Christoph Reichl, Operating Agent of HPT Annex 51 - Acoustic Signatures of Heat Pumps

A man with a burning interest in how to make things quieter reducing the sound emissions

The reduction of CO2 emissions is on everyone’s lips these days. But to make this happen, a multitude of actions are needed. Heat pumps are an environmentally friendly and renewable energy technology, but to further increase the acceptance of these units they must be improved with respect to the noise and vibration emissions they produce. HPT Annex 51 has the purpose to identify the main influencing factors to the acoustic signature of these units during different operating conditions.


Christoph Reichl is a Senior Scientist at the AIT – Austrian Institute of Technology GmbH (AIT). He is also the Operating Agent of the research project HPT Annex 51 – Acoustic Signatures of Heat Pumps.

Aeroacoustics is a branch of acoustics that studies noise generation via either turbulent fluid motion or aerodynamic forces interacting with surfaces. Together with structure borne noise this is exactly what Christoph and his team is dedicated to when it comes to heat pumps such as air-to-water, water-to-air, air-to-air and brine-to-water units.

A passion for reducing sound emissions
Christoph has a broad and deep education in the area. After he got his physics PhD working in the field of solid state physics investigating magnetic order of intermetallic compounds, he continued to do research in the area of numeric simulation. He worked in Aeroacoustics, Computational Aeroacoustics (CAA) and Computational Fluid Dynamics (CFD). As aeroacoustics needs a lot of computational power, he was the driving force to build up the Computational Infrastructure (Linux Cluster) that was needed for the work. So, what was the driving force behind the dedication in this area?

– My passion for the subject was not so profound from the start. I just had a burning interest in how I could make every day appliances quieter reducing the sound emissions. It might be a more silent fan, a car, a helicopter or even an airplane, it is all about acoustics and making things more silent, says Christoph.

Unwanted sound – a barrier to acceptance
Today, climate change is one of the most important issues that concerns us all. It has also affected and deepen Christoph’s work. The reduction of acoustic emissions is important to further increase the acceptance of heat pumps.

– I am convinced that it will become difficult to reach climate goals if we have to rely on equipment that emits disturbing noise. To increase this acceptance and minimize noise annoyance, more focus must be put on acoustics emissions at steady state and transient operation conditions.

The research in HPT Annex 51
The work in HPT Annex 51 revealed several important parameters. Christoph describes the problem as dual: One part concerns the technical functionality of the heat pump and the other part concerns the impact on the environment when installing the heat pump.  It is important to choose the right location for the installation of the outdoor unit of the heat pump to channel the outer noise in the right direction as a constant humming sound can be very frustrating. And here it comes down to the installers of heat pumps and their knowledge about acoustics. In addition to outer acoustic, there are also sound emission on the inside as sound and vibrations can be transmitted through tubes and walls creating unwanted noise. This aspect is also investigated in the Annex.

The work in the Annex goes beyond altering inner and outer sound and aims to understand the most important factors that influence sound emissions. Temperature and moisture of the air used as heat source are two of these factors that influence the operation behaviour of heat pump. Ice formation during cold winter day, as an example, can reduce the efficiency. Although the Annex 51 team knew that acoustics are affected by temperature before they started their work, they did not know to what extent. Today the picture is much clearer thanks to the research.

– Our focus is to understand all the parameters and circumstances that have an impact on the emitted acoustic waves and their dependence on the behaviour of the heat pump. We also investigate this by placing the exact same equipment into different environments. If there are deviations, that is our big “Why”. This comparison has been made in several different countries like Italy, Sweden, Denmark, Germany, Austria and France, and the differences in the measurement data lead to a deeper understanding on the acoustic behavior of heat pumps, says Christoph.

Another aspect when characterising acoustics of heat pumps is correctly describing the acoustic perception and how people are affected by sound emission changes over time. This is also being investigated by the Annex when measuring sound power, the frequency behaviour, defrosting cycles, emission directivity, different operation points and the impact of heat pump placement.

Disseminate information to the right people
A key part and ambition of HPT Annex 51 is to publish a comprehensive documentation to spread the knowledge and to influence regulations and measurements techniques used in the field. One of the problems is that the regulations differ greatly from country to country which is a challenge for heat pump manufacturers.

– Our goal is that our research leads to standardisation. We hope that the information created in our Annex will be used by lobbyists in different networks around the world and also reaches public authority that defines restrictions for sound emissions. Finally, we also have the ambition to produce guidelines that help installers and buyers of heat pumps, says Christoph.