Heat Pump KNX and SG Ready: Vitocal, Aquarea and arotherm Plus Self-Consumption Control
Integrating air-source heat pumps with KNX via the SG Ready standard enables PV self-consumption optimisation — using surplus solar energy to heat the building or DHW instead of exporting at low feed-in tariff rates. Modbus TCP integration extends this to variable setpoint control and real-time COP monitoring.
SG Ready standard: 4 operating states
SG Ready (Smart Grid Ready) is a German BWP (Bundesverband Wärmepumpe) standard for grid-responsive heat pump control. Two dry-contact inputs (S1, S2) on the heat pump PCB define four operating states — no complex communication protocol is needed, only two relay contacts from the KNX system.
| State | S1 | S2 | Meaning | KNX trigger |
|---|---|---|---|---|
| 1 — Blocked | 1 (closed) | 0 (open) | Grid overload — HP off | Never used for PV; utility grid signal only |
| 2 — Normal | 0 (open) | 0 (open) | Default operation | PV surplus < 1 kW |
| 3 — Encouraged | 0 (open) | 1 (closed) | Run at enhanced capacity | PV surplus 2–5 kW |
| 4 — Run-at-max | 1 (closed) | 1 (closed) | Excess PV — maximum capacity | PV surplus > 5 kW |
KNX implements SG Ready with two KNX binary output relay channels — one per contact. The PV inverter surplus power value (Fronius, SMA or Huawei Modbus) drives the KNX logic that sets the relay state.
Viessmann Vitocal SG Ready wiring
The Vitocal 250-A and 300-A PCB terminal block X1.3 provides the SG Ready 2-wire input. Terminals 63 (S1), 64 (S2) and 65 (GND common) accept potential-free contacts — exactly what a KNX relay actuator provides.
Vitocal X1.3 wiring to MDT BJ65-AB08.01 relay actuator
MDT BJ65-AB08.01 relay channel 1:
Common (COM) → X1.3 terminal 65 (GND)
NO output → X1.3 terminal 63 (S1)
MDT BJ65-AB08.01 relay channel 2:
NO output → X1.3 terminal 64 (S2)
(share COM terminal 65 with channel 1)
ETS6 Group Addresses:
GA 11/0/0 → S1 relay (DPT 1.001, 0=open, 1=closed)
GA 11/0/1 → S2 relay (DPT 1.001, 0=open, 1=closed)
KNX PV surplus logic (MDT Logic Module):
PV surplus (GA 9/0/1, watts) > 5000W:
→ Send 1 to GA 11/0/0 (S1 closed)
→ Send 1 to GA 11/0/1 (S2 closed) → State 4
PV surplus 2000–5000W:
→ Send 0 to GA 11/0/0 (S1 open)
→ Send 1 to GA 11/0/1 (S2 closed) → State 3
PV surplus < 1000W:
→ Send 0 to GA 11/0/0 (S1 open)
→ Send 0 to GA 11/0/1 (S2 open) → State 2Panasonic Aquarea SG Ready via native KNX
The Panasonic Aquarea J and K series CZ-CAPRA1 KNX adapter eliminates relay wiring entirely. SG Ready state is set directly via a KNX group object — the adapter translates the KNX DPT value to the Aquarea internal SG Ready state machine.
CZ-CAPRA1 SG Ready group object
ETS6 Application: CZ-CAPRA1 KNX adapter Parameter: SG Ready → Enable Group object: DPT 5.001 (0–100%) Value 0 → SG Ready State 2 (Normal) Value 1 → SG Ready State 3 (Encouraged) Value 2 → SG Ready State 4 (Run-at-max) GA 11/0/2 (Aquarea SG Ready state, DPT 5.001) KNX logic thresholds (same as relay approach): PV surplus > 5000W → send value 2 to GA 11/0/2 PV surplus 2000–5000W → send value 1 PV surplus < 1000W → send value 0 No relay wiring required — Aquarea receives state directly via KNX TP bus from CZ-CAPRA1 adapter
Aquarea advantage: the CZ-CAPRA1 also exposes flow temperature, DHW temperature, operating mode and energy counter as KNX group objects — providing full monitoring without a separate Modbus gateway. For new-build projects with Panasonic heat pumps, CZ-CAPRA1 is the recommended integration approach.
