Vaillant Arotherm Sweet Spot Hot Water Mode

Vaillant Arotherm Sweet Spot Hot Water Mode

Can Sweet Spot hot water mode be an effective alternative to Eco, Normal and Balance modes on the Vaillant Arotherm Heat Pump?

The Vaillant Arotherm has three hot water modes;

• Eco
• Normal
• Balance

And here is a short summary

• Eco is slow but very efficient
• Normal is fast but very inefficient
• Balance is a combination of the two modes.

You can read about the three different modes in more detail here

Arotherm Plus Hot Water Modes

Throughout the whole of 2024 I ran in Eco mode on my 5kW Arotherm and 250L Mixergy cylinder.

You can read a deep dive about all my 2024 hot water runs here

A review of 700 heat pump hot water runs in 2024

The problem with having a large cylinder and small output heat pump like a 5kW model is that Eco mode, with its restricted kW heat output can take a long time.

An Eco hot water run for me with my 5 kW heat pump and 250L cylinder combination can take anywhere from 60 minutes to almost 3 hours depending on how empty the cylinder is and how cold it is outside. 

This can lead to the house temperature dropping a lot whilst the hot water is being reheated, especially in winter.  Remember, just like a gas boiler, when the heat pump is doing hot water it can’t heat the house at the same time.

I went into detail how and why this happens and can be a problem in this article

Why is Eco mode more efficient than Normal mode?

Balance hot water mode is a combination of Normal mode followed by Eco mode, which results in faster reheats.  But it’s not necessarily that much more efficient.

I’ve always suggested that Balance mode is the “wrong way around” because I think Eco mode followed by Normal mode would be a much more sensible combination.

This is because it’s in Eco mode that you can slurp up all the juicy COP available early in the reheat process when flow temperatures are colder.

If we look here at the first 30 minutes of an Eco run at 5C outside (10th Dec 24) you can see how much efficiency we pull when the water is at its coldest.

The blue line from this Open Energy Monitor snapshot shows an Instantaneous COP of 6.2 where I hover the cursor.

https://emoncms.org/energystatsuk?mode=power&start=1733827790&end=1733830210&cop=1

But compare that to the first 30 minutes of this Normal run, also at 5C outside. (14 Jan 25)

https://emoncms.org/energystatsuk?mode=power&start=1736816120&end=1736818290&cop=1

It seems to me that because the compressor is working flat out in Normal mode there isn’t the opportunity to extract the benefit from the cold water and garner the efficiency.

As you get later into the hot water run, the benefits of Eco are still better than Normal, but the gap between the two becomes narrower.

Eco than Normal rather then Normal than Eco would make for a much better version of Balance.

Feature request to Vaillant:

Change behaviour of Balance Mode: Where it’s Eco mode first followed by Balance.   

Plus, you’d also get to choose the return temperature when the switch of modes occurs.

In the existing Balance mode you have to wait until the return temperature hits 45C before the switch happens.  Which can be far too late if you’re only heating to sub 50C target temperatures.

Final little request for this, you get to choose the compressor speed too.  At the moment Eco is fixed at 50 rps on the 5kW model and 60 rps on the larger units.  It would be great to be able to tinker and raise that.

Eco mode is 50 rps, Normal mode is 120 rps, what about in-between?

The usable range of the compressor on a 5kW Arotherm is between 30 rps and 120 rps.

We talked in great detail in the Top of the SCOPS article about where to get all the kW output and COP data at various outside temperatures and compressor speeds.

We are talking about the Czech tables that can be found in the following article.

Smaller Heat Pumps will never reach Top of the SCOPS

Let’s take a look at some COP data in more detail to show why Normal is so bad and why Eco is so good.

I created these graphs by transposing data from the Czech Tables into Excel.  I’ve taken the key COP data points from the Czech Tables at 35C, 45C, 55C and 65C flow temperatures for each compressor speed and created a graph for each outside temperature listed within the data.

As the data sheets only provide 4 flow temperatures I used some Excel magic to infer the missing flow temperatures.  For example,I used 35C and 45C data points to create 36C, 37C, 38C up to 44C etc.

Not perfect, but makes for nice graphs.

As we look through all the graphs you can see why Normal mode (120 rps) is so inefficient as it constantly props up the bottom of the COP graphs.

Whilst 50 rps (Eco mode) is primarily at the top.

