RLT-Opt – Holistic optimization of ventilation and air conditioning systems

General methods for the holistic process optimization of ventilation and air-conditioning systems (RLT stands for “raumlufttechnische Anlagen“; English: Ventilation and air conditioning systems).

© Fraunhofer IOSB
Optimizing the operation of ventilation and air-conditioning systems promises a high potential for energy savings.


The share of energy consumption in buildings throughout the EU is almost 40 % of the total energy demand of the countries. The share of air handling devices (RLT devices), i.e. ventilation and air conditioning systems, is not precisely known. Estimates suggest that over one million RLT devices are in operation. Only a part of these are covered by the obligation of an energy inspection (approx. 250,000) and an energy inspection has only been carried out on an estimated 10% of these (as of 2015). 

An energetic optimization of RLT systems requires an integrative consideration of all trades (heating, cooling, ventilation) of the technical building equipment (TGA). From the practical experience of the consortium (in particular partner IBDM) of approx. 10,000 metrologically examined TGA systems, the result is very disillusioning. Almost all air handling systems (more than 90 %) are not operated according to the energy saving law.

Aim of the project

In the RLT-Opt project, new methods and tools are being developed to enable the inventory and savings potential to be quickly recorded in the first step, so that appropriate optimization measures can be implemented in a targeted manner in the second step. The first step is to analyze the tightness and operating mode of the ventilation and air-conditioning systems. The operation of the ventilation and air-conditioning systems is then successively optimized by means of sensor-based recording and evaluation of key operating parameters.

The following important questions concerning the monitoring and optimization of the operation of air handling systems have not yet been answered or have been discussed only inadequately and will be addressed in the project:

1. Questions regarding sensor technology and monitoring:

  • Which sensor technology is required for monitoring the efficient, hygienic and comfortable operation of air handling systems (mobile or stationary)?
  • How can mobile measurement technology be easily networked in the building?
  • In existing buildings, how can the air tightness of the duct network be tested as easily as possible, leakages identified and air distribution tuned?

2. Questions on operational optimization:

  • Which are frequently recurring topics in HVAC system and operation optimization, so that it is worthwhile to develop generic procedures and software tools for this purpose?
  • Where are the highest optimization potentials and how can they be identified as quickly and easily as possible?
  • How can existing and new plants be optimized efficiently?
  • What role does the user play and how can he be involved?

Project partners

  • Fraunhofer IOSB
  • BUILDING Consultants + Innovators GmbH
  • IBDM GmbH
  • Effizienzbörse Deutschland (German waste management company)

The tools developed in RLT-Opt

As a result of the project, RLT-Opt has created the efficiency board as an independent forum for exchanging information and increasing energy efficiency. The tools developed in the project can be downloaded here [German website].

Systems for Measurement, Control and Diagnosis department

The Systems for Measurement, Control and Diagnosis (MRD) department at Fraunhofer IOSB is responsible for the project.

Project details

RLT-Opt – Holistic optimization of ventilation and air conditioning systems

Funding code: 03EN1037A-D

Project duration: 4/2021 - 3/2024

The RLT-Opt project is funded by the German Federal Ministry for Economic Affairs and Energy (BMWi) as part of the German government's 7th Energy Research Program.

Research topic

Would you like to find out more about this topic? Then visit the research topic page "Energy-optimized operation of buildings and district heating networks" and find out more.