Adapting Variable Air Volume (VAV) Systems for Office Buildings without Active Control Dampers

Licentiatavhandling, 2006

In commercial buildings the building services systems may account for a substantial part of the need of energy. There is, at least in Sweden, a large existing stock of 20 to 30 year old office buildings, which are supplied with centralised constant air volume flow (CAV) air conditioning systems with the primary function to keep the room temperature at an acceptable level summertime. However, since the heat loads are only occasionally at the peak level, considerable amounts of energy can be wasted during periods when cooling is not needed or when rooms are not in use. Substantial energy savings can be achieved by changing the system to variable air volume flow (VAV) operation. Nevertheless, such a change becomes meaningful only if the system as a whole is adapted to the new configuration. The aim of this work is to clarify the prerequisites for a well functioning VAV system in office buildings. A special focus has been that the system solutions should be uncomplicated and possible to implement in existing buildings as well as in new ones. Adopting VAV in office buildings without having to install active control dampers in the duct system is one way of building up a simple VAV system. To obtain this, some special demands must be set upon the VAV variable supply air diffusers. They must have good controlling properties and a low noise level even at a high pressure drop over the device. Also, a comfortable airflow pattern in the room must be ensured within the variable volume flow range in question. One example of uncomplicated system configuration, which in theory has properties that fulfil the demands, was tested both in the laboratory and studied in plants in function, with focus on the indoor climate and the need of energy. Furthermore the influence of a variable air volume flow on duct heat gains was analysed and mathematically evaluated. The results indicate that it is clearly possible to obtain a well functioning system with a technical solution in accordance with the studied one. The indoor climate demands are essentially fulfilled. The noise levels in rooms can be kept at a well acceptable level. The draught appeared to be considerably low despite low supply air temperatures. The field monitoring of energy consumption showed that the tested VAV systems worked energy efficiently. The heat recovery system managed to cover all the heating need of the supply air, i.e. any additional heating with the heating coil was not needed even at low outdoor temperatures.