Founded in 1874, Vaillant, still a family-owned business, was actually one of the first to patent a gas bath boiler with a closed system. Since then, Vaillant has been consistently setting higher standards in the heating market and leading the way in renewable technologies. Vaillant’s extensive range of ground source and air source heat pumps allows homeowners to lower their carbon footprint and energy bills.

The Quiet Mark-awarded Vaillant flexoTHERM 400V + aroCOLLECT is available in 5kW, 8kW, 11kW, 15kW and 19kW models. This means the system is highly adaptable for different house types and sizes. It’s reported to be one of the quietest air source heat pump models on the market, so you can expect minimal sound disruption in your home.

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Technical information

The Vaillant flexoTHERM 400V plus aroCOLLECT is a quiet and highly efficient heat pump system, combining the benefits of both a ground and air source heat pump. The added aroCOLLECT brine collector module adapts the flexoTHERM ground source heat pump to work as an air source heat pump, using energy from air to heat brine. The addition of this module results in highly efficient air-to-water heat exchange where no heat is lost between the outdoors and indoors.

This heat pump system is ideal for residential projects and larger dwellings or developments due to its high power output of up to 19kW. One of its main advantages is that the base system can connect to three different sources: water, ground and air.

The aroCOLLECT module is only available with the 400V flexoTHERM pump, and comes in power outputs of 5, 8, 11, 15 and 19kW. These units can be cascaded with up to seven pumps.

  • Available models: 5kW, 8kW, 11kW, 15kW and 19kW
  • Energy efficiency: ErP A+++ (for both heating efficiency and hot water)
  • Dimensions: 595mm (w) x 1,185mm (h) x 600mm (d) (for all models)
  • Weight: 157–200kg (depending on model)
  • Minimum/maximum temperature heating: 25–65°C
  • Brine fluid type: Ethylene glycol 44% volume

Key features

  • Quiet Mark: The Vaillant flexoTHERM 400V has been awarded the Quiet Mark accreditation as one of the quietest systems on the heat pump market. At just 39.8dB(A) for the 5kW model, it’s as quiet as a modern refrigerator.
  • Integrated EVI super compressor: The flexoTHERM’s compressor injects additional refrigerant into the compression cycle for a higher outlet water temperature, more power and improved efficiency.
  • Green iQ technology: This technology helps future-proof the flexoTHERM 400V in terms of sustainability. All Green IQ Vaillant models are set up for the future, meaning they are internet-ready, sustainably manufactured, made from largely recyclable materials and operate at optimum efficiency.

Pros and cons

Installing an air source heat pump is a large financial investment, so you want to ensure it’s the right energy solution for you and your home. There are many systems on the market to choose from, so you need to weigh up the pros and cons of each. Here are the upsides and downsides of the Vaillant flexoTHERM 400V plus aroCOLLECT.


  • It’s very quiet: The system features fully modulating ventilators that produce minimal sound. The 5kW model can operate at an exceptionally low noise level of 39.8dB(A), which has earned the Quiet Mark accreditation.
  • Heat efficiency: The flexoTHERM 400V for the aroCOLLECT configuration comes in five power outputs depending on your needs: 5kW, 8kW, 11kW, 15kW and 19kW. When installed using VRC 700 controls, both hot water and heating efficiency have an A+++ ErP rating. 
  • Long guarantee: The flexoTHERM 400V + aroCOLLECT comes with a standard two-year guarantee for parts and labour. You can increase this to a seven-year guarantee if you have your heat pump registered via the Vaillant Advance Renewables loyalty scheme.


  • More costly than alternatives: At £5,300 for the unit itself, this heat pump is pricier than other systems on the market.
  • Low operating temperature is not optimal: The Vaillant flexoTHERM has a lowest operating temperature of just −10°C.

How the Vaillant flexoTHERM 400V + aroCOLLECT air source heat pump works

The Vaillant aroCOLLECT air source heat pump module on a teal green background.

It is the aroCOLLECT module that turns the Vaillant flexoTHERM into an air source heat pump. (Image credit: Vaillant)

Because it can work with a variety of innovative renewable technologies such as ground source and water source pump arrays, the flexoTHERM is one of Vaillant’s greenest heating systems.

The pump offers optimum performance and efficiency, being rated A+++ for energy efficiency. The flexoTHERM range also incorporates a state-of-the-art compressor technology called electronic vapour injection (EVI), which can help the system deliver the highest levels of heating performance.

The aroCOLLECT add-on module is a three-phase air collector. The module uses the ambient air temperature to warm the fluid (in this case, brine) like a ground loop system would. This results in higher air-to-water efficiency, as it prevents heat loss between your indoor areas and the outside. aroCOLLECT features a twin unit connector that uses balanced Tichelmann piping. The reverse return system is also ideal for outdoor use in commercial, agricultural and residential projects. 

The system features a set of intelligent weather-compensating controls and is fully compatible with wi-fi. This also means that the heat pump can be operated remotely. The flexoTHERM range is also compatible with the Vaillant wired VRC 700 and wireless VRC 700f controls. You can manage the heat pump using a clear digital display that shows, in real time, how much heat is being absorbed or used.

Installing the Vaillant flexoTHERM 400V and aroCOLLECT is very straightforward. Many integral components are already included in the system, such as ground and heating circulation pumps and a three-way diverter valve. There is also a wide range of accessories available that can complete your setup (for example, water heat exchangers and brine filling stations).

