Vaillant AroTherm Plus 7kW

The Vaillant AroTherm Plus 7kW heat pump is a stylish, compact and highly capable unit that delivers efficient heating and hot water with a user-friendly control interface. While it operates whisper-quiet most of the time, it’s not without its flaws – most notably a noticeable increase in noise during defrost recovery. However, with its flexible settings, excellent tech support and cost-effective performance, it’s a strong contender for those looking to transition to renewable heating.

Introduction and Background

This review focuses on the Vaillant Arotherm Plus VZW 75/6, commonly referred to as the ‘7kW’ model. The unit was installed in November 2024 in a 1930s detached house with approximately 200 square meters of floor area. It replaced a condensing gas boiler dating from around 2007. The house originally had solid walls and metal-framed windows, which remain in some areas. Two modest modern extensions, partial internal wall insulation, 300mm of loft insulation, and mostly UPVC replacement double glazing have improved the insulation, earning the property an EPC rating of ‘C’.

Two full fabric surveys (one free and one paid for, each taking three hours) calculated a heat loss of 16kW at -2°C. Other estimates, without a full survey, ranged from just under 7kW to 16kW. Analysis of gas consumption suggested a heat loss in the region of 6-9kW. A closer examination of one of the full surveys revealed that many of the fabric upgrades had been ignored, and some losses were double-counted. Correcting for these errors, while using standard MCS assumptions, reduced the calculated heat loss to 10.5kW.

During this period, several quotes were sought, ranging from £12,000 to over £20,000 before applying the Boiler Upgrade Scheme (BUS) grant. A delay allowed for a more accurate determination of heat loss based on gas consumption, confirming beyond reasonable doubt that a 7kW unit would suffice. To account for the discrepancy between ‘measured’ and ‘calculated’ heat loss using MCS defaults, it was necessary to assume an air change rate of 0.5 per hour (as opposed to the default 2-3). Anecdotally, some suggest this lower figure is more reasonable in many cases.

The delay also provided time to better understand the technology, with the help of the Renewable Heating Hub and other resources. By the time updated quotes were sought, it was clear what was needed: a 7-9kW machine directly connected to the central heating system without an intervening buffer, low-loss header (LLH), or plate heat exchanger (PHE) – essentially, no ‘system separation.’ A new unvented cylinder was also required. Only four radiator upgrades (including one fan coil) were necessary, as seven had already been upgraded during earlier phases of the work. A 16A dedicated electrical feed was already in place.

Height constraints (the unit had to fit under a window) narrowed the choice of models, and noise requirements eliminated several manufacturers. The published specification for the Vaillant ‘7kW’ model indicates an output of approximately 8.5kW under design conditions, though there are unverified suggestions that this figure does not account for defrost cycles and should be downrated. Its relatively compact size and appearance made it particularly attractive, and several installers recommended it. Additionally, Vaillant’s pre-sales tech support had been prompt and helpful, addressing all my queries. A provisional decision was made to proceed with this unit.

Vaillant was contacted again and provided contact details for three local installers. These installers were given evidence of the measured heat loss, a clear brief that any proposal involving system separation would not be accepted, and other relevant material. A particularly impressive installer identified during the initial work was also asked to confirm their earlier quote. All installers were bound by MCS rules to conduct their own heat loss assessments, but their willingness to consider the provided evidence varied. Ultimately, the choice came down to two installers, and a scoring system was used to make the final decision. The local installer won, primarily due to their proximity.

The Installation

The installation took six days in total, with a single installer completing the entire job. He worked nine-hour days or longer, often without breaks, and was frequently offered tea and biscuits! On one occasion, he stayed until 9 pm to finish fitting and connecting the DHW cylinder, which proved tricky due to the complexity and age of the existing plumbing. The installation was neat and well-executed. The installer programmed a fairly conservative set of parameters and provided a brief overview of how to adjust the ‘user’ settings. I suspect he deduced from our conversations during the installation process that I was reasonably familiar with the technology.