The eagle eyed amongst you will have seen that 80 rps appears closer to the top of the COP graphs than many other compressor speeds.  And on occasions as the flow temperature rises, it even beats 50 rps (Eco).

Could 80 rps be the Sweet Spot we are looking for?

• Eco (50 rps) is slow but efficient
• Normal (120 rps) is fast but inefficient
• Could Sweet Spot (80 rps) be a good compromise?

Let’s look at the kW output of these three compressor speeds, taking snapshots at two extremes; -2C outside and 15C outside.

You can see the issue with Eco mode (50 rps), especially when it’s -2C outside. The average low output is only around 2.5kW (dotted green line).

Although it does pick up between 3.5kW to over 4kW when 15C outside (solid green line).

So it’s full working range is 1.9 kW to 4.2 kW

Normal mode (120 rps) has a working range of between 6.1kW and 8kW, but as we’ve seen from the COP tables, that comes at the cost of efficiency. The red lines in the chart.

Sweet Spot (80 rps) has a working range between 3.8kW and 6.7kW, sitting somewhere between Eco and Normal. The blue lines.

Summary of output ranges

• Eco Mode: 1.9 kW to 4.2 kW
• Sweetspot: 3.8 kW to 6.7 kW
• Normal: 6.1 kW to 8.0 kW

Note: I have focused on the 5kW model here because my unit has these slow Eco mode problems.  The data from the Czech tables suggests similar ratios when comparing compressor outputs on all models in the range; 5 kW, 7 kW, 10 kW and 12 kW.  

We can make a simple summary table for all the main models here showing average kW output for each mode.

Note: these are just average/ballpark figures for each model.  The actual available range very much depends on outside temperature and current flow temperature as seen in the previous summary for my 5kW model.

You can use the Czech Tables to look at the output of each model in more detail.

But these summaries do allow us to use the kW output figures against this handy chart.

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

So you can see why there’s not much to worry about with a 10kW or 12kW Arotherm running Eco mode as they can still kick out 9kW.

How can we access Sweet Spot hot water mode?

So the trick to accessing Sweet Spot hot water mode is to use the Noise Reduction feature of the Arotherm.

Firstly you’ll need to go to your VWZ AI heat pump controller and choose the level of noise reduction that you require.

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.

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

With great power comes great responsibility.

The Compr.noise.reduct. setting can be set between 30% and 60%

On a 5kW model, here is a table showing the compressor speed for some reduction examples in 5% increments.

So if we want to hit 80 rps as per the Czech Tables we’d select 33% reduction.

The next stage is to go onto your Sensocomfort controller and activate a schedule for the noise reduction to kick in.

So really, this Sweet Spot technique can only work if you have fixed times when you do your hot water runs.

For example, I do our hot water runs at 1AM and 1PM each and every day.

Get to the Noise Reduction settings here on the Sensocomfort

• Settings >
• Installer Level >
• 00 >
• Noise reduction mode

Once in there we are going to schedule the noise reduction to activate for one hour to match when we have your hot water scheduled.

Of course you could just activate noise reduction mode for the whole 24 hours so you grab Sweet Spot whenever you do your hot water runs.  But I did find this to be a problem whilst experimenting when it was sub zero outside.  My heat pump needed to go beyond 80 rps at times whilst doing space heating in cold weather, so the heat pump struggled to maintain internal temperature because of the compressor restriction.  

Just a word of caution there if you take this approach.  You can set Noise Reduction 24/7, but it is not advisable in most cases because your ASHP might then not be able to deliver enough heat when temperatures drop.

Once you’ve set your schedule I would then advise copying this schedule to all other days of the week using the “copy settings to” option.

It’s a bit laborious as you have to choose each target day individually.

The final change to make is to set your hot water mode back to Normal mode (on the VWZ AI heat pump controller) .  If you leave hot water in Eco mode, this Sweet Spot technique will not work as Eco (50 rps) will override what we are trying to do.

How will Sweet Spot Hot Water mode work?

Having chosen your noise reduction percentage (use 33% to get 80 rps) and scheduling an hour when this reduction in compressor output will activate at the same time as your hot water run, you are effectively pegging your hot water runs at 80 rps for one hour.

If your hot water run goes longer than an hour the heat pump will drop into Normal mode (120 rps) for the remainder of the run.

If the hot water run finishes in less than an hour then any heating that comes on after will be limited to a maximum of 80 rps until the end of the noise reduction hour.

