Indoor comfort in residential and office buildings is an important concern. Despite, advancement in building automation, it is difficult to achieve the anticipated comfort after post commissioning of existing anticipative building management systems (BMS). Discrepancy between anticipated and actual comfort comes from different sources. Indeed, not only technical or operational failures are responsible for discomfort and for energy expenses. Discrepancies may occur from other sources such as unplanned situations or misusages. Detection and evaluation of these sources of discrepancies and failures is a crucial issue at whole building level. This paper proposes a detailed fault diagnosis and HAZOP (Hazard and Operability analysis) methodology for an office building, integrated with Reactive Building Management System (RBMS). Various causes that account for the gap between potential and perceived comfort are discussed in detail. Diagnosability issues are of particular focus. A so called 'bridge diagnosis approach' is presented in context of reactive management of buildings. The remainder of this paper presents a case study with simulation of different faulty scenarios.