Both before and after the installation, Vaillant contacted me several times to check on progress and ensure everything was running smoothly. This level of support was impressive.

Controls and Operation

The Vaillant Arotherm Plus comes with a wired controller, primarily designed for use by the installer, and a wireless interface, also used for Vaillant boilers, which includes a temperature and humidity sensor. The wireless interface is more user-friendly and aimed at the end user, though many installer settings can also be adjusted through it.

The control system is one of the standout features of the unit. It offers a wide variety of settings, allowing it to be extensively tuned to meet specific requirements. While many of these settings are intended for the installer, the manual provides clear descriptions, making several of them accessible to more advanced end users.

Several modes of space heating control are available, including ‘pure’ weather compensation (with no influence from the measured indoor temperature) and various levels of augmented weather compensation, where the compensation curve is adjusted based on the measured room temperature. I found that operating the heat pump on pure weather compensation, with no room influence, provided the most stable outcome and performed significantly better than my previous gas boiler.

All the emitters have thermostatic radiator valves (TRVs), however the TRV heads on all but two are unscrewed and set to maximum, effectively disabling them. Overall, the heating experience is very satisfactory, with the entire house remaining consistently comfortable regardless of outdoor conditions.

The way Vaillant presents weather compensation is particularly interesting. Their method, also adopted by some other manufacturers, involves defining a series of pre-set curves that can be selected by adjusting a single parameter, rather than the four parameters required by many competitors. While this might seem limiting, it is not in practice and is certainly easier to operate.

The curves are nominally designed for a target room temperature of 20C. The system also includes a ‘target temperature’ setting, which, behind the scenes, shifts the pre-set weather compensation curve up or down depending on the value selected. This works even if room influence is entirely disabled (meaning the controller ignores the internal temperature sensor). This allows the ‘ordinary user’ to set a room temperature as they are accustomed to, while the system adjusts the weather compensation curve accordingly, promoting efficient operation.

Additionally, the built-in scheduler operates in the same way. For example, periods can be programmed where the ‘set temperature’ is reduced or increased, but this actually adjusts the weather compensation curve and, consequently, the flow temperature. This approach is far superior to relying on a room thermostat or turning the unit off to achieve a setback or boost, as it maintains efficiency while preserving a familiar user interface. While some other manufacturers offer equally good functionality, several competitors lack this level of flexibility.

The domestic hot water (DHW) controls are similarly flexible. A variety of settings allow the speed (and thus efficiency) of DHW heating to be adjusted. In addition to ‘DHW priority’ mode, where the tank is reheated as soon as its temperature falls below a set value, scheduled operation is also available. Unlike many heat pumps that rely on an immersion heater for the legionella cycle, the Vaillant heat pump itself performs this function, though this can be disabled if desired.

The wireless controller also provides extensive operational information, including consumption figures. Overall, the controller offers all the flexibility one could reasonably want, and there is no need to consider additional external controls.

Performance and Noise

During the first two or three weeks after installation, I adjusted the weather compensation curve downward from the installer’s programmed value and switched off all room influence. I also turned down a couple of lockshield valves on bedroom radiators to operate them without TRVs. Since then, only minor, occasional tweaks to the settings have been made as I fine-tuned the system to fit the household routine.

The system was nominally designed to operate at a leaving water temperature of 45C at -2C. In practice, it runs slightly lower, at 42C at -2C. During the design process, the default was to oversize rather than undersize radiators when an exact match wasn’t available, so this outcome is not surprising.

The 200-liter Vaillant DHW tank, installed in an airing cupboard, is largely unremarkable. One slightly disappointing feature is that the airing cupboard hardly warms at all, despite my instruction to the installer to omit insulation on the adjacent pipework. However, it’s hard to complain about a cylinder being too well insulated.

