Vaillant aroTHERM Hot Water Modes Performance Compared (Eco vs Sweet Spot)

Vaillant aroTHERM Hot Water Modes Performance

Eco vs Sweet Spot Using Two Years of Real Data

Using over 1,300 real hot water runs across two years, I compared two Vaillant aroTHERM hot water modes, Eco and Sweet Spot, to see which delivers the best combination of efficiency, recovery time and running costs.

One advantage of running a heat pump for several years is that you accumulate a lot of data. This article is an attempt to turn that data into something useful.

This article builds on earlier analysis of over 700 hot water runs from 2024 and extends the dataset to two full years of real world operation.

And is part of my wider series of Vaillant Arotherm guides, aiming to demystify the settings that matter most for efficiency and comfort.

Key Results (TL;DR)

From over 1,300 real hot water runs across two full years:

• Eco mode delivered the highest COP

• Sweet Spot mode reheated the cylinder significantly faster

• The cost difference between the two modes is relatively small

• Electricity tariff timing often has a bigger impact on running cost than hot water COP

The rest of this article explains the data behind those conclusions.

Hot Water Mode Summary

One of the key findings from my earlier Best Heat Pump Hot Water Settings investigation was that electricity tariff timing can have a bigger impact on running costs than small COP differences.

Scheduling hot water reheats during the cheapest electricity periods is therefore one of the most effective ways to reduce running costs.

Normal mode is included in the summary for reference. Full performance data will be added once the 2026 ‘Normal mode’ dataset is complete.

Table of Contents

Use the Table of Contents below to jump straight to the section that matters most or scroll through at your own pace.

Best Vaillant aroTHERM Hot Water Mode

There isn’t a single “best” hot water mode for every system.

• Eco mode generally delivers the highest COP, but reheats the cylinder more slowly.
• Sweet Spot mode sits somewhere in the middle, offering faster recovery with only a small drop in efficiency.
• Normal mode reheats the cylinder fastest but usually delivers the lowest COP.

Which option works best depends on several factors:

• hot water demand

• cylinder size

• recovery time requirements

• electricity tariff timing

The rest of this article looks at the real data behind those trade-offs.

Hot Water Modes Explained (Eco, Normal, Balance and Sweet Spot)

Vaillant provides three official hot water modes:

• Normal

• Eco

• Balance

In addition to those, there is a fourth option I have created and named Sweet Spot mode, created using the Noise Reduction setting.

In simple terms the modes mainly differ in how much compressor output is allowed during hot water reheats.

• Normal allows the highest compressor speed and reheats the cylinder fastest.

• Eco limits compressor output to improve efficiency, which makes reheats slower but increases COP.

• Balance starts in Normal and later switches to Eco, effectively sitting between the two. Good in theory, rubbish in practice.

• Sweet Spot uses the Noise Reduction setting to limit the compressor slightly, creating a middle ground between Eco and Normal.

This article focuses on comparing Eco and Sweet Spot using real-world performance data.

If you want a deeper explanation of the modes and how to configure them, see this Vaillant Arotherm Hot Water Modes guide.

Or if you’re looking for help in setting up the chosen mode, look at Vaillant Arotherm Basic Hot Water Settings.

Better Hot Water Performance, Sizing and Mixing

There are five golden rules when it comes to generating hot water efficiently with a heat pump.

Four of them improve efficiency. The fifth focuses on cost.

Golden Rules of Efficient Heat Pump Hot Water

In practice these are the key principles:

• Reheat from as empty or cold a cylinder as possible

• Choose the lowest target hot water temperature you can get away with

• Use Eco mode or another compressor limiting mode

• Schedule reheats when outside temperatures are warmer

• Run reheats during the cheapest period of a time of use tariff

In practice this final rule can be the most important.

Running hot water during a cheap overnight tariff window will often save more money than chasing small COP improvements.

I cover this in more detail in the Best Heat Pump Hot Water Settings guide.

