In the beginning of 1998 the European project, AIRLESS, was started to develop strategies,principles and protocols to improve and control the petiormance of HVAC-systems and itscomponents for the incorporation in codes and guidelines. The project is divided in 6 tasks.The Hermann-Rietschel-Institute deals with the second task of the project, the maintenance ofHVAC-systems. People stay approximately 90% of their lifetime in enclosed rooms. Theserooms are often served by HVAC systems. In recent years poor indoor climate has causedhealth problems in buildings. One reason for these problems can be wrong maintenance of theWAC-systems. The objective of this task is to develop a new maintenance guideline forbetter indoor air quality. This paper deals with the first two sup-tasks of the task two of theproject. In the first part existing maintenance guidelines will be compared. In the second partinvestigations for defining cleanliness are described. The third and last sub-task, the definitionof a new guideline, is not part of that report. The literature review shows, that the existingguidelines do not address the full spectrum of critical factors. Some of them lack precisespecifications about cleanliness and precise working procedures. Thus, the new maintenanceguideline should be a mixture of existing guidelines and results from the investigations aboutcleanliness and related working procedures. First investigations for definition of cleanlinessshow that it is not easy to give only some values for cleanliness. The first results are presentedin the following chapters.