Customized 4-Pipe FCU Thermostat with Heating-Mode Run-Down Fan Timer

    Project Overview

The ONYX Building project in South Africa required a flexible and reliable HVAC control solution for a 4-pipe Fan Coil Unit (FCU) system, supporting both cooling and heating operation.

The project uses a centralized FCU system where the thermostat is responsible for:

•   ● Cooling and heating control
•   ● Three-speed fan control
•   ● On/off motorized valve control

While the thermostat supports standard cooling and heating functions, the client identified a specific operational requirement only under heating mode, which could not be achieved with a standard FCU thermostat.

To address this, Beris Tech delivered a customized 4-pipe FCU thermostat with a heating-mode run-down fan timer, tailored specifically for the ONYX Building project.

    System Background and Application Requirement

In a typical 4-pipe FCU system:

•   ● Cooling and heating are controlled independently
•   ● The thermostat manages fan speed and valve outputs based on operating mode

Under cooling mode, standard fan stop logic is generally acceptable.

However, under heating mode, the client observed that:

•   ● When heating output is switched off, residual heat remains inside the FCU heat exchanger
•   ● Immediate fan shutdown prevents effective heat dissipation
•   ● Room temperature drops unevenly after shutdown
•   ● Occupant comfort is negatively affected

For this reason, the client required a customized fan control logic applied specifically under heating mode, while keeping standard behavior under cooling mode.

     Customized Heating-Mode Control Logic

The customized logic applies only when the thermostat operates in heating mode.

The required control sequence is as follows:

  ⒈The thermostat operates normally in heating mode and activates the heating output

  ⒉When the controller is switched off (standby mode):  

● Heating output turns off immediately

 ⒊The fan continues running:  

● At the same speed setting as before shutdown 

● For a defined duration of 180 seconds

 ⒋After 180 seconds:

● The fan turns off automatically

  ⒌If the controller is powered back on within the 180-second delay period:

● Normal heating operation resumes instantly

This logic ensures smooth thermal transition without disrupting normal system operation.

      Beris Tech Technical Solution

Beris Tech implemented the above requirement through customized firmware logic integrated directly into the thermostat, without adding any external controller or relay.

Key technical highlights include:

•   ● 4-pipe FCU control with both cooling and heating modes
•   ● Heating-mode–specific run-down fan timer (180 seconds)
•   ● Fan speed memory retained during the delay period
•   ● Independent logic paths for cooling mode and heating mode
•   ● Stable operation under 95–240VAC, 50/60Hz power supply

The customized logic is activated only under heating mode, ensuring standard FCU behavior is maintained in all other operating conditions.

     Thermostat Configuration (Project Version)

•   ● Model: BT02-FCU-4W
•   ● Power supply: 95–240VAC, 50/60Hz
•   ● Application: 4-pipe FCU system
•   ● Functions:
•                   ● Cooling and heating control
•                   ● Three-speed fan control
•                   ● On/off motorized valve control

          ● Customized feature:

  ● 180-second run-down fan timer under heating mode

  ● Communication:  WiFi

     Project Benefits

The customized heating-mode fan run-down logic provides clear advantages for the ONYX Building project:

•   ● Improved thermal comfort after heating shutdown
•   ● Better utilization of residual heat in the heat exchanger
•   ● More natural temperature transition for occupants
•   ● Reduced thermal stress on FCU components
•   ● HVAC operation aligned with real building usage behavior

    Project-Based Customization Capability

At Beris Tech, we understand that standard thermostat logic does not always match real project requirements.

For the ONYX Building project, customization was not about adding hardware, but about refining control logic to fit actual HVAC operation scenarios.

Our engineering team works closely with system integrators and project owners to deliver project-oriented control solutions, especially for FCU applications in commercial buildings.

      Conclusion

The customized 4-pipe FCU thermostat with heating-mode run-down fan timer was successfully applied in the ONYX Building project in South Africa.

This project demonstrates Beris Tech’s capability to provide flexible, engineering-driven HVAC control solutions that go beyond standard thermostat functions.