HomeMaster OpenTherm Gateway — ESPHome Smart Boiler Control

HomeMaster OpenTherm Gateway

OpenTherm or relay. Local control. No cloud. No lock-in.

Tech Specs

Note: Relay outputs are not internally fused. External overcurrent protection must be provided according to applicable standards and installation requirements.

Installation, Environmental & Mechanical

Home Assistant & ESPHome Integration

The OpenTherm Gateway comes with ESPHome pre-installed and supports direct integration with Home Assistant. Wi-Fi onboarding is available via Improv (USB Serial or BLE), without manual flashing for basic deployment.

Getting Started

Improv Wi-Fi Setup (Recommended)

1. Mount & Power - Install on DIN rail and connect power input.
2. Open Improv - Go to improv-wifi.com in Chrome or Edge.
3. Connect Device - Use USB-C (serial) or Bluetooth LE.
4. Enter Wi-Fi - Provide SSID and password, then connect.
5. Auto Discovery - Device appears in Home Assistant / ESPHome Dashboard.

Fallback Access Point

If the device cannot connect to your Wi-Fi network, it automatically starts a fallback Access Point named HomeMaster OT Fallback. This fallback mode is only active when normal Wi-Fi connection fails.

1. Power on the device and wait approximately 60 seconds.
2. Connect to: HomeMaster OT Fallback.
3. Open a browser and navigate to http://192.168.4.1.
4. Enter your Wi-Fi credentials and save.

• The captive portal page may open automatically.
• If it does not open, use http://192.168.4.1 manually.
• The fallback Access Point is active only when normal Wi-Fi connection fails.

One-Click Import (ESPHome Dashboard)

After provisioning, click Take Control in ESPHome Dashboard to import the official configuration from GitHub.

⚠️ Note: After taking control, vendor-managed OTA updates will stop working unless you keep the http_request, ota: platform: http_request, and update blocks from the original configuration in your customised YAML. If you remove these blocks, update via ESPHome OTA or USB instead.

Firmware / YAML Source

Official ESPHome configuration file: opentherm.yaml (GitHub)

Update manifest: manifest.json (GitHub)

• Native Home Assistant API integration
• OTA updates supported
• Improv setup via serial and BLE
• OpenTherm telemetry and control entities

Cabling and Wirings

Power Supply

The OpenTherm Gateway supports three input options: 24 V DC (V+ / 0V), 85-265 V AC (L / N), or 120-370 V DC (L / N + / -). Use only one power input method at a time.

24 V DC input wiring on V+ / 0V terminals.

AC mains input wiring on L / N terminals.

Relay Output Wiring

For boilers without OpenTherm, use the relay output to switch the boiler on/off. Also used to control zone valves, circulation pumps, or two-way and three-way valves.

The relay output exposes C and NC contacts only (normally-closed) and can be used to switch external actuators (zone valves, pumps, backup heaters) through proper external protection devices. System load limits: 3 A @ 250 VAC (resistive, system limit) · 750 VA @ 250 VAC maximum · 90 W @ 30 VDC maximum.

⚠️ External protection required: Relay output MUST be protected by external fuse or circuit breaker. Relay output is not internally fused.

⚠️ Relay output — use restriction (mandatory):
The relay output terminals (C, NC) shall be connected only to:

• the same mains supply circuit as the L/N input of this device, OR
• a SELV (Safety Extra-Low Voltage) circuit, OR
• a Limited Power Source (LPS) circuit per EN 62368-1.

Connection of the relay output to a different mains phase, an isolated mains source, or any circuit at a higher voltage class than the device's L/N input is not permitted. Failure to follow this restriction may cause dielectric breakdown between the device input and relay output circuits and presents electric-shock and fire hazards.

This restriction is required because the printed-circuit-board insulation between the L/N tracks and the C/NC tracks of the Relay board is rated as Basic per EN 62368-1 Table 11/14 (working voltage 250 V r.m.s.). Cross-mains use would require Reinforced insulation that the current hardware revision V1.0 does not provide. The next hardware revision will lift this restriction.

