Register 5000 always returns 32768. What does that mean?

The value 32768 (0x8000) is the ISG substitute value for an unavailable object. This usually means SG Ready is not enabled in the ISG Web interface. Navigate to Settings, Energy Management and set SG Activated to ON and SG Ready Input to Modbus, then retry.

The gateway connects but the temperature register shows implausible values.

The WPM temperature registers use a scaling factor of 10: the raw value 203 represents 20.3 degrees. If your dashboard shows values ten times too large, apply a factor-of-0.1 scaling in your channel configuration. Also confirm the ISG addressing offset: ISG addresses are 1-based, so some tools subtract 1 from the documented address.

Writing to setmode_a or setmode_b has no visible effect on the heat pump.

Two preconditions must both be met before Modbus writes take effect: SG Ready must be activated in the ISG Web energy management settings, and the WPM must be in programmed-operation mode (Programmbetrieb). Confirm both are set before writing to the holding registers.

Is it safe to write any value to the SG Ready setpoint registers at any time?

Write carefully. Sending SG Ready state 4 (maximum heating) for extended periods can push flow temperatures above the heat pump's comfort limits and may affect wear on the compressor. Always validate the target state against your system's operating limits, and restore the default state (2) after a PV surplus event ends.

Free Modbus template: Stiebel Eltron WPM (SG Ready)

The Stiebel Eltron WPM is the heat pump manager and central controller for Stiebel Eltron heat pump systems, covering space heating, cooling, and domestic hot water. This template connects to the WPM via the ISG Web Internet Service Gateway using Modbus TCP on port 502. It reads the active SG Ready operating state and a temperature sensor value, and writes the SG Ready enable flag and channel B input to shift the heat pump into accelerated or maximum-heating mode during PV surplus periods. Supported controllers include WPMsystem, WPM3, and WPM3i.

Name	Address	Function	Type	Unit	Access
getmode Current SG Ready operating state, 1 to 4.	5000	FC04	uint16		R
setmode_a SG Ready enable flag: write 1 to activate Modbus control.	4001	FC16	uint16		W
setmode_b SG Ready input channel B state, write 0 or 1.	4002	FC16	uint16		W
temp Temperature sensor reading, scaled by factor 10.	521	FC04	int16	°C	R