OpenTherm

OpenTherm (OT) is a standard communications protocol used in central heating systems for the communication between central heating appliances and a thermostatic controller. As a standard, OpenTherm is independent of any single manufacturer. A controller from manufacturer A can in principle be used to control a boiler from manufacturer B.

Since OpenTherm doesn’t operate in a standard voltage range, special hardware is required. You can choose from several ready-made adapters or roll your own:

• DIYLESS Master OpenTherm Shield

• Ihor Melnyk’s OpenTherm Adapter

• Jiří Praus’ OpenTherm Gateway Arduino Shield

DIYLESS Master OpenTherm Shield.

Note

This component acts only as an OpenTherm master (for example, a thermostat or controller) and not as a slave or gateway. Your existing thermostat is not usable while you use ESPHome with this component to control your boiler.

Quick glossary

• CH: Central Heating

• DHW: Domestic Hot Water

Hub

First, you need to define the OpenTherm hub in your configuration. Note that most OpenTherm adapters label in and out pins relative to themselves; this component labels its in and out pins relative to the microcontroller ESPHome runs on. As such, your bridge’s in pin becomes the hub’s out pin and vice-versa.

opentherm:
  in_pin: GPIOXX
  out_pin: GPIOXX

Configuration variables:

• in_pin (Required, number): The pin of the OpenTherm hardware bridge which is usually labeled out on the board.

• out_pin (Required, number): The pin of the OpenTherm hardware bridge which is usually labeled in on the board.

• sync_mode (Optional, boolean, default false): Synchronous communication mode prevents other components from disabling interrupts while we are talking to the boiler. Enable if you experience a lot of random intermittent invalid response errors (very likely to happen while using Dallas temperature sensors).

• opentherm_version (Optional, float): OpenTherm version that is required for some boilers to work (message id 124). You don’t need to specify this if everything works.

• id (Optional, ID): Manually specify the ID used for code generation. Required if you have multiple busses.

Note about sync mode

The use of some components (like Dallas temperature sensors) may result in lost frames and protocol warnings from OpenTherm. Since OpenTherm is resilient by design and transmits its messages in a constant loop, these dropped frames don’t usually cause any problems. Still, if you want to decrease the number of protocol warnings in your logs, you can enable sync_mode which will block ESPHome’s main application loop until a single conversation with the boiler is complete. This can greatly reduce the number of dropped frames, but usually won’t eliminate them entirely. With sync_mode enabled, in some cases, ESPHome’s main application loop may be blocked for longer than is recommended, resulting in warnings in the logs. If this bothers you, you can adjust ESPHome’s log level by adding the following to your configuration:

logger:
  logs:
    component: ERROR

Usage as a thermostat

The most important function for a thermostat is to set the boiler temperature setpoint. This component has three ways to provide this input: using a Home Assistant sensor from which the setpoint can be read, using a Number Component, or defining an output to which other components can write. For most users, the last option is the most useful one, as it can be combined with the PID Climate component to create a thermostat that works as you would expect a thermostat to work. See Basic PID thermostat for an example.

Numerical values

There are three ways to set a numerical value:

• As an input sensor, defined in the hub configuration:

  opentherm:
    t_set: setpoint_sensor
  
  sensor:
    - platform: homeassistant
      id: setpoint_sensor
      entity_id: sensor.boiler_setpoint
  

  This can be useful if you have an external thermostat-like device that provides the setpoint as a sensor.

• As a number:

  number:
    - platform: opentherm
      t_set:
        name: Boiler Setpoint
  

  This is useful if you want full control over your boiler and want to manually set all values.

• As an output:

  output:
    - platform: opentherm
    t_set:
      id: setpoint
  

  This is especially useful in combination with the PID Climate component:

  climate:
    - platform: pid
      heat_output: setpoint
      # ...
  

For the output and number variants, there are four more properties you can configure beyond those included in the output and number components by default:

• min_value (float): The minimum value. For a number this is the minimum value you are allowed to input. For an output this is the number that will be sent to the boiler when the output is at 0%.

• max_value (float): The maximum value. For a number this is the maximum value you are allowed to input. For an output this is the number that will be sent to the boiler when the output is at 100%.

• auto_max_value (boolean): Automatically configure the maximum value to a value reported by the boiler. Not available for all inputs.

