The circulation accelerator

As an engineer, Louis Opländer’s goal was always to achieve the greatest possible benefit when using technology. He had a particular interest in people’s quality of life. His work was driven by a desire to improve well-being and hygiene. People’s living conditions played a special part in this endeavour. In contrast to the USA, for example, the predominant form of heating in 1920s Germany was the unhealthy coal-fired stove. At least the business with heating systems for residential buildings and the middle class had evolved increasingly after the First World War, and enormous progress had been made as compared to the time before the war. However, Louis Opländer was convinced that the triumph of central heating in private homes was yet to come. This opinion was contrary to wide sections of the heating sector that relied mainly on large scale industry. He was proved right.

Gravity heating

The evolution from the steam heating to the hot-water heating system did not happen abruptly. It took years for the advantages of hot-water heating systems to ultimately prevail. But one challenge remained in the form of the inertia caused by the large quantity of water in the gravity system. In addition to the Opländer company, scientists were also searching for a solution to this problem.

At the “XI. Kongreß für Heizung und Lüftung” (11th heating and ventilation congress) in Berlin in 1924, Dr Melchior Wirtz gave a lecture on the topic of “accelerated circulation”. Wirtz was a distinguished private lecturer whose scientific focus was on the physics of heating systems. He would certainly have discussed the problem of accelerated circulation with Louis Opländer. Another company had built an axial pump as a circulation accelerator, but its crucial disadvantage was that the pump was powered by a water jet that acted on a Pelton wheel. Since the costs for water increased even more than the circulation speed in the system, this solution was not economical.

The world’s first circulation accelerator

This was the state of development when Wilhelm Opländer joined his father’s company in 1926. Father and son worked together to tackle the challenge of developing a functional, economical circulation accelerator – a pump. They had realised that this required electricity, the cost of which had become reasonably affordable in the meantime. The pump was to be as small and light as possible in order to install it in existing pipe networks. The Opländers identified the pipe elbow as a suitable installation location. And so they welded on the first prototypes in this location and conducted trials with pumps that were operated by electric motors.

In 1928, Wilhelm Opländer finally succeeded in developing the circulation accelerator. One year later, he was granted a patent on a “circulation accelerator consisting of a propeller in the lines of a hot-water heating system”. This invention was a milestone for heating technology and the birth of Wilo. The pump simultaneously increased the level of comfort and reduced material consumption, as considerably smaller pipe cross-sections could now be used. It opened up entirely new possibilities in heating systems.


Nevertheless, by 1934, there were already eight different types of the Wilo pump” that were operated with either three-phase current, alternating current or direct current. The simplest model, the S25, weighed 12 kilogrammes, the motor had a power consumption of 33 watts, and the pump could move up to 300 litres of water per hour. The most powerful pump, the N 156, with the pressure of a 100 centimetre water column, weighed 70 kilogrammes and was able to transport up to 45,000 litres of water per hour with its output of 900 watts. Despite the variety of models, only around 400 pumps had been installed at this time. But the breakthrough was imminent.

The reservations about the new technology decreased with every year that it functioned reliably and efficiently. Great efficiency, smooth running, very low power consumption for the time and therefore minimum operating costs, easy installation – it was a revolution. Not a revolution that came overnight, but one that required some time. Soon, the company was no longer selling hundreds, but thousands of “Wilo pumps”.