There are varying methods of controlling single zone VAV systems including how and when they cool or heat and at what flow the supply fan will operate. The three main types of fan control to be looked at are:
- Constant-speed fan
- Two-speed fan control
- Variable-speed fan control
Each method of control is modelled separately in TAS Systems. The constant-speed fan model simply has no controller connected to the fan, meaning that it runs at the sized load whenever the system is in operation. Due to TAS using an hourly sampling system the two-speed fan control cannot be modelled as it would in a real system. This is because it would require instantaneous adjustment and calculation with relation to the flow rate. For this reason, the two-speed fan is modelled the same as the variable speed fan. This can be done because, if the fan speed is say 0.5x the full capacity, this means that a two speed fan will have been doing the same overall work but the speed output TAS gives can be interpreted as a proportion between the two fan speeds that it operates at. The two-speed fan and the variable speed fan are distinguished in their energy consumptions by their differing part load characteristics. Load in a variable speed fan is not directly proportional the power. With a two-speed fan however, it can be assumed that each fan is operating at peak load, allowing it to be modelled by a directly proportional part load characteristic. The variable speed fan differs from this, in that under low loads it uses less power, meaning that it saves on power usage relative to the other two fan control methods. The part-load characteristics are shown below in figure 7.