arotherm Plus (DHW being Domestic Hot Water) and what difference does each one make.

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.

You may also find this article interesting that explores a way of getting a halfway house between Eco mode and Normal mode.

What I call Sweet Spot hot water mode.

Vaillant Arotherm Sweet Spot Hot Water Mode

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 second winter in this article:

Second Winter With Our Vaillant ASHP

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:

https://emoncms.org/energystatsuk

My heat pump is part of an ever growing community of heat pumps being monitored and tracked by the https://heatpumpmonitor.org/ initiative.

Summary of Modes

• Normal: Max. compressor speed 120 rps possible.
• Eco: The max. compressor output is reduced to 50 rps (3.5kW, 5kW & 7kW)/40 rps (10kW & 12kW). 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 around 50%
• Balance: no limit on compressor until return temp hits 45C, then 50% compressor limit until finished

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

In normal mode, the heat pump runs 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.

29% full to 100% full (so around 180L of my 250L Mixergy cylinder), target 48C at 3C outside temperature.

Which took around 60 minutes to run, peaking at 7kW-8kW heat output (for max 2.5kW electrical input).

Normal mode is not very efficient, but it is fast!

ECO Mode

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

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 190L of my 250L cylinder (24% to 100%) heated to 48C at 3.6C outside.

This run took around 130 minutes because the heat output was capped at around 3kW (compared to 7kW of a ‘normal’ run).  But electrical input only reached only just above of 1kW output (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.

Can your house cope with the heating off for almost two hours 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.

Eco mode is certainly the most efficient way to heat the water, but it can come with issues, especially on smaller heat pumps.

The Problem with Eco Hot Water Mode

You might be interested in this year long analysis where I ran Eco mode throughout the whole of 2024.

A review of 700 heat pump hot water runs in 2024

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 return 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.

In my opinion, Balance is the wrong way round.  Eco first and then Normal would be a better way, so you could soak up all the good efficiency early in the heating cycle.

How to extend run times past 2 hours

When you drop into Eco mode for hot water you reduce how hard the compressor needs to work.  This reduces both the electricity in and the heat output to lower levels than Normal mode.

But doing this means your hot water run times are going to be longer.

On smaller models, you might even hit the default 120 minute limit.  

To combat this, you can set the “Max cyl charge time” setting to off

This should allow runs to extend past 2 hours.  

This option, along with “Cyl. charge. anti-cycl time” are on your indoor controller (installer menu); domestic hot water menu

• Max cyl charge time : off
• Cyl. charge. anti-cycl time: 0 min

The “Cyl. charge. anti-cycl time” setting controls how long the heat pump waits to start the hot water run again after pausing following “Max cyl charge time” (say you had it at 120 mins).

You could put “Cyl. charge. anti-cycl time” at 30 mins to allow the heating to come on for 30 mins before hot water tried to complete.

The obvious downside of having longer hot water run times is that you house will not be heating during this time.  Your heat pump can only do one job at a time.

So if you have a house with a high heat loss you may not want to have the heating off for such long periods.

Conclusion

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

If you want performance then choose Eco mode.  The difference is quite telling.

If you compare the COP on the Eco versus Normal runs you can see how much more efficient Eco is.  Eco gives you more heat output for less electrical input.  So better COP.

You can also see via the blue line on the graph how the COP plays out.  This is a feature on the Open Energy Monitor heatpump app called “Instantaneous COP” that is amazingly useful.

On Eco you get really good early COP, as high as 6, in the early part of the run that gradually tapers down.

But on Normal, COP is poor from the start, like the heat pump just isn’t efficient working its nuts off like that.

It is a quandary though because if you go Eco, it’s going to take twice as long.

As I’ve only got a 5kW Arotherm, logic dictates I can only get a maximum of 7kW out of that on Normal hot water runs (3kW on Eco), hence the longer recharge times.  If you had a 12kW model and you could get 15kW out of heat and Normal and 10kW on Eco, 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)

During the second full year with the Arotherm I’ve stuck with Eco and my performance (COP) numbers have gone through the roof.  I’m willing to take the heating being off for a little bit longer than runs on Normal.

You can look a full second year review here:

Whole House Running Costs and Stats 2024

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 and the performance / timing advantages and disadvantages.