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The Vaillant flexoTHERM is available in two unit sizes

The Vaillant flexoTHERM is available in two unit sizes (Image credit: Vaillant)

German manufacturer Vaillant is best known for its boilers, but its venture into renewable heating systems has produced several good options for homeowners; it makes some of the best heat pumps on the market. The flexoTHERM series are known as multi-source heat pumps. Multi-source heat pumps can harness power from the ground, water or air. 

The Vaillant flexoTHERM ground source heat pump is designed with state-of-the-art Enhanced Vapour Injection (EVI) compressor technology to deliver high heating performance with great efficiency. The Vaillant flexoTHERM is Quiet Mark accredited and supplied with a multiMATIC 700 Vaillant controller for even greater gains in energy efficiency. 

The flexoTHERM is (as the name suggests) highly flexible. It can be combined with multiple units to heat a large space or build a hybrid system run via a single controller. 

Vaillant provides installation and commissioning support and offers an extended range of accessories, including a water heat exchanger, passive cooling and brine filling station.

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Technical information

The Vaillant flexoTHERM can be controlled via an app

The Vaillant flexoTHERM can be controlled via an app (Image credit: Vaillant)

The Vaillant flexoTHERM is suitable for ground collectors or boreholes. It’s available in 230V (5kW, 8kW or 11kW models) or 400V (5kW, 8kW, 11kW, or 15kW models). With an integrated EVI super compressor, the Vaillant flexoTHERM injects additional refrigerant into the compression cycle for greater efficiency and power. You can connect your Vaillant flexoTHERM to a passive cooling module to assist with climate control, removing thermal energy stored in an airtight building to discharge it into the ground.

  • Guarantee options: 5-year parts and labour guarantee 
  • Price range: £8,000-£11,000
  • Efficiency: A+++ (when installed using VRC 700 controls)
  • Max power: 19kW

Key features

Highlights: Quiet Mark-approved, making no more noise than a refrigerator. 

Key benefit: Can be cascaded with up to 7 Vaillant flexoTHERM units to heat large homes or commercial properties.

Maximum flow temperature: 65°C.

Dimensions: 1183mm (h) x 595mm (w) x 600mm (d) for a 15kW unit.

Noise level: 45.2 dB(A)

Pros and cons 

A ground source heat pump is an expensive but worthwhile investment for your home, but it’s important to weigh up the pros and cons. There are several advantages to the Vaillant flexoTHERM ground source heat pump to consider:

  • Low noise levels: The Vaillant flexoTHERM is MCS accredited and Quiet Mark approved, emitting no more noise than the average refrigerator. 
  • Ability to cascade: It’s possible to cascade up to 7 Vaillant flexoTHERM units together to efficiently heat large homes or commercial properties. 
  • Highly efficient: The Vaillant flexoTHERM has a A+++ ErP rating when installed with Valliant’s VRC 700 or 700f thermostats.
  • High seasonal SCoP: A high and efficient Seasonal Coefficient of Performance (SCoP) value makes the unit highly effective, even in a number of weather conditions. 
  • Flexibility: The Vaillant flexoTHERM has a number of accessories that enable connection to either air or water sources. 
  • App control: You can manage your Vaillant flexoTHERM with a convenient app to achieve even greater efficiencies. 

There are some downsides to the Vaillant flexoTHERM to consider, however. 

  • Cost: The Vaillant uses boreholes, which makes it more expensive and inconvenient to install than some of the other leading models on the market. 
  • Output: The Vaillant flexoTHERM isn’t as powerful as some of the other heat pumps on the market with a similar price tag.

How does the Vaillant flexoTHERM heat pump work? 

The Vaillant flexoTHERM incorporates a EVI (Enhanced Vapour Injection) compressor to provide maximum heat transfer and an optimal Seasonal Coefficient of Performance (SCoP) that is much higher than a standard heat pump cycle. 

The Vaillant flexoTHERM is a highly efficient ground source heat pump

The Vaillant flexoTHERM is a highly efficient ground source heat pump (Image credit: Vaillant)

EVI compressors also ensure higher efficiencies while using considerably less power. The compression process is split into two stages to increase capability and the temperature lift. This enables the Vaillant flexoTHERM heat pumps to achieve higher outlet flow temperature at low source temperatures. 

