Why BMS Controllers Fail
Understanding Ageing Electronics in Building Management Systems
Building Management Systems are designed to operate continuously for many years, often controlling critical HVAC and ventilation systems across large commercial buildings.
However, like all electronic equipment, BMS controllers eventually begin to fail as their internal components age.
Many commercial buildings in Sydney and Melbourne are currently operating control systems that were installed 15–25 years ago. As these systems age, intermittent faults and reliability issues become increasingly common.
Understanding why controllers fail can help property managers plan maintenance strategies and avoid unexpected building disruptions.
The Role of BMS Controllers
BMS field controllers are the small computers installed throughout a building that manage equipment such as:
- Air handling units
- Chillers and boilers
- Cooling towers
- Pumps and fans
- Variable speed drives
- Ventilation systems
These controllers continuously read sensor values and send commands to plant equipment to maintain temperature, airflow and energy efficiency.
Because they operate 24 hours a day for many years, they are exposed to constant electrical and thermal stress.
Electronic Ageing Inside Controllers
At the heart of every controller are microchips made from silicon transistors. These tiny electronic components switch on and off thousands of times per second as they process control logic.
Over long periods of operation, several ageing processes begin to occur.
Small leakage currents can develop within the semiconductor structure of the chip. As this occurs, the electrical signals inside the controller become less stable.
In digital electronics a signal is interpreted as either a “0” or a “1” depending on voltage level. As components age, these voltage levels can drift, causing the controller to misinterpret signals.
This is one of the reasons older controllers may begin to behave unpredictably.
Capacitor Degradation
Another common cause of controller failure is the ageing of electrolytic capacitors.
Capacitors are used to smooth power supply voltages and stabilise internal circuits. Over time the electrolyte inside these components slowly dries out, reducing their effectiveness.
When this happens controllers may begin to experience:
- random resets
- unstable operation
- communication dropouts
- complete failure to start
Capacitor degradation is one of the most common causes of electronic equipment failure after 15–20 years.
Thermal Stress in Plant Rooms
Many controllers are installed inside plant rooms where temperatures can fluctuate significantly.
Heat is one of the main factors that accelerates electronic ageing. Repeated heating and cooling cycles cause microscopic expansion and contraction within circuit boards and solder joints.
After many years these stresses can lead to:
- cracked solder joints
- loose internal connections
- intermittent controller faults
These problems often appear as faults that occur randomly and are difficult to reproduce during testing.
Communication Failures in Older Networks
Controllers in a BMS communicate with each other using networks such as BACnet, Modbus or LonWorks.
When controllers begin to fail internally, they may start to drop off the network intermittently.
Symptoms can include:
- controllers appearing offline
- communication errors
- points updating slowly
- data inconsistencies
These faults are often mistaken for network issues when the real cause is ageing hardware.
Signs a Controller May Be Failing
Property managers or facility teams may notice several early warning signs:
- HVAC equipment starting or stopping unexpectedly
- plant not responding to commands
- increasing alarm activity
- controllers disappearing from the network
- graphics showing incorrect data
These symptoms usually indicate that the system is beginning to reach the limits of its design life.
Managing Ageing BMS Systems
Although controller failures become more common with age, a complete BMS replacement is not always required.
Often the most effective approach is a planned upgrade strategy, which may include:
- replacing critical plant controllers
- upgrading communication networks
- modernising the supervisory software
- improving monitoring and trending
A staged approach allows buildings to improve reliability while controlling capital costs.