Vaillant Arotherm Plus Heat Pump: The good, the bad and the ugly.

Friends, it’s been just over a year now since we had our 5 kW Vaillant Arotherm Plus heat pump installed.

The Good

In short, I love the heat pump: it is super quiet; uses low GWP propane as a working fluid; and can even heat water to 70 °C if I should ever desire.

And it is has worked well with a seasonal average COP (sCOP) of 3.6 in its first heating season during which the internal temperature of the house has been steady at ~21.5 °C 24/7. And we have lots of pleasantly hot hot water.

But, not everything is good. And this article is about the things that are seriously bad and the things that are downright ugly.

The Bad

There is no User Manual! There is lots of excellent engineering documentation and installation instructions, but I personally would appreciate a relatively short document that explained how to adjust various aspects of the heat pump.

Click for a larger version. Engineering documentation for the Vaillant Heat Pump. But no user manual.

For example, if one uses a thermostat, it is relatively easy to programme a setback period overnight where the temperature is lower. But it would nonetheless be nice to have instructions.

But if you switch to using weather compensation instead of a thermostat? There is no explanation of how the weather compensation interacts – if it all – with the thermostat setting. And the settings are four or five layers down in a menu system that is labrythine in its obscurity. A manual would be helpful.

The SensoComfort controller looks great: sleek and black. But using it is a nightmare. Each time one attempts to achieve a particular task one has to decide whether it should be looked for under a variety of confusing menu headings:

• Installation
• Basic System Diagram Configuration
• HP control module configuration
• Heat Pump 1
• HP control module
• Circuit 1
• House
• Domestic Hot Water

There is no logic to this and it’s just guesswork every time because there is no manual!

The Ugly

The Vaillant SensoAPP does just about have some basic functionality.

For example, it allows one to set a period of absence or trigger a boost to the domestic hot water. However, it frequently fails to do even these basic tasks, commonly reporting a variety of errors.

But the one thing I would like the App to do would be tell me the Coefficient of Performance (COP) of the heat pump. The COP tells the owner or an engineer, how well the heat pump is working.

The COP is the ratio of the amount of heat delivered to the house, to the amount of electrical energy used to operate the pump. Typically COP lies in the range 2 (poor) to 5 (outstanding) and this provides the most significant measure of a heat pump’s performance.

Ideally, the App would report the COP for hot water and for space heating separately. But actually there is just nothing!

In my opinion it is scandalous that the App does not report the COP.

There are signs that the App should be able to show the COP, but then using the available data it gives erroneous answers. In short, when it comes to monitoring heat pump performance, it is literaly useless.

Let me explain.

The App offers no direct readout of COP– which is disappointing – but the ‘Information’ screen on the SensoAPP appears to offer the opportunity to see the electrical consumption and the thermal output (called the ‘environmental yield’) for both space heating and domestic hot water.

Click image for a larger version. The information page on the Vaillant sensoApp looks like it should have all the information one needs to calculate the COP.

Using these data it should be possible to evaluate the COP. Sadly this is not the case.

In an attempt to do this I downloaded the weekly data for the electrical consumption and checked it against the completely independent MMSP monitoring system I have installed.

The weekly consumption information screen for DHW looks like the figure below. It is highly suspicious to me that the data appear to be exactly whole numbers of kWh every day – but the screen tells me that I used 8 kWh of electricity that week for domestic hot water, and that is the figure I recorded.

Click on the figure for a larger version. The electrical consumption for domestic hot water in Week 41 of 2022 as reported by the Vaillant sensoApp. Notice that the daily consumptions are all exact numbers of kWh.

My weekly MMSP data runs Saturday to Friday while the Vaillant data runs Sunday to Saturday, so we might not expect the data to be identical, but the data (below) are similar. I was hopeful when I saw this correspondence.

Click on the figure for a larger version. The blue curve shows the weekly electrical consumption (kWh/day) as self-reported by the Vaillant App. The red curve shows the same quantity as measured by an independent monitoring system.

Over the 61 weeks since installation the Vaillant reported consumption of 2,147 kWh – 4.3% less than the MMSP system. Not great agreement.

However, if one looks at the thermal data – the environmental yield – the data are both dodgy and missing.

How can they be both dodgy AND missing? As the screen grab below shows, the graph suggest the environmental yield is an exact whole number of kWh every day – something which is very unlikely. This makes the data seem dodgy to me.

But in this case we can also add up the daily yield very easily – it comes to 8 kWh that week. However the App does not do that summation for me – it simply states that the total heating over the entire installation time is 721 kWh. The weekly data are just missing!

Click on the figure for a larger version. The environmental yield for domestic hot water in Week 41 of 2022 as reported by the Vaillant sensoApp. Notice that the daily yields are all exact numbers of kWh.

While I can add up the data in the bar chart above quite easily, this not possible for other screens such as that below – which again simply states the total yield over the entire installation period.

Click on the figure for a larger version. The environmental yield for space heating in Week 41 of 2022 as reported by the Vaillant sensoApp. Notice that the daily yields are all exact numbers of kWh.

This means that it is impossible to work out the COP.

I did try working out the overall COP since installation, but the results were not believable. The Vaillant self-reported average COP is 2.0 whereas the MMSP monitoring system indicates an answer closer to 3.51 .

The good, the bad and the ugly

Summarising, the heat pump is fantastic, and works well.

But I only know that because I have an independent monitoring system.

If I didn’t have independent monitoring I would literally have no idea how well the heat pump was working.

And there has been no improvement or new software updates in the last year

Overall, this is shockingly bad.

Tags: Heat Pump, Software, Vaillant