The Daikin FWEC3 (model FWEC3A) is an Advanced Plus LCD electronic controller for indoor fan coil units. It communicates via Modbus RTU over RS-485 and provides control of operating mode, temperature setpoints, fan speed, and unit on/off state through TapHome.
The controller supports 2-pipe and 4-pipe fan coil systems with up to 38 system configurations (parameter P00), 3 or 4 fan speeds or modulating fan operation, and optional water temperature monitoring. A master-slave RS-485 network supports up to 247 slave controllers.
Hardware connection
RS-485 wiring
The FWEC3 electronic board provides RS-485 terminals labeled A(-), B(+), and GND for Modbus RTU communication.
Connect using a 3-wire RS-485 connection:
- A(-) on FWEC3 → A-/D- on TapHome RS-485
- B(+) on FWEC3 → B+/D+ on TapHome RS-485
- GND on FWEC3 → GND on TapHome power supply (mandatory)
Cable specification:
- Type: AWG 24, shielded 2-conductor cable (0.511 mm diameter)
- Termination resistor required on both ends of the RS-485 bus (master-slave setup)
- Ground the shield at one end only to avoid ground loops
The GND wire must be connected between the FWEC3 controller and TapHome. Without proper grounding, communication errors will occur. Use single-point grounding for the cable shield.
Wiring diagram
The top diagram shows a typical 2-pipe, 3-speed fan coil wiring with the FWEC3 controller. The RS-485 bus terminals (A, B, GND) are located on the lower-left area of the electronic board, alongside sensor inputs (SA, SW, SU) and digital/analog outputs.
The bottom diagram shows a 4-pipe configuration using the EPIMSB6 expansion board for additional valve and motor outputs. The RS-485 connection remains on the main FWEC3 board.
Configuration
Modbus address (P02)
The Modbus slave address is configured via parameter P02 on the controller:
- Switch the thermostat off
- Press the Up and Down keys simultaneously to enter the password screen
- Enter password 10 and confirm
- Scroll to parameter P02
- Set the desired Modbus address (1–254 for slave, 255 for master)
- Confirm and exit configuration mode
The default slave address is 1. After changing P02, power-cycle the controller for the new address to take effect.
Communication parameters
The FWEC3 uses fixed communication settings that must match the TapHome Modbus RTU CCU configuration:
| Parameter | Value |
|---|---|
| Baud rate | 9600 |
| Parity | None |
| Data bits | 8 |
| Stop bits | 2 |
Supervisor enable pattern
The FWEC3 uses a supervisor enable pattern on register H:50. Each control function (on/off, mode, setpoint, fan speed) has a dedicated enable bit in the high byte of H:50. The corresponding control bit in the low byte has no effect until its enable bit is set.
The TapHome template includes four service actions on the module level to enable supervisor control:
| Service action | Bit | What it enables |
|---|---|---|
| Enable On/Off supervisor | H:50 bit 8 | Unit on/off control via State device |
| Enable mode supervisor | H:50 bit 9 | Cool/Heat mode switching via Mode device |
| Enable setpoint supervisor | H:50 bit 13 | Temperature setpoint override via Thermostat |
| Enable fan speed supervisor | H:50 bit 15 | Fan speed control via Fan Speed device |
After importing the template, run all four service actions before attempting to control the fan coil unit. Without enabling the supervisor bits, write commands from TapHome are silently ignored by the controller.
Device capabilities
Operating mode and state control
- Mode — selects the operating mode: Cool, Heat, Fan, or Dry. Reads bit 1 of the status register (A:0) and writes via supervisor bit 1 in H:50. A service attribute shows whether mode supervision is currently enabled.
- State — turns the fan coil unit on or off. Reads bit 0 of the status register (A:0) and writes via supervisor bit 0 in H:50. A service attribute shows whether on/off supervision is currently enabled.
Temperature monitoring and control
- Thermostat — provides room temperature regulation. Reads the current room temperature from A:2, the current setpoint from A:8, and room humidity from A:3. The write script automatically selects the correct setpoint register based on the current operating mode: H:52 for cooling or H:53 for heating. Service attributes expose the setpoint supervision status and the minimum/maximum setpoint limits for both cooling (H:54–H:55) and heating (H:56–H:57).
- Water Temperature — reads the water pipe temperature from register A:4 with automatic division by 100 for conversion to degrees Celsius. This sensor is only available when a water temperature probe is installed (parameter P04=1).
Fan speed control
- Fan Speed — selects from five speed levels: Off, Very low, Low, Medium, and High (values 0–4). Reads the current speed from A:1 and writes to H:58. For units with modulating fan control, H:58 accepts a percentage value instead. Service attributes show the modulating fan mode (Off, Manually forced, Automatic) from H:60, and whether fan speed supervision is enabled.
Additional capabilities
The FWEC3 controller also exposes registers for the active temperature setpoint (A:7), system configuration readback (A:5), analog output monitoring (A:15–A:16), economy mode with setpoint correction (H:50 bit 3 + H:59), hourly programming schedules (H:51), keypad lock (H:50 bit 5), and dehumidification status (A:0 bit 12). These can be added in a future template update.
The module ReadScript monitors bit 5 of the status register (A:0) and reports a “General alarm on unit” error in TapHome when an alarm condition is detected on the fan coil unit.
Troubleshooting
Controller does not respond to mode/setpoint/fan changes
This is the most common issue. The supervisor enable bits in H:50 must be set before BMS control works:
- Open the Daikin FWEC3 module in TapHome
- Run all four service actions: Enable On/Off supervisor, Enable mode supervisor, Enable setpoint supervisor, Enable fan speed supervisor
- Retry the control command
No Modbus communication
- Check RS-485 wiring: Verify A(-), B(+), and GND are correctly connected and the GND wire is present
- Verify slave address: Confirm parameter P02 matches the Slave ID configured in TapHome (default: 1)
- Check communication settings: TapHome must be set to 9600 baud, no parity, 8 data bits, 2 stop bits (note: 2 stop bits, not 1)
- Termination resistor: For long bus runs or multiple units, ensure termination resistors are present at both ends of the RS-485 bus
Water temperature reads zero
The water temperature sensor (A:4) requires a physical water probe to be installed and parameter P04 to be set to 1 (available). If P04=0, the register returns 0.
Thermostat writes to wrong setpoint register
The template automatically selects H:52 (cooling) or H:53 (heating) based on the current mode bit (A:0 bit 1). If the mode changes between writing setpoints, the setpoint may be written to the previous mode’s register. Ensure the mode is stable before adjusting the setpoint.
