LG ThermaV (2021+)

Modbus RTU

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Last updated: 03. 2026

The LG ThermaV (2021+ models) is an air-to-water heat pump that communicates with TapHome via Modbus RTU over RS-485. The template provides heating/cooling mode control, domestic hot water (DHW) on/off switching, five temperature sensors (water inlet, water outlet, room air, DHW tank, solar collector), and two target temperature setpoints for space heating and DHW.

The heat pump connects directly to TapHome Core via RS-485 without requiring the PMBUSB00A Modbus gateway — the outdoor unit has a built-in Modbus RTU slave interface.

Hardware connection

RS-485 wiring

Connect the RS-485 A and B signal lines from the outdoor unit PCB to TapHome Core:

A (RS-485+) → A+/D+ on TapHome RS-485
B (RS-485-) → B-/D- on TapHome RS-485

Use shielded twisted pair cable for the A/B signal lines.

Important — Register type swap in LG documentation
The official LG installation manual has the holding register and input register function codes swapped. What the manual labels as “Holding register (0x03)” is actually an input register, and vice versa. Community sources have confirmed and corrected this. The TapHome template uses the correct register addresses.

Technical specifications

Protocol: Modbus RTU Slave, RS-485
Baud rate: 9600, no parity, 1 stop bit, 8 data bits
Default Slave ID: 1 (configurable via thermostat display)

Configuration

Enabling Modbus communication

Modbus communication must be enabled via the DIP switch SW1 located inside the outdoor unit:

Power off the outdoor unit
Open the outdoor unit electrical panel
Locate DIP switch SW1 on the PCB
Set the following DIP switch positions:
- DIP 1: ON — enables Modbus interface
- DIP 2: ON — slave mode
On SW2, set DIP 1: ON — slave designation
Power on the outdoor unit

Slave ID configuration

The default Slave ID is 1. To change it, navigate to the thermostat display settings. When importing the template in TapHome, enter the matching Slave ID in the import parameters.

The Slave ID can also be set to 2 on some model variants. If communication fails with Slave ID 1, try Slave ID 2.

Modbus settings in TapHome

Configure the TapHome Modbus RTU module with:

Baud rate: 9600
Parity: None
Stop bits: 1
Data bits: 8
Slave ID: 1 (or as configured on the heat pump)

Device capabilities

Heating and cooling mode

Heating & Cooling Mode (C:0 + H:0) — a combined multi-value switch that controls both the power state and operation mode of the heat pump. The switch reads coil register C:0 for the on/off state and holding register H:0 for the active mode.

Available modes:

Value	Mode	Description
Off	Stop	Heat pump turned off (C:0 = 0)
Auto	Automatic	Automatic heating/cooling selection (H:0 = 3)
Heating	Heating	Space heating mode (H:0 = 4)
Cooling	Cooling	Space cooling mode (H:0 = 0)

The write logic handles the power state transition: when switching from Off to any mode, the template first enables the operate coil (C:0 = 1) before setting the operation mode.

Domestic hot water control

DHW On/Off (C:1) — enables or disables domestic hot water production. This is a simple on/off switch using coil register 1. When enabled, the heat pump heats the DHW tank to the target temperature.

Temperature monitoring

The template provides five temperature sensors, all using automatic /10 conversion (raw register value divided by 10 to get °C):

Water Inlet Temperature (Input 2) — water temperature entering the heat pump from the heating system
Water Outlet Temperature (Input 3) — water temperature leaving the heat pump to the heating system
Room Temperature (Input 7) — room air temperature from heating circuit 1, polled every 15 seconds
DHW Tank Temperature (Input 5) — domestic hot water tank temperature
Solar Collector Temperature (Input 6) — solar collector temperature, available only on AWHP models with a solar collector installation, polled every 15 seconds

Target temperature setpoints

Two temperature setpoints allow direct control from TapHome:

Target Temperature (Circuit 1) (H:2) — heating/cooling target temperature for the primary heating circuit. Range 30–50°C for heating. The register uses Int16 with /10 scaling (value × 10 for write, /10 for read).
DHW Target Temperature (H:1) — domestic hot water target temperature. Range 45–60°C. Uses the same /10 scaling.

The target temperature setpoints can be adjusted directly from TapHome and will be reflected on the heat pump thermostat display.

Additional capabilities

The LG ThermaV heat pump also supports silent mode (reduced outdoor unit noise), DHW tank disinfection (anti-legionella cycle), emergency stop, and emergency operation (backup heater only) via additional coil registers. The system provides extensive read-only status information through discrete input registers, including water flow, compressor, defrost cycle, DHW heating, solar pump, and backup heater status. Additional sensor registers for outdoor air temperature, water pressure, compressor RPM, and refrigerant pressures are available. A second heating circuit with independent target temperature is also supported. These can be added in a future template update.

Troubleshooting

No Modbus communication

Verify DIP switches: Confirm SW1 DIP 1 and DIP 2 are both ON, and SW2 DIP 1 is ON
Check RS-485 cables: Ensure A and B are correctly connected and not swapped
Confirm Slave ID: Default is 1 — verify TapHome import parameter matches the heat pump setting
Check baud rate: Must be 9600 baud, no parity, 1 stop bit in TapHome

Register type confusion

The official LG installation manual has holding and input register function codes swapped. The TapHome template already uses the correct (community-verified) register addresses. If referencing the LG manual directly, swap the register types.

