Functional Availability of HVAC Systems

The capability of HVAC systems is of interest when contracting specific function criteria such as room temperature. The probability of maintaining the criteria reflects the risk the building contractor will take when giving guarantees. The level of knowledge and use of probability in the HVAC sector is low since there are just a few tests in this area. These facts make it difficult for all actors (and especially the building contractor) to increase guarantees and develop new types of performance-oriented contracts. In the last decade, the author has studied how reliability theory can be applied in the HVAC sector (Myrefelt 1996, 2004 and 2005). The starting point of this work was a study of the reliability of HVAC installations. This showed large variations, especially between different operating companies and covered many different aspects. This study was based on a large amount of buildings and seven different operating organizations. With time the Functional Availability (which quantifies the probability of a systems capability both to be in an operational state and at the same time maintain the intended levels of the functions) appears as an interesting option for performance measure. This is especially so in office buildings with computerized operation and control equipment where, typically, measured values are available. This paper describes results from a practical test in an office building during one week using the concept of Functional Availability to characterize the capability of the HVAC system to maintain room temperature, and the capability of the HVAC system to maintain the supply air temperature, heating and cooling. The results from the study showed that the use of Functional Availability gives valuable information about the HVAC operational state. This study also gives valuable information about the difficulties in setting acceptable and useful function criteria. The Functional Availability of the room temperature varies between rooms on the same floor, between hours of the same day and between days during the week. The results indicate that the Functional Availability concept can be used to calculate and show deviations from proper performance of the HVAC systems as a daily and hourly value. This means that this concept (as a lowest acceptable mean value for longer periods of time and perhaps also a minimum daily value) could be an option as a performance measure since it can be used as an on-line tool to enable operation personnel to survey the daily operation of the HVAC system.