An add-on Wi-Fi interface is required (in addition to the wireless controller) for internet-connected operation. An iPhone/Android app allows remote control of the system and provides various information, including a direct readout of the COP for the day, week, month, year or any other selected period, as well as the ability to download historical data.

There is also the option to link this data to Home Assistant via third-party integration for more extensive monitoring or control, if desired. This is a cost-effective and relatively easy-to-set-up alternative to using OpenEnergyMonitor, though the latter exposes more variables. Neither option is as convenient as Mitsubishi’s MelCloud offering. It’s worth noting that linking to an external monitoring system is not necessary, but it does provide interesting insights.

In operation, the outdoor unit is whisper-quiet most of the time and, for the most part, completely inaudible inside the house or even outside beyond a few meters. This is a stringent test, given that there are two fairly large windows only 30 cm from the unit. However, when the compressor operates above 60% modulation (and particularly above 80%) the unit emits an audible buzz. This occurs primarily during the recovery from defrost and, to a much lesser extent, for short periods when heating DHW or when the outside temperature drops below zero. For the vast majority of the time, even in cold weather, the unit operates well below the noise threshold.

While the noise level is still quieter than a domestic fridge, this is a disappointing feature. It’s inevitable that heating systems work harder in cold conditions, so some increase in noise is expected. At the time of writing, there are suggestions that this is a ‘known problem,’ and Vaillant has advised that it is being investigated, so improvements may be forthcoming. It’s unclear whether competitor products have similar noise profiles.

The excess noise can be completely suppressed by activating noise reduction mode. Fortunately, this can be scheduled, though it limits output power. I found it practical to use this mode at night, even during very cold weather, ensuring no disturbance to occupants or neighbors.

One further design flaw, shared by many popular heat pumps, is the casing. The unit has a perfectly flat and level top, which does not shed water. It’s surprising that only a minority of heat pump manufacturers consider rainwater runoff when designing their casings.

COP and Running Costs

The claimed monthly COPs so far are 4.6 (December 2024), 3.7 (January 2025), and 3.8 (February 2025). However, the accuracy of these values is unclear.

More usefully, I have a reasonably accurate record of how my gas consumption varied with outside temperature. This allows me to make a direct and robust comparison of the actual running costs of the air source heat pump (ASHP) with what the costs would have been had I retained the gas boiler over the same period.

My average electricity import cost (February 2025) is 19.3p/kWh, and my gas price is 6.06p/kWh. Over the period from mid-November 2024 to mid-February 2025, I estimate that the ASHP cost 12% less to run than the gas boiler would have during the same period. These figures are based on actual outdoor temperatures and the electricity consumption reported by the heat pump itself, which I have cross-checked against my electricity meter and found to be reasonably accurate.

My electricity tariff is EON Next Drive, which offers seven hours of cheap-rate electricity (in exchange for a slightly higher daytime rate). When calculating the average unit price, I excluded car charging cycles, which occur only at night and would otherwise distort the proportion of nighttime versus daytime consumption in a way that unfairly favors the heat pump. That said, without an electric vehicle (EV), I could have opted for the Ovo heat pump add-on (15p/kWh for heat pump consumption only at the time of writing), which would have made the heat pump even more cost-effective. However, overall, including the EV, this would have been more expensive.

I have not yet adjusted the heat pump schedule to take full advantage of the nighttime tariff, so there may be scope for further optimisation. Nevertheless, I am already satisfied with the results, particularly as this period covers the coldest months of the year when heat pumps are least efficient.

Summary

In summary, the Vaillant 7kW heat pump is a mostly well-designed unit with a user-friendly and flexible control interface that operates cost-effectively, at least in my setup. The increase in noise during certain circumstances, particularly during defrost recovery, is disappointing, though it can be managed by scheduling noise reduction mode. Vaillant’s tech support has been very helpful, unlike some competitors who either refer you to your installer or refuse to answer questions altogether.