HomeMaster OpenTherm Gateway — ESPHome Smart Boiler Control

Domestic Hot Water

The gateway controls hot water over OpenTherm the same way it controls heating. In Home Assistant you get a Boiler DHW Enable switch, a Boiler DHW Setpoint number (35–65 °C, 1 °C steps) and two sensors that are off by default until you enable them: Boiler DHW Temperature and Boiler DHW Flow Rate (l/min). A Boiler DHW Active binary sensor shows when the boiler is heating water.

With a combi boiler that is all you need: set the tap-water temperature from Home Assistant, turn hot water off while you are away, see when the boiler switches to DHW.

With a single-circuit boiler and an indirect cylinder the boiler stays in charge of the cylinder: it runs the 3-way valve, decides DHW priority and any anti-legionella cycle. The gateway sets the DHW setpoint and enable and reports what the boiler is doing. It does not drive the valve itself and has no dedicated cylinder sensor input — one of the two 1-Wire inputs is free for that if you want the cylinder temperature in Home Assistant.

When Home Assistant Is Down

The setpoints are ESPHome number entities that live on the gateway, not in Home Assistant. If Home Assistant is unreachable, the gateway keeps sending the last CH and DHW setpoints to the boiler and the boiler keeps heating. If the gateway itself loses power, the boiler stops receiving OpenTherm messages and falls back to its own panel, as the OpenTherm standard requires — it does not stop heating. The relay contact (C–NC) is closed when the gateway is unpowered, so a pump or valve wired to it stays energised.

What does not survive a Home Assistant outage: automations that recompute the setpoint, for example the weather-compensation curve from the build article. The last value they wrote stays in force. For a heating controller in winter this is the point: no cloud in the loop, and the boiler never depends on a server being up.

Which Boilers

Any boiler with an OpenTherm terminal. Boilers on eBus, Navien or other proprietary buses are not supported over OpenTherm. Boilers without OpenTherm can be switched on and off through the relay output — no modulation, but the same Home Assistant control. To see which OpenTherm data your boiler actually reports, run the OpenTherm scanner first.

Weather Compensation

The stock firmware exposes the CH flow setpoint; the outdoor-reset heating curve is added with the configuration from Weather-compensated heating with the OpenTherm Gateway. It needs an outdoor temperature source — a DS18B20 on one of the 1-Wire inputs, or a sensor from Home Assistant.

FAQ

Does the OpenTherm Gateway control hot water in Home Assistant?

Yes — DHW enable, DHW setpoint 35–65 °C, and DHW temperature and flow rate sensors.

Does it work with a single-circuit boiler and an indirect cylinder?

Yes. The gateway sets the DHW setpoint and enable; the boiler runs the cylinder, the valve and priority itself.

What happens to the boiler when Home Assistant is down?

The gateway keeps the last setpoints and the boiler keeps heating; if the gateway loses power the boiler falls back to its own panel.

Is there a failsafe?

The relay's NC contact is closed when the gateway is unpowered, and the boiler reverts to panel control without OpenTherm messages.

OpenTherm or a relay thermostat?

OpenTherm modulates the burner and sets the flow temperature; a relay only switches the boiler on and off. Use OpenTherm where the boiler has it, the relay where it does not.

Which boilers are compatible?

Any boiler with an OpenTherm terminal; eBus and proprietary buses are not.

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 gateway is not an RS-485 device. It talks OpenTherm to the boiler and reaches Home Assistant over the network on its own, so it works with or without a MiniPLC or MicroPLC in the panel. To size a panel that also carries RS-485 modules, use the System Builder.

New to this kind of system? Two guides cover the ground: building a Home Assistant automation system and a wired ESPHome smart home. Two guides go deeper: weather-compensated heating and a full OpenTherm register scan.

Going deeper on this module: what still works when Home Assistant is down, wiring a new house before the walls close, weather-compensated heating, STR-3221-R1, AIO-422-R1 and the MiniPLC.

Connectors & terminal map

Top row (1 WIRE | OT)

Pos	Label	Group	Function
1	Gnd	ONEWIRE	1-Wire ground
2	D1	ONEWIRE	1-Wire bus 1 data
3	D2	ONEWIRE	1-Wire bus 2 data
4	+5V	ONEWIRE	1-Wire sensor supply, max 50 mA
5	O+	OT	OpenTherm bus positive
6	O-	OT	OpenTherm bus negative

Bottom row (24Vdc | 220Vac | RELAY)

Pos	Label	Group	Function
1	0V	POWER_DC	24 V DC return
2	+V	POWER_DC	24 V DC input
3	L	POWER_AC	AC line 85-265 V (MAINS)
4	N	POWER_AC	AC neutral (MAINS)
5	C	RELAY	Relay common
6	NC	RELAY	Relay normally closed

Ports & service interfaces

Id	Type	Note
USB-C

Housing notes

• No RS-485 bus. The gateway talks OpenTherm to the boiler and reaches the controller over the network.
• Relay exposes C and NC only. There is no NO terminal and the contact is CLOSED when the relay is de-energised, so the load is powered by default.
• Two 1-Wire buses (D1, D2) share one Gnd and one +5V.
• The +5V rail is 50 mA - one third of the WLD and STR rails.
• Dual supply, 24 V DC or 230 V AC. Use one at a time.

Cabling and Wirings

Power Supply

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

Relay Output Wiring

⚠️ NC relay (reserve): The relay output is dry-contact and exposes only C and NC (no NO). When the device is unpowered or the relay is switched OFF, the NC contact is closed and the load is energized. This channel is a reserve for the installer — use it where equipment must switch on if the panel loses power. For failsafe-off loads (pumps, valves, heaters), use a module with NO/SPDT contacts (e.g. DIO-430-R1).

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.

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).

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.

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

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 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)