What does Sweet Spot mode look like in action?

This run here is with a 33% noise reduction schedule in place for 1 hour, so the heat pump is doing this hot water run on my 5kW Arotherm at 80 rps.

On this particular run my 250L Mixergy cylinder started at 37% full, so we had around 150L to heat to a target temperature of 45C.

https://emoncms.org/energystatsuk?mode=power&start=1737939120&end=1737943820&cop=1

This run took 60 minutes to complete with a COP of 3.26 at an average heat output of 4.8kW

Let’s look at an Eco mode run for comparison

https://emoncms.org/energystatsuk?mode=power&start=1733788410&end=1733795250&cop=1

With this run we started from 38% full, so almost the same amount as the Eco run and the outside temperature was almost the same too, around 5.2C

So this makes for a great comparison.

You can see via the instaCOP blue line on the graph how the COP is slightly better on this run.  But the run times are longer.

This Eco run took 1 hour and 35 minutes to complete with a COP of 3.55 at an average heat output of 3.27kW

Going back through my emoncms history I was also able to find a Normal run when 5.5C outside starting at 30% full, so similar.

• Normal: 50 mins, 2.85 COP, avg 6.9kW output
• Sweet Spot: 60 mins, 3.26 COP, avg 4.8kW output
• Eco: 95 mins, 3.55 COP, avg 3.3kW output

I’ve been running the Sweet Spot tests for about a month now and this is about the average level of speed / performance ratio difference I’ve seen between Sweetspot and Eco.

Sweet Spot is around a third faster than Eco, with a COP reduction of about 0.3 to 0.5 per run, so your heating is off for less time in Sweet Spot.

So a reasonable middle ground compared to the COP losses seen in Normal mode, especially as the speed gains (Normal versus Sweet Spot) seem negligible?

What about really long runs?

When you schedule noise reduction mode for only an hour, that first hour of your hot water run is done at your chosen compressor reduction value (say 33% for 80 rps).

When the run goes past an hour in length then the remainder of the run drops into Normal mode (120 rps) until completion.  This is why we need to set the default hot water mode back to Normal on the VWZ AI controller.

Depending on the size of your cylinder and size of your heat pump you may never see a hot water run an hour long.

For example, with a 12kW heat pump and 150L cylinder this is highly unlikely.

But like me with a 250L cylinder and only a 5kW heat pump this happens quite often, especially if we’ve drained the cylinder leaving it quite empty.

Let’s see what that looks like.

This run was from 15% full, so around 210L of water to re-heat when it was 5.2C outside.

https://emoncms.org/energystatsuk?mode=power&start=1738111780&end=1738118390&cop=1

As we’d only got 1 hour of noise reduction (33%, so 80 rps) and the whole run took 1 hour and 20 minutes you can see the final portion dropped into Normal mode (120 rps).

In this Eco mode run, again from around 17% full and 5.3C outside, we can see it took 2 hours and 10 mins.

https://emoncms.org/energystatsuk?mode=power&start=1733827580&end=1733837020&cop=1

• Sweet Spot: 1 hour 20 mins, 3.30 COP, avg 5.2 kW output
• Eco: 2 hour 10 mins, 3.85 COP, avg 3.3 kW output

Despite there being an element of “Normal” at the end of the Sweet Spot run, the overall COP was down around 0.5. 

But the loss in COP came because the Sweet Spot run took almost an hour less than Eco.

That’s the compromise and benefit.

In the case of this run dropping into Normal at the end, perhaps I could experiment further by increasing my Noise Reduction period to be slightly more than an hour?  Maybe 70 minutes, 80 minutes or even 90 minutes?  So that most of even my longest runs are done at 80 rps?

Reverse Balance Mode

Remember at the beginning of the article where I said I thought that Balance mode was the wrong way around?

Well using this method of manipulating Noise Reduction schedules we can create our own reverse balance mode.

Here is a hot water run where I scheduled only 30 minutes of Noise Reduction mode (at 33% reduction for 80 rps).

https://emoncms.org/energystatsuk?mode=power&start=1737636580&end=1737642010&cop=1

Because the hot water run lasted more than 30 minutes, the rest of the run completed in Normal mode (120 rps).

So rather than us creating Reverse Balance (Eco then Normal), we’ve created Sweet Spot then Normal.

Let’s call this Sweet Spot Reverse Balance hot water mode (name patent pending, LOL).

