Home Assistant custom integration for Stiebel Eltron LWZ and Tecalor THZ integral heat pumps (e.g. LWZ/THZ 303/304/404) connected via their serial service interface (USB, 115200 baud) — no ISG/IP module required.

Protocol knowledge is derived from the FHEM module 00_THZ.pm (GPL-2.0, immi / Robert Penz et al.). This project is a clean re-implementation, not a port: one declarative register table, strict telegram serialization, robust reconnect handling and per-group poll intervals.

• ~50 sensors: all temperatures, pressures, humidity, fan speeds, thermal/electrical power, flow rate, operating hours, fault memory
• ~30 binary sensors: compressor, pumps, booster stages, valves, EVU release, defrost, filter-change and service indicators
• Write access (verified with read-back): operating mode, room and DHW setpoints, fan stages, unscheduled ventilation, heating-curve parameters
• climate entity (heating circuit thermostat) and water_heater entity
• 12 energy/heat meters, energy-dashboard ready
• Configurable poll intervals per register group (status / parameters / history / energy) — the serial interface is served strictly one telegram at a time, mirroring the field-proven FHEM timing
• Config flow with connection test, options flow, diagnostics, en/de translations
• Extras in dashboard/: a control dashboard, FHEM-style graph dashboard templates and a heating-curve custom card

• Home Assistant 2026.6 or newer
• The heat pump's USB service port mapped into the HA container (e.g. devices: ["/dev/lwz304:/dev/lwz304"] in docker-compose, ideally via a udev alias) — or a ser2net bridge (TCP)
• Exclusive port access: stop FHEM or any other reader first

Tested on a LWZ 304 Trend (firmware 5.09). Firmware profile 4.39/5.39 (LWZ/THZ 30x/40x); the 2.x firmware family is not supported.

1. HACS → Custom repositories → add https://github.com/mochlast/lwz-thz as type Integration
2. Install Stiebel Eltron LWZ / Tecalor THZ, restart Home Assistant

Copy custom_components/lwz_thz/ into your HA config/custom_components/ directory and restart.

Settings → Devices & Services → Add integration → Stiebel Eltron LWZ / Tecalor THZ → pick serial port (or ser2net host) — the connection is verified by reading the firmware version. Poll intervals and HC2/solar support are configurable via the integration's Configure dialog.

The protocol layer is HA-independent and ships with a CLI for bring-up and debugging:

uv run scripts/thz_cli.py ports
uv run scripts/thz_cli.py --port /dev/ttyUSB1 get sFirmware sGlobal
uv run scripts/thz_cli.py --port /dev/ttyUSB1 get all --dump
uv run scripts/thz_cli.py --port /dev/ttyUSB1 energy all
uv run scripts/thz_cli.py --port /dev/ttyUSB1 params
uv run scripts/thz_cli.py --port /dev/ttyUSB1 set p07_fan_stage_day 2
uv run scripts/thz_cli.py --port /dev/ttyUSB1 monitor --interval 60

Uses uv:

uv sync            # venv with dev dependencies (incl. HA test harness)
uv run pytest -q   # 130+ tests: protocol golden tests, HA component tests
uv run ruff check .

Repository layout:

• custom_components/lwz_thz/ — the integration
  • thzprotocol/ — HA-independent async protocol library (framing, handshake, transports, register tables, CLI); extractable to PyPI
• scripts/thz_cli.py — CLI wrapper
• tests/ — pytest suite
• dashboard/ — dashboard templates + heating-curve custom card. The control dashboard (heizung.json) is a Lovelace template built on the HACS cards Mushroom, button-card and stack-in-card; its advanced settings (weekly time programs, unscheduled-ventilation durations) open in browser_mod popups, so install browser_mod and add its integration before importing the dashboard.

This integration stands on the shoulders of the FHEM community, which reverse-engineered and maintained the THZ/LWZ serial protocol for over a decade:

• 00_THZ.pm — the FHEM THZ module by immi, based on the protocol analysis by Robert Penz
• FHEM wiki: Tecalor THZ Wärmepumpe
• FHEM forum thread — years of register documentation and firmware findings

License: GPL-2.0 (inherited from the FHEM reference).

This project is not affiliated with Stiebel Eltron or Tecalor. Writing parameters to your heat pump is at your own risk.