Paper 9
Functional Availability of HVAC
Systems
Sonny
Myrefelt
Skanska
Sverige AB, Box 1219, 351 12 Växjo, SWEDEN
Abstract
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 system’s 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.
Key words: HVAC
reliability, fault monitoring, Functional Availability.
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