Vaillant arotherm plus SG Ready and Modbus
The Vaillant arotherm plus uses the same relay-based SG Ready wiring approach as Vitocal — terminals S1 (pin 10) and S2 (pin 11) on the arotherm plus PCB terminal block. The arotherm plus also supports Modbus TCP via the VR940F module, enabling richer control beyond the fixed SG Ready state machine.
arotherm plus Modbus TCP register map (VR940F)
Register 1: Flow setpoint (°C×10, read/write)
Register 5: Current COP (×100, read-only)
COP 350 = 3.50 (multiply by 0.01)
Modbus control vs SG Ready comparison:
SG Ready: 3 fixed states (Normal / Enhanced / Max)
Modbus: Variable setpoint writes every 60 seconds
e.g. 37°C (economy), 45°C (boost), 50°C (DHW)
For PV optimisation via Modbus:
PV surplus 1–3 kW → setpoint 39°C
PV surplus 3–6 kW → setpoint 45°C
PV surplus > 6 kW → setpoint 50°C (DHW)
Night rate tariff → setpoint 42°C (pre-heat buffer)Modbus setpoint control allows continuously variable flow temperature optimisation rather than the binary on/off of SG Ready states — improving COP during partial PV surplus conditions and enabling weather-compensated setpoint schedules.
Heat pump COP optimisation with KNX weather compensation
Heat pump COP is highest at mild outdoor temperatures and low flow temperatures. A Vitocal 250-A at 10°C outdoor and 35°C flow achieves COP 4.5–5.0; at -5°C outdoor and 50°C flow, COP drops to 2.5–3.0. KNX weather-compensated setpoint control maximises seasonal COP (SCOP).
Weather compensation logic — MDT Logic Module
Input: GA 8/2/0 (Outdoor temperature, °C, DPT 9.001)
Output: GA 11/1/0 (HP flow setpoint, °C, DPT 9.001)
→ Modbus write → VR940F register 1
Linear compensation curve:
Outdoor +15°C → flow setpoint 30°C (min HP output)
Outdoor +5°C → flow setpoint 37°C (mild heating)
Outdoor 0°C → flow setpoint 42°C
Outdoor -5°C → flow setpoint 47°C
Outdoor -10°C → flow setpoint 50°C (maximum)
ETS6 MDT Logic Module: linear interpolation function
Input range: -10°C to +15°C
Output range: 50°C to 30°C (inverse relationship)
Update interval: every 15 minutes (Modbus write)HP buffer tank and short-cycle prevention
Heat pumps require a buffer tank (50–200 litres depending on HP capacity) between the HP primary circuit and the secondary heating circuit. KNX buffer tank temperature monitoring prevents short-cycling by ensuring the HP runs for a minimum of 10 minutes per start.
Buffer tank controller logic — ETS6
Buffer tank top sensor: Pt1000 or NTC → MDT SCN-TM55P.02 temperature input module → GA 8/3/0 (Buffer Top Temperature, DPT 9.001) HP start condition: IF GA 8/3/0 < 35°C AND GA 11/2/0 (HP running) = 0 AND time since last stop > 10 minutes THEN write mode 1 (heating) to HP Modbus register 100 HP stop condition: IF GA 8/3/0 > 50°C THEN write mode 0 (standby) to HP Modbus register 100 Minimum run time interlock: HP start timestamp stored in ETS6 variable HP stop only permitted if (now - start) > 10 minutes Prevents compressor short-cycling — protects compressor
Buffer tank sizing: minimum buffer volume = HP capacity (kW) × 10 litres. A 10 kW heat pump requires a minimum 100-litre buffer tank. Undersized buffer tanks cause short-cycling regardless of KNX logic — the buffer temperature swings too quickly for the minimum run time interlock to be effective.
Commissioning test sequence
Perform the following test sequence after ETS6 download and Modbus gateway configuration. Test SG Ready states before enabling the live PV surplus logic.
| Step | Action | Expected result |
|---|---|---|
| 1 | Manually trigger S2=1 in ETS6 Group Monitor | HP display shows SG Ready State 3 — enhanced capacity mode |
| 2 | Trigger S1=1, S2=1 | HP runs at maximum power output — State 4 confirmed |
| 3 | Set both relays to 0 | HP returns to normal scheduling — State 2 |
| 4 | Write 40°C to flow setpoint Modbus register (if Modbus equipped) | HP flow temperature rises to 40°C within 15 minutes |
| 5 | Simulate PV surplus: set Fronius GA 9/0/1 to -4000W in Group Monitor | KNX logic triggers SG Ready State 3 automatically |
| 6 | Simulate PV surplus > 5000W | KNX logic triggers SG Ready State 4 automatically |
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