The Stiebel Eltron WPL series — WPL 17, WPL 25, and WPL 50 ACS — are air-source heat pumps designed for residential and light commercial buildings. Integrating these units into a KNX building automation system opens up two complementary approaches: the SG Ready relay interface for simple grid-interactive control, and the ISG web internet service gateway for full Modbus TCP data exchange.

SG Ready Terminal Wiring

SG Ready is a standardised 2-bit interface (S1 and S2) defined by the German Heat Pump Association (BWP) that allows the grid or a smart home system to signal the heat pump into one of four operating states. On the WPL 17/25/50 ACS, the SG Ready terminals are located on the control PCB at connector X1, terminals 31 (S1) and 32 (S2). Both terminals are potential-free digital inputs rated for 24 V AC/DC contact closure.

The four SG Ready states are encoded as follows: State 1 (S1=0, S2=0) is the hard block state — the heat pump suspends operation. State 2 (S1=1, S2=0) is normal energy management — the heat pump operates at its internal programme. State 3 (S1=0, S2=1) is the recommended-on state — the heat pump raises the heating setpoint by the configured delta (typically +3°C) to consume surplus PV energy. State 4 (S1=1, S2=1) is the maximum operation state — the heat pump runs at full capacity and charges the DHW cylinder to 60°C.

For KNX control, an MDT Glastaster II or MDT SCN-REIG8.03 relay actuator provides the contact closures. The actuator occupies two KNX binary outputs: one GA for S1 (e.g. GA 8/4/1, DPT 1.001) and one GA for S2 (GA 8/4/2, DPT 1.001). ETS6 logic converts a 2-bit SG Ready state value (0-3) arriving from a PV surplus logic GA (GA 8/4/0, DPT 5.010) into the correct S1/S2 relay combination.

ISG Web Modbus TCP Register Map

The ISG web is a DIN-rail internet service gateway for Stiebel Eltron heat pumps. It connects to the heat pump via the internal CAN bus and exposes a Modbus TCP server on port 502. This interface provides read/write access to all operational parameters, making it far more capable than the SG Ready relay approach for building management integrations.

Key Modbus TCP registers on the ISG web (holding registers, function code 3/16): Register 1 = Flow temperature setpoint (°C, scale 0.1, read/write). Register 4 = Coefficient of Performance (COP, scale 0.1, read-only). Register 10 = DHW setpoint (°C, scale 0.1, read/write). Register 100 = Operating mode (0=off, 1=automatic, 2=heating, 3=cooling, 4=DHW only, read/write). Register 200 = Active fault code (0 = no fault, read-only).

Intesis IN701KNX Gateway Configuration

The Intesis IN701KNX200O000 gateway bridges Modbus TCP to KNX. In the MAPS software, add the ISG web as a Modbus TCP device (IP address, port 502, unit ID 1). Create signal objects for each register: flow setpoint as Analog (DPT 9.001, scale 0.1), COP as Analog (DPT 9.001, scale 0.1), DHW setpoint as Analog (DPT 9.001), mode as Integer (DPT 5.010), fault as Binary (DPT 1.001, reg > 0). Assign KNX GAs: GA 8/3/0 = flow setpoint, GA 8/3/1 = COP, GA 8/3/2 = DHW setpoint, GA 8/3/3 = mode, GA 8/3/4 = fault active. Export the Intesis ETS6 add-on file and import into ETS6 for group address assignment.

KNX PV Surplus Self-Consumption Logic

The PV surplus control uses a Fronius Symo inverter providing AC power measurement via Modbus TCP, with power exported to KNX GA 9/0/0 (W, DPT 14.056) by a separate Intesis gateway. A KNX logic module (MDT SCN-LOG16.02 or IP logic in ETS6) computes surplus power as PV output minus house base load. When surplus exceeds 2 kW, the logic writes SG Ready state 3 to GA 8/4/0; when surplus exceeds 5 kW, it writes state 4. At night or during grid stress events (utility signal input), state 1 (hard block) can be activated.

For DHW anti-legionella scheduling, a KNX weekly time controller (MDT SCN-TS1.01) triggers a one-shot write of DHW setpoint 65°C to GA 8/3/2 every Tuesday at 02:00, holding for 60 minutes before returning to the normal 45°C setpoint.

SHM-Plus Thermal Buffer Sizing

Stiebel Eltron recommends a thermal buffer store (Speicher) of at least 50 litres per kW of heat pump output. For the WPL 25 (25 kW), a minimum 1250-litre buffer is appropriate. The buffer decouples the heat pump from short heat demand spikes, reducing compressor cycling and improving seasonal COP. When SG Ready state 4 is active, the buffer charges to the maximum setpoint (55°C flow), storing thermal energy for later discharge without the compressor running during peak-tariff periods.

Commissioning Test Sequence

Step 1: verify Modbus TCP connectivity from MAPS software — confirm ISG web responds at the configured IP and all five registers return valid values. Step 2: apply SG Ready state 3 via KNX Group Monitor write to GA 8/4/0 value 2; verify flow setpoint increases by 3°C on GA 8/3/0. Step 3: apply state 4; verify DHW setpoint writes 60°C to ISG web register 10 and the heat pump enters DHW mode. Step 4: simulate fault by disconnecting ISG web network cable; verify GA 8/3/4 remains at 0 (fault register still reads 0 from last polled value) then reconnect. Step 5: log 24-hour COP from GA 8/3/1 and verify seasonal plausibility (COP > 3.0 at 7°C outdoor temperature).

Comparison: SG Ready Relay vs ISG Web Modbus

SG Ready relay wiring requires no additional gateway hardware (only relay actuator, ~€80), offers four coarse operating states, is independent of network connectivity, and suits projects where only basic surplus heating control is needed. ISG web Modbus TCP requires the ISG web gateway (€350) plus the Intesis IN701KNX gateway (€450), but provides full parameter read/write, COP monitoring, fault visibility, and setpoint scheduling — essential for BREEAM energy metering, commissioning records, and tenant energy portals. For new-build projects with KNX infrastructure already present, the ISG web integration delivers substantially more value and is recommended over the relay-only approach.