This Cylinder Sizing and Mixing article also explains why higher cylinder temperatures can sometimes increase usable hot water due to mixing with cold water.

In practice there is always a balance between:

• efficiency

• recovery time

• electricity cost

• comfort

With that in mind, let’s look at the real data.

Real Heat Pump Hot Water Data and Test Setup

This analysis uses two full years of operational data from my Vaillant aroTHERM heat pump.

The hot water schedule has been consistent across both years.

We run two hot water reheats per day:

• 1am overnight

• 1pm during the day

We are a family of four including two teenagers, so a single cylinder reheat simply does not meet daily demand.

Running two reheats ensures sufficient hot water while still allowing each cycle to operate efficiently.

The data has been pulled directly from my Open Energy Monitor feed.

Cylinder Temperature Strategy

During 2024 I experimented slightly with cylinder target temperatures.

January to April
48°C

May to December
45°C

By May it became clear we were finishing the day with excess hot water at 48°C, so the target was reduced to 45°C.

Across 2025 all runs were set to 45°C.

Sweet Spot mode was introduced in February 2025, so January still ran in Eco mode before switching.

Dataset Overview

This article will be updated as additional years of data become available.  I plan to add a year of Normal mode data after the end of 2026.

System Setup

For context, the system used for this analysis is:

• Heat Pump: Vaillant aroTHERM Plus 5kW

• Hot Water Cylinder: 250L Mixergy smart cylinder

• Household: Family of four (two teenagers)

• Hot Water Schedule:

  • 1am reheat

  • 1pm reheat

• Cylinder Target Temperature:

  • 48°C Jan–Apr 2024

  • 45°C May 2024 onwards

Whilst I try to stick to the ‘golden rules’ and reheat from as empty as cylinder as possible, the reheats starts at whatever state they find at 1am and 1pm.  If there’s been 4 showers the cylinder will be pretty empty.  If only one shower, not so empty.  You will see this shown in the graph later.

My heat pump system has been running since October 2022, providing a large dataset of real operational performance.

Monitoring is via the combination of a Sontex heat meter, SDM electricity meter and the Open Energy Monitor application which provide billing grade accuracy.

Headline Performance Result

The first step is to compare overall yearly performance.

This provides a high level comparison of COP before we dig deeper.

COP vs Outside Temperature

To explore how outdoor temperature affects performance, I grouped both datasets into 1 °C bands using the lower bound.

For example:

• 3 °C band = 3.0 to 3.9 °C

• 10 °C band = 10.0 to 10.9 °C

The table below shows the average COP for Sweet Spot and Eco in each temperature band, along with the number of runs in each group.

Observations

A few clear patterns appear when the data is grouped this way.

• Eco mode shows a higher COP in almost every temperature band

• The difference is typically around 0.25 to 0.50 COP

• The largest gains appear around 10 °C to 17 °C, which is typical shoulder-season weather

• The 24 °C band briefly reverses the pattern, but the sample sizes there are very small, so this is likely statistical noise rather than a real trend

Grouping the data by outdoor temperature therefore confirms a consistent pattern: Eco mode delivers slightly better efficiency across most operating conditions.

Across much of the useful operating range, Eco is roughly 0.2 to 0.5 COP higher than Sweet Spot.

All 1,300 hot water runs in one graph

A useful way to visualise performance is to plot each hot water run against outside temperature.

Here is a graph showing over 1,300 hot water runs across two years.

X axis
Outside temperature

Y axis
COP

Points
Individual hot water runs

Colours
Eco vs Sweet Spot

The trend lines highlight the overall behaviour of each mode.

This chart shows two key things:

• COP increases as outside temperatures rise

• Eco mode consistently sits slightly above Sweet Spot

Hardly ground breaking analysis, but nice to see it in graph format.

As mentioned earlier, you can see the variation in COP even at the same outside temperature in the same mode.  This will primarily be because of how empty (or full) the cylinder was at the start of the run.  There is a deep dive into how much difference that makes in the 700 runs in one year article.

