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The influence of unstable control on energy use and thermal comfort
Diploma Thesis in the Engineering Programme
Building and Civil Engineering
JENS RYBERG, MARTINA JORDESTEDT
Department of Energy and Environmental Engineering
Division of Building Services Engineering
Chalmers University of Technology
ABSTRACT
The purpose of the study is to investigate the possible effects of an unstable
ventilation system and its influence on energy use and thermal comfort. The report
also explores how qualified personal working with HVAC view and handles system
control.
In order to meet the requirements in energy efficiency and a comfortable indoor
climate control regulation is a prerequisite. With optimized increased complexity of
the climate systems, it becomes more challenging to control the system and the risk of
unstable control increases. No previous studies about what affect an unstable system
can have on energy use and thermal comfort have been found.
The report is based on several tests were the controlling variable has been increased
until the system becomes unstable. Two different types of unstable systems have been
investigated. During the first trials the flow was unstable while the temperature where
constant and during the second trials the temperature was unstable but the flow
constant.
The results indicate that the energy use, both for the fans and the air heater, is
drastically increased when the system is unstable. The results on the affects in thermal
comfort showed no noticeable differences during the stable and unstable condition
under the tests. This indicates that a system can be unstable without anybody noticing
it but will most likely increase the energy.
Qualified people believe that unstable control is quite common in buildings. One
possible consequence of unstable control is shortened lifespan for components in the
system.
The knowledge about control theory among HVAC workers in Sweden is generally
assessed as low. The lack of willingness among the property owners to invest in
improvements in ventilation system varies and often depends on the length they
intend to keep the building. Cost aspects and lack of knowledge among the property
owners results in that many buildings HVAC system is not up to date.
Key words: Unstable control, thermal comfort, energy use, ventilation system
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