⚠️ Relay default state on power-up: The relay output is dry-contact and exposes only C and NC. When the device is unpowered or the relay is switched OFF, the NC contact is closed and the load is energized. Verify this is safe for your installation before connecting any boiler call-for-heat, pump, or valve to NC.

Relay dry-contact wiring example.

1-Wire Sensor Wiring

Use both 1-Wire buses to monitor multiple points — flow temperature, return temperature, buffer tank, or room sensors. Works independently of the boiler type.

Two independent 1-Wire channels are available for DS18B20-compatible temperature sensors. Use daisy-chain (bus) topology only — star wiring is not supported. Keep sensor stubs ≤ 0.5 m. See Cable Recommendations below for full wiring guidance.

⚠️ One sensor per bus by default. The shipped configuration does not pin sensor addresses. With multiple sensors on the same 1-Wire bus, ESPHome reads the first sensor it discovers — assignment is non-deterministic across reboots. For multiple sensors per bus, set explicit address: values in YAML (visible in ESPHome logs at boot).

1-Wire sensor bus connection example.

OpenTherm Bus Wiring

Connect OT+ and OT− to the boiler OpenTherm interface for full modulation control, setpoint management, fault monitoring and flame status reporting in Home Assistant.

Keep OT wiring separated from high-voltage conductors.

OpenTherm OT+ / OT- bus connection.

Cable Recommendations & Shield Grounding

General Routing Rules

• Route low-level signal cables (1-Wire/OT) separately from mains, relay output wiring, contactors, and power wiring.
• If crossing power cables is unavoidable, cross at 90 degrees.
• Keep cable runs as short as practical and avoid long parallel runs with high-current conductors.

Temperature Cable (1-Wire)

• Recommended: shielded 3-core (+5V / DATA / GND).
• High-EMI / long runs: shielded pairs + overall shield (e.g., LI2YCY PiMF 2x2x0.50).
• Keep sensor stubs ≤ 0.5 m. Use daisy-chain (bus) topology only — star wiring is not supported.

OpenTherm Cable

• Use twisted pair cable for OT+ / OT-.
• Overall shield recommended in industrial/noisy cabinets.
• Recommended cable types: J-Y(ST)Y 2×2×0.5 mm² or LI2YCY PiMF 2×2×0.50.

Shield Grounding

• Default recommendation: bond cable shield(s) to cabinet PE/EMC ground at controller side only.
• Do not connect shields directly to signal terminals (1-Wire/OT).
• If both ends are equipotential bonded cabinets, both-end shield bonding may be used with proper 360-degree clamps.

System Architecture & Pinout

OpenTherm Gateway functional block diagram.

Pinout and front terminal reference.

Safety and Installation Notes

• Disconnect all power before installation or wiring changes.
• Use only one power input method at a time (24 V DC or AC/DC L/N input).
• Install only inside a closed control cabinet; protect all terminals from accidental contact.
• Use external overcurrent protection (fuse/breaker) for relay circuits and mains inputs.
• Relay outputs are not internally fused; use external contactors for loads above 3 A and for high inrush/inductive loads.
• Relay output use restriction: terminals C and NC shall be connected only to the same mains supply as L/N, or to SELV / Limited Power Source circuits. Cross-phase or cross-source mains connection is not permitted with hardware revision V1.0.
• L / N terminals may carry hazardous voltage; installation by qualified personnel only.
• Follow local electrical code and boiler manufacturer OpenTherm wiring requirements.

Compliance & Certifications

The OpenTherm Gateway module is CE marked. ISYSTEMS AUTOMATION S.R.L. (HomeMaster® brand) maintains the technical documentation and a signed EU Declaration of Conformity (DoC) available for download in the Documents and Resources section above.

• EMC Directive 2014/30/EU — EN 55032:2015 (Class B emissions), EN 55035:2017 (immunity), tested by Idvorsky Laboratories Ltd. (Job #1648)
• Low Voltage Directive 2014/35/EU — EN 62368-1:2020 + A11:2020, in-house dielectric and isolation testing by ISYSTEMS AUTOMATION compliance laboratory
• RoHS Directive 2011/65/EU — EN IEC 63000 technical documentation
• HomeMaster® — registered EU trademark (EUTM No. 019082911, EUIPO, registered 15 January 2025)