Temperature readings showing incorrect values

All temperature registers use a scale factor of 0.1 (divide by 10). If raw values appear in TapHome, verify the template is correctly imported with the /10 conversion scripts.

Communication intermittent or unstable

Some LG ThermaV model variants or firmware versions have limited Modbus support
USB RS-485 adapters must explicitly support Modbus RTU protocol
“Cleanup recv buffer before send” errors indicate configuration issues — verify baud rate and parity settings

Available devices

LG ThermaV Module

Heating & Cooling Mode Multi-value Switch

Combined power and operation mode switch — Off, Auto, Heating, Cooling

Values / States: Off · Auto · Heating · Cooling · Heating

Heating & Cooling Mode

Read switch state

var operate := MODBUSR(C, 0, Bool);
var operateMode := MODBUSR(H, 0, Int16);

if operate = 0
    return(0); # off
else
    if operateMode = 3 
        return(1); # auto
    elseif operateMode = 4 
        return(2); # heating
    elseif operateMode = 0 
        return(3); # cooling
    else
        return(NaN);
    end
end

Write switch state

var operate := MODBUSR(C, 0, Bool);
var operateMode := Mu;

if operate = 0 and operateMode > 0
    MODBUSW(C, 0, Bool, 1); # turn on
end

if operateMode = 0
    MODBUSW(C, 0, Bool, 0); # off
elseif operateMode = 1 
    MODBUSW(H, 0, Int16, 3); # auto
elseif operateMode = 2 
    MODBUSW(H, 0, Int16, 4); # heating
elseif operateMode = 3 
    MODBUSW(H, 0, Int16, 0); # cooling
else
    # magic happened
end

DHW On/Off Switch

Enables or disables domestic hot water production

Water Inlet Temperature Temperature Sensor Read-only

Water Outlet Temperature Temperature Sensor Read-only

Room Temperature Temperature Sensor Read-only

DHW Tank Temperature Temperature Sensor Read-only

Domestic hot water tank temperature

Solar Collector Temperature Temperature Sensor Read-only

Solar collector temperature — available on AWHP models with solar installation only

Target Temperature (Circuit 1) Variable

Heating/cooling target temperature setpoint for the primary circuit (30–50°C)

DHW Target Temperature Variable

Domestic hot water target temperature setpoint (45–60°C)

Connection: Modbus RTU • 9600 baud• 8N1 • Slave ID: $[SlaveId]

Possible improvements (20)

C:2 Silent Mode — Coil 2 — 0=Off, 1=On; reduces outdoor unit noise at night
C:3 DHW Disinfection — Coil 3 — triggers anti-legionella tank disinfection cycle
C:4 Emergency Stop — Coil 4 — emergency stop of heat pump operation
C:5 Emergency Operation — Coil 5 — triggers emergency operation (backup heater only)
H:0 (Control Method) Control Method — Holding 1 in community sources — 0=Water outlet, 1=Water inlet, 2=Room air temperature control
H:4 Shift Value Auto Mode (Circuit 1) — Auto mode temperature shift -5 to +5 K for circuit 1
H:5 Target Temperature (Circuit 2) — Heating/cooling setpoint for second heating circuit, range 20–40°C, scale 0.1
H:7 Shift Value Auto Mode (Circuit 2) — Auto mode temperature shift -5 to +5 K for circuit 2
Input 0 Error Code — Error code register — 0=no error, other values reference LG error table
Input 1 ODU Operation Cycle — Outdoor unit operation cycle counter
Input 4 Backup Heater Outlet Temperature — Backup heater outlet temperature, scale 0.1 °C
Input 8 Current Flow Rate — Water flow rate in L/min, scale 0.1
Input 9 Flow Temperature (Circuit 2) — Flow temperature for second heating circuit, scale 0.1 °C
Input 10 Room Air Temperature (Circuit 2) — Room air temperature from circuit 2, scale 0.1 °C
Input 12 Outdoor Air Temperature — Outdoor ambient air temperature, scale 0.1 °C
Input 13 Water Pressure — System water pressure in bar, scale 0.1
Input 22 High Pressure — Refrigerant high-side pressure in bar
Input 23 Low Pressure — Refrigerant low-side pressure in bar
Input 24 Compressor RPM — Compressor speed, raw value × 60 = RPM
Discrete 0–16 Status Registers (17 discrete inputs) — Water flow, water pump, compressor, defrost, DHW heating, silent mode, cooling, solar pump, backup heater steps, error status, emergency operation availability, mix pump — all binary read-only status flags

Download Template

Sources

LG PMBUSB00A Modbus RTU Gateway — Register Map
www.lg.com 2026-03-18
PDF
LG ThermaV Modbus Setup Guide — DIP switches, register corrections
www.yourwizblog.com 2026-03-18
Home Assistant LG ThermaV Modbus Integration — Complete register map
github.com 2026-03-18