In the last few months, we noticed that in one of our sites, quite a few air handling units (AHU) have temperature control issues. From the trend from building automation system, we found the valve oscillates. As you can see from the graph below, the valve oscillates from the 50% open to 0% open position. As a result, the discharge air temperature fluctuates up and down a well. When the temperature rises above the setpoint, the valve then starts to close which will drop the discharge air temperature.
The AHU is a 20+ years old system equipped with a steam heating coil with pneumatic actuator. All field control devices such as temperature sensors are relatively old. The control system was installed a few years ago, but we are not sure if it was commissioned properly. Among one of the many different solutions to solve the malfunction, we think the reprogram of the control strategy and tuning of the PID loop is the best choice. We do not need install or change any field device and we do not have to shutdown the unit.
After reprogramming and tuning, the supply air temp was controlled properly on the setpoint. The heating valves are no longer haunting all over the place.
With a steady air temperature of the supply air to the space, the terminal unit will be able to control space temperature smoothly. Also, with less movement of the valve stem, the lifetime of the control valve will be longer. Most importantly, we did not need to invest in anything. Instead, we recommissioned the system and got a very good result.