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Quick Integration Guide: HomeMaster OpenTherm Gateway & Viessmann Vitodens 100-W

The HomeMaster OpenTherm Gateway is a DIN-rail mountable ESP32 device pre-flashed with ESPHome. It acts as a translator between Home Assistant (via Wi-Fi) and your boiler's OpenTherm digital bus.

1. Physical Installation & Wiring

⚠️ Safety Warning: Always isolate the mains power supply to the boiler before opening the casing.

+──────────────────────────┐             +──────────────────────────┐
│  HomeMaster Gateway      │             │  Viessmann Vitodens 100-W│
│                          │  2-core     │                          │
│     [ OT1 ] [ OT2 ] ─────┼─────────────┼─────> [ OT ] Terminals   │
│   (Polarity Independent) │             │  (On main control board) │
└──────────────────────────┘             └──────────────────────────┘

1. Mounting: Clip the HomeMaster Gateway onto your DIN rail or secure it in an electrical enclosure near the boiler.

2. Wiring to Boiler: Run a standard 2-core cable (e.g., 0.75mm² alarm wire or bell wire) from the OT1 / OT2 terminals on the HomeMaster gateway to the OpenTherm (OT) terminals on the Viessmann main PCB board. Note: OpenTherm is polarity-independent; it doesn’t matter which wire goes to which terminal.

3. Power: Power the HomeMaster gateway using its designated power input (typically a 5V micro-USB/USB-C or a 12V/24V DC terminal block, depending on your exact sub-model).

2. Boiler Commissioning (Crucial step)

The Vitodens 100-W will ignore the physical OpenTherm connection until you toggle its internal operating profile.

1. Press the Menu and OK buttons simultaneously for approximately 4 seconds to enter the Service/Installer Menu.

2. Scroll to parameter C.4 (Operating Mode).

3. Change the value from its default factory setting to 14 (OpenTherm Mode).

4. Save and exit. Restart the boiler if prompted.

3. Home Assistant Configuration

Because the HomeMaster comes pre-flashed with ESPHome, integration is almost completely automated:

1. Power on the HomeMaster gateway.

2. Open Home Assistant. Go to Settings > Devices & Services.

3. The gateway will appear as a "Discovered" ESPHome device. Click Configure and add it.

4. This will instantly expose entities such as climate.opentherm_hub, sensor.boiler_flame_status, sensor.boiler_water_temperature, and controls for your target flow temperature (number.boiler_max_target_flow_temp).

⚠️ Critical Viessmann OpenTherm "Gotchas"

The combination of the Viessmann firmware and OpenTherm protocol introduces a few strict rules that will prevent your system from working if ignored:

Gotcha 1: The "No Ignition" Status Trap

Viessmann’s firmware is incredibly protective. If Home Assistant sends a call for heat (ch_enable: true), the boiler will report it is active but the burner will not light (0% modulation) unless it simultaneously receives a specific set of dummy room metrics.

• The Fix: In your ESPHome configuration or automation, you must continuously pass a valid Room Temperature (t_room) and Target Room Temperature (t_room_set). If these fields are missing or read 0, the boiler assumes a telemetry error and refuses to ignite.

Gotcha 2: The DHW (Hot Water) Priority Override

On the Vitodens 100-W, Domestic Hot Water (DHW) production takes absolute hardware priority over Central Heating (CH).

• The Behavior: When a tap is turned on or a hot water cylinder is heating, the boiler will completely ignore OpenTherm commands sent by Home Assistant and pump the flow temperature to maximum to satisfy DHW.

• The Fix: Ensure your Home Assistant automations expect sensor.boiler_water_temperature to suddenly spike during DHW usage, and do not let your multi-room zone logic misinterpret this as a runaway central heating loop.

Gotcha 3: The Minimum Flow Temp Cycling Loop

Modulating boilers hate short-cycling (turning on and off rapidly). If your Home Assistant smart TRVs only call for a very low flow temperature (e.g., 35°C / 95°F) because only one tiny room is slightly cold, the Vitodens 100-W's physical minimum firing rate might be higher than the heat that room can dissipate.

• The Behavior: The boiler fires up, instantly overshoots the 35°C target because the volume of water is too small, shuts off, waits 3 minutes, and tries again. This will rapidly wear out the boiler's spark igniter.

• The Fix: Implement a logic floor in Home Assistant. Do not engage a "Call for Heat" unless the calculated target flow temperature is at least 40°C (104°F), or ensure at least two radiator zones are open simultaneously to give the thermal mass somewhere to go.