Drawing on theories from studies of Facilities Management (FM) and social studies of science and technology as well as explorative qualitative research methods the purpose of this paper is to examine energy operations of large, existing buildings and how knowledge development and knowledge transfers may provide lessons for how FM of existing buildings may contribute to transition towards zero emission buildings. In this energy operation of existing buildings is described as an important measure towards sustainability. The relevant actors explored for this study are related to large non-residential buildings (university), and different types of large housing facilities (health rehabilitation and student housing), aiming to provide practical insight to professional development and learning processes related to energy operation of all types of large buildings. The preliminary findings and originality and value of this paper is the emphasis on how in-house FM may be applied to create added economic and environmental value through energy management, and how energy efficiency operation of existing buildings may contribute to transitions towards zero emission buildings. Inspired by literature and the case studies conducted the paper argues for further research investigating formal and informal meeting spaces and involved actors important for stimulating knowledge sharing and innovations in and among FM actors, and the possibility of knowledge exchange and cooperation between in-house and external FM organisations towards new and improved management of complex facilities and potential zero emission buildings.