And this graph will look even more interesting in early 2027 after I add a whole year of ‘Normal Mode’ hot water runs to it.  Although I bet you can imagine where the trend line will be already?

Why Eco Mode Takes Longer

The performance differences between Eco, Sweet Spot and Normal mode come down to one simple thing:

• compressor output limits

Each hot water mode chooses or restricts the maximum compressor speed, which directly limits how much heat the heat pump can deliver.

From the Vaillant performance tables we can see roughly how the output changes.

Typical Output Ranges (5kW aroTHERM)

So Eco mode is operating at roughly half the heating output of Normal mode, which explains why it takes much longer to reheat the cylinder.

Sweet Spot sits between the two.

I have a a big deep dive into the compressor limits of all the modes in the dedicated Sweet Spot Mode Article.

Typical Output by Heat Pump Size

Hot Water Reheat Time Comparison

To understand how these output differences affect hot water recovery, we can use a simple rule of thumb.

The table below shows the approximate time required to heat water from 10°C to 50°C depending on heating power.

Time (in minutes) to Heat Water

Eco vs Sweet Spot: Is the Efficiency Worth It?

Looking at two full years of data, the answer is fairly clear.

Eco mode is the most efficient way to heat hot water on a Vaillant aroTHERM heat pump.

Limiting the compressor output reduces the load on the system and improves COP. Over hundreds of runs this consistently shows up in the data.

However the difference is not dramatic.

Sweet Spot mode sacrifices a little efficiency but gives faster recovery times, which can make the system easier to live with, especially in a busy household.

In my case we run two reheats per day because a single cylinder reheat simply cannot keep up with a family of four that includes two teenagers.

So the real question becomes whether the extra efficiency of Eco mode is worth the slower reheats.

When you translate the electricity difference into pounds and pence, the gap is surprisingly small as we will show next. Even across an entire year the cost difference is typically tens of pounds rather than hundreds depending on electricity prices.

That leads to a useful rule of thumb; Tariff timing often matters more than hot water COP.

Running hot water during the cheap window of a time of use tariff will usually save more money than squeezing out the last few percent of efficiency from the heat pump.

So the practical takeaway is:

• Eco mode gives the best efficiency

• Sweet Spot mode gives faster reheats with only a small COP penalty

• Electricity tariff timing can matter more than either of those

For most households the best setup will depend on a combination of:

• hot water demand

• cylinder size

• recovery time requirements

• electricity tariff

In 2026 I have switched the system to Normal mode, which will give us a third full year of data.

Once that dataset is complete we will be able to compare Eco, Sweet Spot and Normal mode across multiple years of real world operation.

Cost Comparison

Here are the totals from the two years of measured data.

Eco mode clearly delivers the higher COP.

However the total heat produced in each year is slightly different, so to make a fair comparison we need to standardise the hot water demand.

Standardising the Heat Demand

For the comparison I’ve assumed a yearly hot water demand of 3200 kWh of heat.

That figure aligns closely with the real measured output in the dataset.

As a reference point, heating 250 litres of water from 10 °C to 45 °C requires around 10.2 kWh of heat.

If that amount of water was heated every single day (10.2 kWh x 365), the theoretical yearly demand would be:

3723 kWh of heat per year

The measured dataset is slightly lower than that.

That’s completely normal in real households because hot water demand varies. Some days use less hot water, there are holidays when the house is empty, and incoming mains water temperature is often higher than 10 °C.

So for this comparison we standardise on 3200 kWh of hot water heat per year.

Note: we are a high hot water usage household.  When I look on heatpumpmonitor.org our house is in the top 10 from around 100 systems that have separate hot water monitoring enabled.

So bear this in mind when look at these usage and cost metrics.  Your hot water usage could well be lower than ours.

Whole Year Electricity Requirement

Using the measured COP values we can estimate the electricity required to produce that heat.

Eco mode therefore saves roughly 91 kWh of electricity per year compared with Sweet Spot.

