Heat Pump Integration with KNX via Modbus
How to connect Daikin Altherma, Vaillant aroTHERM, Nibe, and Mitsubishi Ecodan heat pumps to KNX for weather compensation, COP monitoring and smart tariff control.
Why integrate a heat pump with KNX?
A modern heat pump has its own controller and thermostat. So why connect it to KNX? Four reasons:
Weather compensation
KNX outdoor weather station (Theben TR608) provides outdoor temperature to the heat pump via Modbus — enabling fine-tuned heating curves beyond the heat pump's own internal sensor.
Zone setpoint coordination
KNX sends heating/cooling mode and flow temperature setpoints to the heat pump based on zone demand — preventing the heat pump from heating when no zones are calling.
COP monitoring and energy optimisation
Read heat pump power consumption and heat output via Modbus → calculate real-time COP → adjust operating schedule to run during low-tariff periods.
Presence-based control
KNX occupancy data (sensors or phone-based presence) triggers heat pump setback when the building is empty — not just the floor heating zones, but the heat source itself.
Heat pump Modbus interfaces
| Manufacturer | Model | Modbus type | Interface |
|---|---|---|---|
| Daikin | Altherma 3 H/R | Modbus TCP | P1/P2 to Modbus adapter (DCOM-LT/S) |
| Vaillant | aroTHERM plus | Modbus TCP/RTU | VR921 eBUS–Modbus gateway |
| Nibe | F2040/F2120 | Modbus TCP | NIBE Uplink or direct RS-485 Modbus |
| Mitsubishi | Ecodan R32 | Modbus RTU | PAC-IF07B-E Modbus adapter |
| Bosch | Compress 7000i | Modbus TCP | Bosch MX300 Modbus module |
| Viessmann | Vitocal 250-A | Modbus TCP | VitoConnect 100 or Modbus adapter |
| Stiebel Eltron | WPL/WPC series | Modbus TCP/RTU | ISG (Internet Service Gateway) optional |
Note:Heat pump Modbus availability varies by model year and region. Always verify the specific model's Modbus register map before designing the integration. Open Energy Monitor's heatpump monitor project and OpenEMS publish community-verified register maps for many EU heat pump models.
Commonly used Modbus registers
Exact register addresses vary per manufacturer — always use the specific register map. However, these data points are typically available across most systems:
| Data point | Read/Write | KNX DPT | Use |
|---|---|---|---|
| Flow temperature | R | 9.001 (°C) | Actual supply water temperature |
| Return temperature | R | 9.001 (°C) | Return temperature (COP calc) |
| Outdoor temperature | R | 9.001 (°C) | Outdoor ambient (from HP sensor) |
| Flow temp setpoint | R/W | 9.001 (°C) | Write to override weather curve |
| Operating mode | R/W | 20.102 (HVAC) | Heating / Cooling / DHW / Standby |
| Compressor on/off | R | 1.001 | Compressor running state |
| Active power (kW) | R | 9.001 (kW) | Electrical input power |
| Heat output (kW) | R | 9.001 (kW) | Thermal output power |
| DHW setpoint | R/W | 9.001 (°C) | Hot water temperature setpoint |
| DHW temp actual | R | 9.001 (°C) | Current hot water tank temperature |
| Error code | R | 7.001 | Active fault code (0 = no fault) |
Weather compensation curve via KNX
Weather compensation adjusts the heating flow temperature based on outdoor temperature. The colder outside, the higher the flow temperature needed to maintain room temperature. Most heat pumps have an internal curve — but KNX can provide a finer control or override it:
Weather compensation logic (KNX + Modbus)
Outdoor temp (from Theben TR608 KNX weather station)
↓
KNX logic block (MDT Logikmodul or Home Assistant)
↓
Flow temp setpoint calculation:
T_outdoor = -10°C → T_flow = 45°C
T_outdoor = 0°C → T_flow = 38°C
T_outdoor = +10°C → T_flow = 32°C
T_outdoor = +15°C → T_flow = 28°C (minimum)
↓
Write T_flow setpoint to HP via Modbus TCP register
↓
Heat pump adjusts compressor speed accordinglyThe specific curve depends on the building's thermal insulation (U-value, heat loss coefficient) and the floor heating system design temperature. A well-insulated passive-standard house may only need T_flow = 30°C even at -10°C outdoor — significantly improving heat pump COP.
Tariff-based scheduling and smart charging
In countries with time-of-use electricity tariffs (most EU markets), it pays to run the heat pump during off-peak hours (typically 23:00–07:00) and pre-heat the building and hot water tank:
KNX + heat pump panel design
We build panels with Modbus TCP gateways pre-configured for your heat pump model — Daikin, Vaillant, Nibe, or Mitsubishi — and KNX integration for full building energy management.
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