Reading a heat pump via Modbus gives you dozens of status bits: compressor on, hot water, defrosting, heating element, cooling. It is tempting to build automations and calculations on them straight away. On my LG Therma V R290, three of those LG Therma V Modbus bits could not be trusted.
1. Hot water mode
What I saw: the hot water tank heated up from 21 to 49 °C, while the bit for hot water mode was off.
What I do now: hot water is “active” if the bit is on, or if the tank heats up faster than 3 K per hour while the compressor is running. That derived sensor does lag a few minutes behind; so don’t tie it to a snapshot from another meter.
2. The electric heating element
What I saw: during disinfection (heating the tank to 60 °C every so often to prevent legionella) the bit for the tank heating element turned on. Home Assistant counted 3 kW of electrical consumption, and the “Hot water” status stayed stuck afterwards. But there is no element connected in my tank. The compressor reached the 61 °C by itself.
The explanation: the LG drives the relay for an element, even if nothing is connected to it. The bit says what the controller wants, not what actually happens.
What I do now: a switch in Home Assistant, “tank element connected”, is off. As long as it is off, the element bit counts nowhere: not in the COP, not in the costs and not in the hot water detection.
3. Cooling
What I saw: the cooling bit is always on, even with cooling not enabled on the Control Unit.
What I do now: the operating state in Home Assistant looks at the register for the operating mode (0 is cooling), not at that bit.
Two more things
- Without contact with the outdoor unit, the LG reports 1200 °C for flow and return. See keeping error values out of your graphs.
- Turning off the disinfection coil does not abort a running disinfection. The LG’s internal schedule finishes it.
The lesson: check LG Therma V Modbus bits first
First compare every status bit with a measurement you can explain yourself (a temperature that rises, a power reading that runs) before you build anything on it. And what you see on the hardware itself takes priority over what the data appears to say.
More about the LG: Heat pump via Modbus. All incidents: Lessons from practice (in Dutch).
These values apply to one installation. Check them against your own documentation; leave work on 230/400 V to a qualified electrician.
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