Annual Cost Comparison

The table below converts that electricity use into yearly running costs at different electricity prices.

Even at higher electricity prices the difference between the two modes remains relatively small.

At 30 p per kWh, Eco mode saves around £27 per year compared with Sweet Spot.

Daily Cost Comparison

To make the numbers easier to visualise, we can also look at the daily cost.

Assuming 250 litres of hot water heated from 10 °C to 45 °C (10.2 kWh of heat):

So the real-world difference between the two modes is only a few pence per day.

What This Means in Practice

Eco mode clearly delivers the highest efficiency and COP.

However the financial saving compared with Sweet Spot mode is relatively small. Across a full year the difference is only around £5 to £27 depending on electricity prices.  And this is for our high usage home.

That means the choice between Eco and Sweet Spot often comes down to reheat time and convenience rather than pure efficiency.

Sweet Spot mode reheats the cylinder faster, which may be preferable in homes with higher hot water demand.

I can certainly vouch for having the space heating off less, by using Sweet Spot over Eco.  Ultimately for us, home comfort wins.

Electricity Tariffs Often Matter More Than COP

One of the key findings from my earlier investigation into Best Heat Pump Hot Water Settings was that electricity tariff timing can have a much bigger impact on running costs than small COP differences.

Running hot water during a cheap off-peak window can easily save more money than chasing the highest possible COP.

For example:

• COP 2 at 10p electricity can be cheaper than

• COP 4 at 25p electricity

Which looks like:

• 10.2 kWh heat at COP 2 = 5.1 kWh of electricity (x 10p) = 51p
• 10.2 kWh heat at COP 4 = 2.55 kWh of electricity (x 25p) = 64p

In other words, cheaper electricity often matters more than higher efficiency.

That’s why one of the golden rules from that investigation is simple:

Schedule hot water reheats during the cheapest electricity periods available on your tariff.

This was also referenced into our Best Heat Pump Tariff deep dive where we looked strategies and the equipment you have to save money running your heat pump.

The Bigger Picture

This is also where the wider renewable setup starts to make a difference.

If you have solar panels, battery storage, a heat pump and a time-of-use tariff working together, hot water becomes one of the easiest loads to optimise.

Running hot water when electricity is cheapest, or when solar generation is high, can reduce costs far more than small efficiency differences between hot water modes.

It’s a good example of how the renewable quintessentials work together: solar, battery storage, a heat pump, an EV and a smart electricity tariff.

You can read more about the Holy Trinity and Renewable Quintessentials in this dedicated article.

Automation Opportunities

If you are struggling with the concept of time of use tariffs and scheduling, especially with the likes of Octopus Agile and EDF FreePhase where the price changes even within the same day, consider automation.

Products like Havenwise can do a great job of doing all that heavy lifting for you.

Take a look at my Havenwise Review for more details of what’s possible.

Key Takeaways

Looking across two full years of real data the resaults are pretty clear.

• Eco mode consistently delivers the highest efficiency and COP.
• Sweet Spot mode provides faster reheats with only a small efficiency penalty.
• Normal will be even faster but with a further hit on efficiency.

The overall cost difference between modes is relatively small compared with the impact of electricity tariff timing.

For many homes the best choice will depend on:

• hot water demand

• cylinder size

• recovery time requirements

• electricity tariff

At the end of 2025 my preference and recommendation would be to go with Sweet Spot if you are able to schedule it.  Forgo the slight performance loss compared to Eco, but gain the speed so that your hot water recharges are faster and the heating is not off as long.

During 2026 I am running the system in Normal mode, which will add a third year of real world data to this comparison.

You can keep up with how that is going over on my live monitoring feed: https://emoncms.org/energystatsuk

Once complete we will be able to compare Eco, Sweet Spot and Normal modes directly.  Both from a cost point of view as well as in house comfort.

One final big thanks to Trystan and Glyn at Open Energy Monitor for their amazing product.  None of this analysis would be possible without it.