Stiftung Warentest Names Lambda EU10L the Most Efficient Air-Source Heat Pump in 2025/2026 Test

Stiftung Warentest evaluated ten air-source heat pumps in its 2025/2026 test. The Lambda EU10L, with a thermal output of 10 kW, received the highest score in the final ranking. The publication named it the most efficient heat pump in the sample: the model delivered the best results both when paired with radiators and with underfloor heating.

Test Results

According to Lambda, in the energy efficiency and environmental properties category in heating mode, the EU10L took first place among all ten participants. On average, the other tested pumps consumed 20% more electricity. In terms of noise, the model ranked among the quietest in the comparison — a low sound emission level was built into the design from the outset. Stiftung Warentest rated ease of operation as "very good" (1.4), including control and configuration.

3K-Process Technology

Lambda Wärmepumpen GmbH (Tyrol, Austria) develops and manufactures air-source heat pumps. The company was founded in 2019 on the basis of a research project. A key element of its product development is the patented 3K process, which optimizes heat exchange between outdoor air and the refrigerant for the most efficient use of available ambient energy. High efficiency and low noise levels are, according to Lambda, the primary design targets since the company's inception.

Production and Distribution in Remagen

From the very beginning, Lambda has collaborated with Zewotherm. At Zewotherm's head plant in Remagen (Germany), identical Lambda pumps are produced in an in-house workshop and sold through a three-tier distribution network. Lambda, in turn, offers its pumps through its own sales channels. Zewotherm integrates the heat pump into a comprehensive energy system: the model is compatible with photovoltaic installations, battery storage, as well as underfloor heating and cooling systems.

Photo: From Tyrol to Remagen — the identical Zewo-Lambda pump produced at Zewotherm's head plant for three-tier distribution in Germany. Left to right: Andreas Ziegler (Zewotherm), Florian Fuchs (Lambda Wärmepumpen GmbH), Silke Ziegler (Zewotherm), Daniel Knapper (Lambda Deutschland GmbH). Photo: Zewotherm.