FixAtlas
Back to Manuals
Building AutomationAdvanced

Siemens DESIGO Room Controller RXC21

Installation and programming guide for Siemens DESIGO RXC21 fan coil and room controller. Covers point configuration, setpoint programming, and integration with central BAS.

Siemens RXC21.1

Safety Warnings

  • •Do not modify control parameters without understanding system-wide impact
  • •Incorrect configuration can cause occupant discomfort or equipment damage

Physical I/O

  • 3 × Universal inputs (temperature, CO2, or binary)
  • 4 × Triac outputs (for fan stages, valve, damper)
  • 1 × DC 0-10V output (for modulating devices)
  • LON or KNX bus connection

Standard Fan Coil Configuration

  • AI1: Room temperature sensor (NTC10K)
  • AI2: Outdoor temperature (optional)
  • Q1-Q3: Fan speed low/med/high
  • Q4: Cooling valve (open/close or modulating)
  • Y1: Heating valve (0-10V modulating)

Commissioning Steps

  1. Assign device address on LON/KNX bus
  2. Set application program (fan coil 2-pipe or 4-pipe)
  3. Set heating/cooling setpoints
  4. Set deadband and night setback offsets
  5. Test each output manually
  6. Verify sensor readings vs. calibrated reference

P-gain and I-time Tuning

  • Default P-gain: 2.0 (increase if hunting, decrease if slow)
  • Default I-time: 600s (reduce for faster response)
  • Oscillation: Increase P-gain or I-time

Structured Troubleshooting

Common problems, likely causes, diagnostic steps, and safe resolution paths.

Siemens RXC21 — no communication to central controller

High confidence
Likely Causes
  • LON or BACnet wiring fault between RXC21 and controller
  • Device not commissioned — neuron ID or BACnet instance not bound
  • RXC21 firmware version incompatible with network protocol version
  • Termination resistors not installed on LON trunk
Diagnostic Steps
  1. Check communication status LED on RXC21 face — no blink = no traffic received
  2. Verify LonTalk or BACnet cable connections at RXC21 terminal block
  3. Confirm neuron binding or BACnet enrollment in BAS commissioning tool
  4. Check LON trunk for 105Ω termination resistors at each end
Resolution Paths
  • Repair communication wiring and verify polarity
  • Re-commission RXC21 in BAS software
  • Update RXC21 firmware if version mismatch
  • Install 105Ω LON termination resistors at trunk ends
BASroom controllersiemensdesigofan coilroom automation