The technology can achieve particularly high seasonal yield levels compared to other units, and works at high production temperatures. The production of hot water is managed through the geoSTOR and uniSTOR storage. Air-water, water-water or a passive cooling system can be configured through a range of accessories. 

Add-ons for the Vaillant flexoTHERM ground source heat-pumps

Vaillant offers several accessories including the fluoCOLLECT water-to-water stainless-steel heat exchanger for water source adaptation, or aroCOLLECT for air-water adaptation. Passive cooling modules can be added to enable passive cooling via the flexoTHERM pump. All accessories are available at an additional cost and are subject to certain conditions.

How does the Vaillant flexoTHERM ground source heat pump compare to other heat pumps?

The Vaillant flexoTHERM is suitable for domestic or commercial use

The Vaillant flexoTHERM is suitable for domestic or commercial use (Image credit: Vaillant)

The Vaillant flexoTHERM series is a solid option for larger homes or commercial properties, or homeowners that want to create a complex hybrid system controlled via a single controller. However, it’s not a practical option for smaller homes or apartments. 

If you own a flat, you should steer clear of the Vaillant flexoTHERM and opt for the Kenso Shoebox range. This small but efficient unit fits easily in an airing cupboard and comes in below £4,000. 

If you are looking for something a little more powerful than the Vaillant flexoTHERM, you should look at the Viessmann Vitocal 350-G. It offers single-stage heating outputs from 20.5-42.3kW and two-stage from 41 to 84.6kW. It costs more or less the same as the Vaillant flexoTHERM, but with fewer opportunities to customise/accessorise the unit. 

What does the Vaillant flexoTHERM ground source heat pump cost?

The Vaillant flexoTHERM ground source heat pump can be classed as premium, and it’s not the cheapest on the market. Here’s how it compares to other units:

Manufacturer

Cost

Worcester-Bosch Greenstore LECP

£8,000

Viessmann Vitocal 350-G

£10,000

Vaillant flexoTHERM

£11,000

Maintaining the Vaillant flexoTHERM

The Vaillant flexoTHERM is suitable for larger homes in a variety of climates

The Vaillant flexoTHERM is suitable for larger homes in a variety of climates (Image credit: Vaillant)

The Vaillant flexoTHERM heat pump is low maintenance, but to keep your heating system in good working order, it’s best to have your ground source heat pump professionally serviced at least once a year. 

Annual servicing is a requirement while the system is under the guarantee in order to maintain the warranty. An annual service starts at £250 for heat pump systems and £300 for a hybrid system, excluding VAT. Your service technician will examine the ducts, filters and coils and check for leaks or worn parts that may lead to equipment failure. 

What the professionals say

“Vaillant ground source heat pumps are efficient, reliable and quiet heating systems, and the flexoTHERM model is available in a range of outputs. The flexoTHERM is one of the best heat pumps for commercial properties and larger homes, as up to seven individual units can be installed together to meet greater power needs. At the same time, Vaillant’s GreeniQ technology helps ensure maximum efficiency and reliability.”

“The upfront costs of a ground source heat pump and the installation are much higher than a conventional boiler. However, they can last twice as long as boilers and could help to lower your heating bills. Plus, RHI payments can potentially see you make a return on that investment.

Vaillant ground source heat pumps are efficient, reliable and quiet heating systems available in a range of outputs to suit the heating demands of different homes.”

Customer reviews

“Very happy with the new Vaillant air-source heat pump system in our self-build house. Good looking too!”

“The installation although by its nature was a bit disruptive, was very well handled and went smoothly. The plumbers and electrician from Pink Heating were knowledgeable, friendly and careful to do the best job possible with the minimum possible disruption. As it is now edging into summer, the system has not really been tested yet, but all seems fine so far and works well. The flexibility of control of individual room temperatures via a phone app is excellent.”

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

Our methodology

In order to compile this review, we reviewed several professional thinkpieces, media articles, reports and hundreds of customer reviews. We then scored the Vaillant flexoTHERM using a scoring system totalling 100 points against the criteria most consumers would consider before making a purchase, including efficiency, cost and brand reputation. 

The score consisted of the following: 

  • 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 5 points;
  • Awards and accolades (e.g. Which? Best Buy Awards) for a total of 5 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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