This article is focused on plan optimization issues of preventive maintenance based on risk assessment. The aim is to propose a cohesive system layout which involves the maintenance optimisation as a whole and achieve better results compare to simple applying the individual methods. This system is divided into five fundamental phases which are consequently following one after another and guarantee the order and structure of the process. Every phase consist of individual sections which thoroughly describe particular execution and procedures. The proposed system should help readers to obtain sufficient information in order to understand the different parts of the system and enable them to make their own opinion on synergy of systematic methodology which deals with optimization of maintenance. All these aspects are very important in practice due to the complexity of most ventilation, heating and air conditioning systems.