You can see we get good COP for the first half hour, then as the heat output rises (yellow) the COP drops.  But with higher output we do get quicker re-heat.  As always, it’s a compromise.

So depending on your own requirements you can alter the length of the Noise Reduction period and level of compressor reduction to suit your household needs.

For example, you could create a true Reverse Balance by having the first 30 minutes with a 60% noise reduction, giving 50 rps (same as Eco) and then let the reminder be in Normal (120 rps).

I’ve settled on 90 minutes of Noise Reduction

So back to my original “5kW heat pump, Eco mode and 250L cylinder” problem with mega long runs leaving the house heating off for hours.

For info, some of my real lengthy Eco runs through 2024 got close to 3 hours long.  Especially at sub zero outside when defrost cycles interrupt the re-heat.

The colder outside temperatures also heavily affect the output of the heat pump as we saw from the various output charts early in the article.

This Eco mode run (50 rps) was 2 hours and 30 mins long as we started at only 6% full and it was 3.5C outside.  So we had around 225 litres of water to heat.

https://emoncms.org/energystatsuk?mode=power&start=1731630040&end=1731640650&cop=1

Let’s compare a Sweet Spot run with the noise reduction set to the first 90 minutes of the run, so 80 rps.

This one starts at just 5% full too and the outside temperature is the same as the Eco run, 3.5C.

The Sweet Spot run completed in 1 hour and 40 minutes.  So a good 50 mins quicker than the same Eco run.  Meaning the heating came back on almost an hour earlier than in Eco.

COP was 3.25 on this Sweet Spot re-heat compared to 3.79 on the longer Eco run.

https://emoncms.org/energystatsuk?mode=power&start=1739537710&end=1739544960&cop=1

• Sweet Spot: 1 hour 40 mins, 3.25 COP, avg 4.9 kW output
• Eco: 2 hour 30 mins, 3.79 COP, avg 3.3 kW output

You can see in those final 10 minutes of the Sweet Spot run that the Noise Reduction period ended so the re-heat dropped back into Normal mode to finish off.

Having played with all sort of Noise Reduction and Normal combinations across a whole month of testing, I think this is where I’m going to leave things for now.

90 mins of Noise Reduction with 33% reduction for 80 rps.

It seems the best balance of speed versus performance for my set-up of a 250L cylinder and a 5kW Arotherm.

Round up and Shout Outs

In reality, if you have a 10kW or 12kW Arotherm you’ll likely be fine running Eco mode all the time as those models can still kick out 9kW in Eco.

Sweet Spot is probably more for owners of 3.5kW, 5kW or 7kW heat pumps, especially those that also have larger cylinders that want faster re-heats, are willing to forgo some efficiency but don’t want to drop to Normal mode.

If you have a 5kW and you don’t mind really long runs then stick with Eco as it’s one of the golden rules of efficient hot water generation.

• Reheat from empty cylinder / lowest temperatures (high hysteresis) – no quick top-ups
• Choose lowest target hot water temperature you can get away with
• Use Eco mode (or some other low compressor / quiet mode)
• Schedule reheats when highest outside temperature

Whichever way you decide to use Sweet Spot you just have to remember that it’s a compromise.

• Sweet Spot will never be as efficient as Eco, but it will always be faster.
• Sweet Spot will never be as quick as Normal, but it will be more efficient.

Sweet Spot (noun)

“a point, range, or particular set of conditions that will achieve the most desirable or effective outcome”

Have fun playing.  

I’d love to hear what timing, schedule and compressor reduction combination you find works best for you.

A massive thank you has to go to @FinchampsteadPH on Twitter / X for planting the seed a long time ago about noise reduction hot water runs and also helping me proof read the article.

Him and I have had plenty of back and forth on the subject, which ultimately led to this article.  I know he’s been running 80 rps runs on his 7kW unit for a while now with great success.

The Mixergy.io platform has been a great help creating the data, being able to track starting state of charge etc.  That is a really unappreciated feature by many of the Mixergy.

Finally, big thanks to Open Energy Monitor and their amazing Heatpumpmonitor.org platform.

https://heatpumpmonitor.org/

We would have never been able to simulate, view and revise “Sweet Spot” without the OEM heat pump monitoring platform.

You can view my Open Energy Monitor heat pump data going back to November 2023.

https://emoncms.org/energystatsuk