How the Vaillant flexoTHERM 400V + aroCOLLECT air source heat pump compares to others

This air source heat pump is ideal for residential projects deemed unsuitable for ground arrays or boreholes. Plus, you can cascade the system to suit larger residential and commercial projects if you require more heating power. You can install up to seven flexoTHERM heat pumps together on a large property.

Compared to other systems, the Vaillant flexoTHERM 400V is powerful, quiet and reliable.

One of the system’s main advantages, however, is that it’s very adaptable. This flexibility is almost incomparable in the heat pump market. Connect the system to the aroCOLLECT brine collector module to create an air source heat pump. You can also connect it to a ground loop, turning it into a ground source heat pump, or the fluoCONNECT module, which converts it into a water source heat pump.

However, if you’re looking for an air source heat pump that works as a standalone unit without having to add a separate module to work, you might want to consider the Viessmann Vitocal 100-A, which offers high output options like the flexoTHERM. These units come with a lower starting price than the flexoTHERM, though it does need to be installed alongside a cylinder to provide hot water.

Another alternative is the Mitsubishi Ecodan Monobloc air source heat pump, which has a much lower price tag than the flexoTHERM but doesn’t offer the best output.

How much does the Vaillant flexoTHERM 400V + aroCOLLECT air source heat pump cost?

The Vaillant flexoTHERM 400V costs around £5,300 for the unit itself. With installation and the addition of the aroCOLLECT module, this price will likely increase to about £11,800.

Although this upfront cost can seem relatively high, the best air source heat pumps can lead to significant savings over time by way of lower heating bills. You can also expect your heat pump to function with very little maintenance and reduced carbon emissions. The Vaillant flexoTHERM range is also fitted with weather compensation controls, which can maximise efficiency and reduce expenditure.

Manufacturer and productCost (excluding installation and labour)
Mitsubishi Ecodan MonoblocFrom £2,600 to £5,815
Viessmann Vitocal 100-AFrom £3,041 to £6,667
Vaillant FlexoTHERM 400V + aroCOLLECTFrom £5,400 to £11,800

Maintaining your Vaillant flexoTHERM 400V + aroCOLLECT air source heat pump

Maintenance should be carried out by competent and qualified engineers. All Vaillant products come with a comprehensive set of instructions and schematic drawings to troubleshoot common issues. Generally, you can use the Live Monitor to see the current status of the product and see if there are any faults. To keep the unit working as intended, make sure you keep all filters, coils and fans clean, and ensure there is proper air flow (not impeded by debris, dust, etc).

What the professionals say

“aroCOLLECT features a twin unit connection using balanced Tichelmann piping, reverse return system and is ideal for outdoor use in agricultural, commercial and residential projects. flexoTHERM and aroCOLLECT are MCS accredited, so may qualify for the RHI scheme.”

“Connect with a standard pre-plumbed cylinder for an eco-friendly, low cost hot water solution. With its sleek design and pre-plumbed architecture, the space requirements for the pre-plumbed hot water cylinder have been reduced dramatically.”

What customers say

A person controlling the Vaillant flexoTHERM 400V and aroCOLLECT air source heat pump via a mobile app.

The Vaillant flexoTHERM plus aroCOLLECT air source heat pump can be controlled via a smartphone app. (Image credit: Vaillant)

“This is a superb heating system for modern day carbon free living. The system is very versatile and easy to use. Vaillant is a renowned company that delivers the best service. We had a gas boiler made by Vaillant years ago. It lasted 30 years. The casing rusted a bit over the years BUT the machinery worked until we removed it, to be replaced by the air heat pump system. This should be testimony to the high quality of the products Vaillant offer.”

“I am very pleased with the installation of my new air source heat pump, and with its quiet running. I am yet to find out how good the central heating system will be because it is summer time just now. The members of the installation company team were excellent and very professional.”

“Very pleased with Vaillant air source heat pump. Overnight costs reduced by two thirds.”

Final recommendation

The Vaillant flexoTHERM 400V is a well-engineered product that is both sustainable and incredibly flexible. You can turn it into an air source heat pump with the aroCOLLECT module.

To ensure the best performance possible, it’s always best if your house is well insulated so you can avoid unnecessary heat loss. A Vaillant installer can carry out a full heat-loss calculation and recommend the flexoTHERM 400V model that is most suitable for your heating needs.

The system is easy to install, runs very quietly and supports a range of accessories such as brine filling stations, plate heat exchangers and passive cooling options.

If you’re looking for an excellent UK air source heat pump with a proven track record of efficiency and performance, the Vaillant flexoTHERM 400V + aroCOLLECT is a perfect heat pump solution.


We considered several factors when reviewing the Vaillant flexoTHERM 400V + aroCOLLECT air source heat pump. Looking at professional and customer reviews plus technical documentation, we assessed this heat pump using five main criteria: design, durability, efficiency, performance and guarantee options. These criteria were considered with the following weightings:

  • The warranty period offered by the brand, for a total of 10 points;
  • The overall reputation of the brand based on online reviews, for a total of 15 points;
  • Trustpilot reviews, for a total of five points;
  • Awards and accolades (eg, Which? Best Buy Awards), for a total of five points;
  • Customer service performance, for a total of 10 points;
  • The price range, for a total of 15 points;
  • Efficiency ratings, for a total of 15 points;
  • Installation and maintenance network and reviews, for a total of 15 points; and
  • The total warranty period, for a total of 10 points.

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