The Buderus GB192i is a gas condensing boiler from the Logamatic system range, widely installed in central European residential and small commercial buildings. Unlike heat pumps or solar inverters, the GB192i does not have a native Modbus TCP or BACnet interface. Integration with KNX therefore requires an adapter for the proprietary EMS plus (Energy Management System) 2-wire bus that the Logamatic controller uses internally. This article covers the EMS plus bus adapter options, the open-source EMS-ESP approach, Modbus TCP register mapping, and KNX heating zone control.

EMS Plus Bus Characteristics

The EMS plus bus is a 2-wire half-duplex serial interface running at 9600 baud with Buderus-proprietary framing. It is accessible via the boiler service port — a 3-pin DIN connector typically located behind the front panel. Pin 1 is EMS+, Pin 2 is EMS-, and Pin 3 is a 12 V supply. Official Buderus accessories (Logamatic 4000/5000 system controllers, web gateways) use this bus internally. Third-party integration is possible via adapter hardware that decodes the EMS protocol.

EMS Plus Adapter Options

Three adapter approaches exist. The Buderus MX25 web module (OEM, ~€250): provides a REST API over Ethernet, suitable for cloud-based integrations but without native Modbus TCP — requires additional gateway software. The Iventum EMSHelper RS485 adapter (~€120): converts EMS plus to Modbus RTU for direct connection to Modbus-capable gateways. The EMS-ESP open-source firmware on a Wemos D1 Mini (ESP8266 dev board, ~€10 hardware cost): connects to the EMS bus via a 2-resistor, 2-capacitor level-shifter circuit and runs the EMS-ESP firmware which decodes all EMS plus messages and exposes the data via MQTT and Modbus TCP.

EMS-ESP Implementation

EMS-ESP (available on Github, maintained by the EMS community) supports over 80 Buderus, Nefit, and Sieger boiler models. Hardware: Wemos D1 Mini, EMS bus interface circuit (two 100-ohm resistors, two 10 µF capacitors, one BC547 transistor — full schematic in the EMS-ESP documentation). Connect the interface circuit to the boiler service port EMS pins. Flash EMS-ESP firmware via the Arduino IDE or EMS-ESP flasher tool. On first boot, connect to the EMS-ESP Wi-Fi access point and configure: Wi-Fi credentials, static IP, and Modbus TCP enable (port 502). EMS-ESP auto-discovers the attached boiler model and populates its entity list with all available data points.

Modbus TCP Register Assignments

EMS-ESP provides a configurable Modbus TCP register map (configured via the EMS-ESP web UI under Settings > Modbus). Recommended register assignments: Register 1 = Flow temperature setpoint (°C × 10, read/write — write sets the boiler target flow temperature). Register 2 = Current flow temperature (°C × 10, read-only — actual measured flow temperature). Register 3 = Flame active (0 or 1, read-only — 1 = burner is on). Register 4 = Modulation percentage (0-100, read-only — burner modulation level). Register 5 = Active fault code (0 = no fault, non-zero = fault code from boiler display). Register 6 = DHW temperature measured (°C × 10, read-only). Register 7 = DHW mode (0=off, 1=on, 2=eco, read/write).

Intesis IN701KNX Configuration

In MAPS, add EMS-ESP as a Modbus TCP device (static IP, port 502, unit ID 1 — EMS-ESP default). Create signal objects: flow setpoint (Reg 1, scale 0.1) → GA 8/0/0 (DPT 9.001, bidirectional), current flow temp (Reg 2, scale 0.1) → GA 8/0/1 (DPT 9.001, read-only), flame active (Reg 3) → GA 8/0/3 (DPT 1.001, read-only), modulation (Reg 4) → GA 8/0/5 (DPT 5.001, read-only), fault code > 0 as binary → GA 8/0/4 (DPT 1.005 alarm, read-only). The fault binary is derived in MAPS via a threshold comparison: if Register 5 value > 0, output = 1.

KNX Heating Zone Control

The GB192i serves multiple underfloor heating zones via a manifold with thermally-actuated valve heads. Each zone's valve is controlled by a KNX output channel (MDT AKK-16UP.01 actuator module, one output per zone). A KNX room thermostat in each room provides a heat call binary output (DPT 1.001). The heat call GAs from all zones are combined by a KNX logic module (OR function): if any zone is calling for heat, the logic writes a 1 to the boiler enable GA (GA 8/0/2, DPT 1.001). The boiler enable GA is mapped via Intesis to EMS-ESP Modbus Register 8 (boiler enable command). Additionally, when a heat call is active, the logic computes a flow setpoint based on the number of active zones (more zones = lower setpoint to maintain efficiency) and writes this to GA 8/0/0 (flow setpoint).

Comparison with Viessmann ViCARE Modbus

The Viessmann ViCARE Modbus adapter provides functionally similar data (flow setpoint, measured temperatures, fault status) for Viessmann Vitodens boilers. The EMS-ESP approach for Buderus achieves the same end result — Modbus TCP data available to an Intesis IN701KNX gateway — at lower hardware cost for the adapter, but requires manual firmware installation. The Iventum EMSHelper is the plug-and-play commercial equivalent for projects where open-source firmware is not acceptable.

Commissioning Steps

Step 1: flash EMS-ESP, connect to boiler service port, and verify the EMS-ESP web UI shows the boiler entity list with live values (flow temp, modulation). Step 2: enable Modbus TCP in EMS-ESP web UI, confirm MAPS can read all configured registers. Step 3: write Register 1 = 450 (45.0°C setpoint) from MAPS test mode, verify boiler display shows 45°C target. Step 4: trigger a KNX heat call from ETS6 Group Monitor (write GA 8/0/2 = 1) and verify flame active (Register 3) transitions to 1 within 60 seconds. Step 5: monitor GA 8/0/4 (fault) remains 0 during normal operation; trigger a test fault via boiler diagnostic mode and verify BewO alert fires.