Part 2 of the series "Smart Home in Practice". Part 1 made the case for running this home locally. Parts 3, 4 and 5 show the implementation: wiring, register hunting, diagnostics.
On 21 July 2026 at 05:05, my heat pump stopped talking to me. It kept heating, stubbornly and reliably as ever, but it stopped answering: the cloud gateway connecting the unit to the vendor app and to Home Assistant no longer got a network connection. Seen from the outside, the machine simply vanished from one moment to the next.
My hub's recorder history shows the cut precisely: 588 state changes until 21 July, then nothing. Four entities disappeared from the system: the climate control with operating mode and flow target, the hot water control, silent mode and the main switch. Five automations that heated by season and price and pushed hot water into solar surplus have been idle ever since.
The backstory: a maintenance contract and many emails
The unit, an LG Therma V R290 Monobloc, was installed by Thermondo, together with a maintenance contract and the cloud connection. My experience with their support: there is no phone number with a human behind it, only email, and replies to my requests came late or not at all. When the maintenance contract then went up in price considerably, I cancelled it.
This is where the story becomes relevant for anyone who does not own a Thermondo-installed heat pump: a broken piece of cloud hardware plus a support desk that does not respond means, in the cloud model, that functions you paid for disappear without replacement. It is exactly the dependency argument from Part 1, except that this time it hit the most expensive device in the house instead of an eight-euro Zigbee sensor.
I wanted to get away from vendor clouds anyway. The gateway failure turned that intention into a deadline.
What a heat pump actually does
For everyone who checks out at the words "refrigerant circuit", here is the short model, which is all the rest of the series needs. A heat pump is a fridge running backwards. A refrigerant (in modern units often R290, better known as propane) evaporates outside at low temperature, absorbing heat from the outdoor air. A compressor squeezes the gas, which makes it hot. It hands that heat over to the heating water, condenses back into a liquid, and starts over. The electricity the unit consumes essentially drives the compressor, while the larger share of the heating energy comes from the air.
Four terms are enough to read on: flow is the warm water heading to your heating, return is the cooled water coming back. The compressor is the heart of the machine and the only part that consumes serious power. And the unit serves two separate jobs: the heating circuit for the house and the domestic hot water in the tank.
The real depth, with pressures, superheat and what they reveal, follows in Part 5, backed by actual measurements.
The ways back to control
Without the cloud gateway there are several ways to reach the unit again. I checked them all:
- A new cloud gateway would have been the most convenient path: set up the integration, everything works again. The price is the next round of the same dependency, so I ruled it out.
- The official Modbus gateway (LG PMBUSB00A) translates between the unit and a local bus, but costs a three-digit sum and is hard to get hold of.
- SG-Ready is the simplest local interface: two switching inputs through which an energy manager recommends operating states to the unit, from blocking it to overshooting. No data comes back, but it is robust and standardised across manufacturers.
- The third-party controller port on the indoor unit: an RS485 terminal behind which the unit speaks Modbus RTU. That is an industrial protocol from the seventies: simple, local, no cloud involved. All it takes is a microcontroller board with an RS485 transceiver for around 35 euros.
It became the last path, with an ESP32 and the open-source firmware ESPHome. The manufacturer intends the port for external controllers. Still, connecting wires inside the indoor unit is work on a heating system. It can affect warranty and statutory guarantees, and the wiring happens at your own risk. When in doubt, it belongs in the hands of a qualified installer. Live parts are off limits either way: the data terminals sit on a separate terminal block.
What came out of it
So much for the decision. The implementation turned out to be a story of its own. The short version: the unit does speak Modbus, but it follows none of the published register maps, and the documentation is silent about what it does instead. What followed was detective work with measurements instead of a manual, and the result is more than the cloud ever offered: all of the unit's temperatures, refrigerant circuit pressures and superheat, operating mode, silent mode and hot water locally readable and switchable, as ready-made firmware on GitHub.
Part 3 shows the wiring and the three details without which not a single byte flows. Part 4 tells how to identify unknown Modbus registers systematically without endangering the machine. And Part 5 walks through what the new diagnostic values reveal about the unit.
Sources
- LG: Therma V R290 Monobloc (German), product page of the series
- LG: Therma V R290 Monobloc, installation manual (PDF, German), terminal assignments and energy states
- German Heat Pump Association: SG Ready (German), the standardised interface for energy managers
- ESPHome: project website, open-source firmware for ESP32 microcontrollers
- GitHub: lg-therma-v-esphome-modbus, the finished firmware including register map and docs