SG Ready Heat Pump Grid Optimisation with KNX: Four Operating States

SG Ready standard: four operating states

The SG Ready interface (VDE 0755-95, published by BAFA Germany) uses two binary input contacts (S1 and S2) to define four operating states for the heat pump. The heat pump manufacturer implements the response to each state — the interface itself is purely digital (voltage-free contacts). Both inputs are read by the heat pump control board, typically via low-voltage 24V dry contact terminals labelled SG Ready, Smart Grid, or S1/S2.

KNX wiring: MDT AKD-0800.01 relay actuator

The MDT AKD-0800.01 is an 8-channel DIN rail relay actuator (230V, 16A per channel, KNX TP). For SG Ready use, two channels are wired to the heat pump S1 and S2 terminals. The SG Ready inputs on the heat pump side are dry contact inputs — they do not carry mains voltage. Use the normally-open (NO) contact of the MDT relay channel, connecting one relay terminal to the SG Ready terminal and the other to the 24V supply or neutral as specified by the heat pump manufacturer.

MDT AKD-0800.01 (Channel 1 = S1, Channel 2 = S2):
  Channel 1 NO terminal → Heat pump S1 terminal
  Channel 1 COM terminal → Heat pump SG Ready COM/24V
  Channel 2 NO terminal → Heat pump S2 terminal
  Channel 2 COM terminal → Heat pump SG Ready COM/24V

KNX Group Addresses:
  GA 3/2/1 — DPT 1.001 — S1 relay (Channel 1)
  GA 3/2/2 — DPT 1.001 — S2 relay (Channel 2)

State mapping (write to both GAs simultaneously):
  State 1 (block):   GA 3/2/1 = OFF, GA 3/2/2 = OFF
  State 2 (normal):  GA 3/2/1 = ON,  GA 3/2/2 = OFF  (default)
  State 3 (PV/cheap): GA 3/2/1 = OFF, GA 3/2/2 = ON
  State 4 (forced):  GA 3/2/1 = ON,  GA 3/2/2 = ON

ETS6 configuration:
  AKD-0800.01 Channel 1: switch object, GA 3/2/1
  AKD-0800.01 Channel 2: switch object, GA 3/2/2
  Power-on behavior: restore last value (ensures State 2 after KNX restart)

Heat pump brands supporting SG Ready

SG Ready compliance is required for BAFA subsidy eligibility in Germany and is increasingly standard across European heat pump brands. The four-state behaviour varies slightly between manufacturers in how aggressively they respond to State 3 and State 4 — check the specific heat pump service manual for setpoint offset values.

PV surplus State 3: Victron Cerbo GX Modbus trigger

The Victron Cerbo GX (central hub for Victron ESS systems) exposes real-time grid power via Modbus TCP on register 820 (com.victronenergy.grid: AC Power, W, signed). A negative value means the system is exporting power to the grid — surplus solar after battery and loads have been served. This is the cleanest signal for SG Ready State 3 activation: it accounts for battery state and existing loads without complex recalculation.

Victron Cerbo GX Modbus TCP: IP 192.168.1.40, port 502
Register 820 (unit ID 100): Grid AC Power (W), int16, signed
  Positive = importing from grid
  Negative = exporting to grid (solar surplus)

MDT Modbus Gateway configuration:
  Poll register 820 every 10s → KNX GA 9/0/5 (DPT 9.001, W)

KNX logic (MDT SCN-LCRM.01):
  Input: GA 9/0/5 (grid power, W)
  Condition: value < -500 (exporting more than 500W) for 120s
    → Write GA 3/2/1 = OFF (S1 open)
    → Write GA 3/2/2 = ON  (S2 closed)
    → SG Ready State 3 activated

  Condition: value > -200 (surplus dropped below 200W) for 300s
    → Write GA 3/2/1 = ON  (S1 closed)
    → Write GA 3/2/2 = OFF (S2 open)
    → Return to SG Ready State 2 (normal)

120s activation delay: prevents State 3 on brief solar clouds
300s deactivation delay: allows heat pump to complete cycle

The result: when there is more than 500W of solar surplus for 2 continuous minutes, the heat pump raises its DHW setpoint from 55 degrees C to 60 degrees C and its heating setpoint by 2-3K, consuming the surplus solar electricity for thermal storage in the water cylinder and building mass. This is effectively free hot water from solar that would otherwise be exported at a low feed-in tariff rate.

Grid peak shedding: State 1 for demand charge avoidance

In buildings with a demand-charge tariff (grid operators billing for 15-minute peak demand in kW, in addition to energy in kWh), activating SG Ready State 1 stops the heat pump compressor and reduces peak power draw. The heat pump contributes 2-12 kW depending on model — significant on a 45 kW demand threshold.

The VDE 0755-95 specification limits continuous State 1 (hard block) to a maximum of 2 hours per day to protect heat pump components and occupant comfort. The KNX logic must enforce this time limit — implement a timer that automatically returns to State 2 after 30 minutes of State 1 even if the demand threshold is still exceeded. The thermal mass of the building typically provides 30-60 minutes of heating comfort buffer before room temperatures noticeably drop.