Heat pumps are now the most common Modbus TCP device in modern EU smart homes — yet most manufacturers don't advertise this capability prominently. Daikin Altherma, Vaillant aroTHERM Plus, and Nibe F-series all expose full Modbus TCP interfaces that give you read/write access to every parameter the internal controller touches. Paired with a KNX Modbus gateway or Home Assistant, you can implement weather-compensated setpoints, tariff-based scheduling, and real-time COP monitoring without any proprietary gateway hardware from the heat pump vendor.

Why Modbus instead of KNX native?

Most heat pump manufacturers do not produce KNX modules — and the few that do (Nibe Uplink KNX, Vaillant senso, Daikin BRP15B61) offer limited object sets: on/off, mode, and a single setpoint. The Modbus interface, by contrast, exposes the complete register table: leaving water temperature (LWT) setpoint, domestic hot water (DHW) setpoint, outdoor temperature sensor reading, compressor frequency, flow rate, COP calculation input registers, energy meter pulse counts, and error codes. For any serious integration, Modbus is the right choice.

Daikin Altherma Modbus TCP

The Daikin Altherma 3 (ERGA/EHBH series) exposes Modbus TCP on port 502 by default when connected via the BRP069A61 LAN adapter or the built-in LAN port on newer units. Key registers for heating control: register 2 (holding) = leaving water temperature setpoint in 0.1°C increments (write 200 = 20.0°C); register 3 = operating mode (0=standby, 1=heating, 2=cooling, 3=DHW); register 6 = DHW setpoint (write 550 = 55.0°C); register 100 = outdoor temperature reading (read only); register 106 = compressor running state (0/1). Daikin also provides D-CHECKER software (Windows) for direct Modbus register monitoring without a gateway — useful during commissioning to verify register values before wiring into KNX.

Vaillant aroTHERM Plus Modbus

The Vaillant aroTHERM Plus uses Modbus RTU on RS-485 (not TCP) via the VR 900 or eBUS adapter. RS-485 to Ethernet converters (Moxa NPort 5110A or similar) bridge this to TCP for gateway compatibility. Key registers: 1 = LWT setpoint (0.1°C), 3 = operating mode, 5 = DHW setpoint, 40 = outdoor temperature, 50 = flow temperature measured, 60 = return temperature, 100 = current power draw (W). Vaillant's register documentation is available via their OEM partner portal — request access through a registered Vaillant installer account.

Weather compensation via KNX

The standard weather compensation curve adjusts LWT setpoint based on outdoor temperature: at -10°C outdoor, LWT = 45°C; at +10°C outdoor, LWT = 30°C; at +20°C outdoor, heating off. KNX outdoor temperature sensor (e.g. Theben LUNA 120, MDT SCN-TS1UP.03) sends measured outdoor temperature to Home Assistant via the KNX integration. A HA automation looks up the heating curve table (configured as a template helper) and writes the resulting LWT setpoint via Modbus TCP to register 2. This overrides the heat pump's own weather compensation with a custom curve tuned to the building's thermal characteristics — typically improving comfort while reducing energy consumption.

Tariff scheduling

EU electricity tariffs increasingly include time-of-use pricing: cheap-rate hours (typically 22:00–06:00 or weekends) cost 40–60% less than peak-rate hours. KNX weekly scheduler triggers: at cheap-rate start → write Modbus register 3 = heating mode, register 2 = LWT setpoint +3°C above normal (pre-heat the thermal mass); at cheap-rate end → write register 2 = normal setpoint; at peak-rate morning peak → write register 2 = normal setpoint -1°C (run slightly cooler, reducing consumption). For DHW: schedule a full DHW cycle (55°C) during cheap-rate hours only. This alone reduces heating electricity cost by 20–35% in split-tariff environments with no additional hardware beyond the Modbus gateway.