Heatpump Modbus Register 117: Outdoor Unit Low Pressure

Technical Definition

The register Outdoor Unit Low Pressure at address 117 is used to monitor outdoor unit low pressure on the heatpump.

• Protocol: Modbus RTU
• Data Type: U WORD
• Unit: kPa
• Access: Read Only

How to Read (Python pymodbus)

Use this code snippet to read this specific value via RS485:

from pymodbus.client.sync import ModbusSerialClient

client = ModbusSerialClient(method='rtu', port='/dev/ttyUSB0', baudrate=9600)
client.connect()

# Read Outdoor Unit Low Pressure (Address: 117)
# Note: Check if your device uses 0-based or 1-based addressing
result = client.read_input_registers(address=117, count=2, unit=1)

if not result.isError():
    print(f"Raw Value: {result.registers}")
else:
    print("Error reading register")

client.close()

Integration (Home Assistant)

Paste this into your configuration.yaml:

- platform: modbus_controller
  name: "Outdoor Unit Low Pressure"
  address: 117
  value_type: U_WORD
  unit_of_measurement: "kPa"

Connection Diagram

Here is the standard RS485 wiring for this configuration:

graph TD
    PLC[Host Controller / Home Assistant]
    Device[heatpump]

    PLC -- "A+ (D+)" --> Device
    PLC -- "B- (D-)" --> Device
    PLC -- GND --> Device

💡 Engineer's Insight

Analysis: Outdoor unit low pressure. Units: Bar/kPa. Monitors the low-pressure side of the refrigerant loop.

Troubleshooting: Zero might indicate a pressure sensor failure, loss of refrigerant, or system shutdown.