• auto_min_value (boolean): Automatically configure the minimum value to a value reported by the boiler. Not available for all inputs.

The following numerical values are available:

• t_set: Control setpoint: temperature setpoint for the boiler’s supply water (°C)

  • Default min_value: 0

  • Default max_value: 100

  • Supports auto_max_value

• t_set_ch2: Control setpoint 2: temperature setpoint for the boiler’s supply water on the second heating circuit (°C)

  • Default min_value: 0

  • Default max_value: 100

  • Supports auto_max_value

• cooling_control: Cooling control signal (%)

  • Default min_value: 0

  • Default max_value: 100

• t_dhw_set: Domestic hot water temperature setpoint (°C)

  • Default min_value: 0

  • Default max_value: 127

  • Supports auto_min_value

  • Supports auto_max_value

• max_t_set: Maximum allowable CH water setpoint (°C)

  • Default min_value: 0

  • Default max_value: 127

  • Supports auto_min_value

  • Supports auto_max_value

• t_room_set: Current room temperature setpoint (informational) (°C)

  • Default min_value: -40

  • Default max_value: 127

• t_room_set_ch2: Current room temperature setpoint on CH2 (informational) (°C)

  • Default min_value: -40

  • Default max_value: 127

• t_room: Current sensed room temperature (informational) (°C)

  • Default min_value: -40

  • Default max_value: 127

• max_rel_mod_level: Maximum relative modulation level (%)

  • Default min_value: 0

  • Default max_value: 100

  • Supports auto_min_value

• otc_hc_ratio: OTC heat curve ratio (°C)

  • Default min_value: 0

  • Default max_value: 127

  • Supports auto_min_value

  • Supports auto_max_value

Switch

Switches are available to allow manual toggling of any of the following seven status codes:

• ch_enable: Central Heating enabled

• dhw_enable: Domestic Hot Water enabled

• cooling_enable: Cooling enabled

• otc_active: Outside temperature compensation active

• ch2_active: Central Heating 2 active

• summer_mode_active: Summer mode active

• dhw_block: Block DHW

If you do not wish to have switches, the same values can be permanently set in the hub configuration, like so:

opentherm:
  ch_enable: true
  dhw_enable: true

This is useful when you’d never want to toggle it after the initial configuration.

The default values for these configuration variables are listed below.

To enable central heating and cooling, the flag is only sent to the boiler if the following conditions are met:

• the flag is set to true in the hub configuration,

• the switch is on (if configured),

• the setpoint or cooling control value is not 0 (if configured)

For domestic hot water and outside temperature compensation, only the first two conditions are necessary.

The last point ensures that central heating is not enabled if no heating is requested as indicated by a setpoint of 0. If you use a number as the setpoint input and use a minimum value higher than 0, you must use the ch_enable switch to turn off your central heating. In such a case, the flag will be set to true in the hub configuration and the setpoint is always larger than 0, so including a switch is the only way you can turn off central heating. (This also holds for cooling and CH2.)

Binary sensor

The component can report boiler status on several binary sensors. The Status sensors are updated in each message cycle, while the others are only set during initialization, as they are unlikely to change without restarting the boiler.

• fault_indication: Status: Fault indication

• ch_active: Status: Central Heating active

• dhw_active: Status: Domestic Hot Water active

• flame_on: Status: Flame on

• cooling_active: Status: Cooling active

• ch2_active: Status: Central Heating 2 active

• diagnostic_indication: Status: Diagnostic event

• electricity_production: Status: Electricity production

• dhw_present: Configuration: DHW present

• control_type_on_off: Configuration: Control type is on/off

• cooling_supported: Configuration: Cooling supported

• dhw_storage_tank: Configuration: DHW storage tank

• controller_pump_control_allowed: Configuration: Controller pump control allowed

• ch2_present: Configuration: CH2 present

• water_filling: Configuration: Remote water filling

• heat_mode: Configuration: Heating or cooling

• dhw_setpoint_transfer_enabled: Remote boiler parameters: DHW setpoint transfer enabled

• max_ch_setpoint_transfer_enabled: Remote boiler parameters: CH maximum setpoint transfer enabled

• dhw_setpoint_rw: Remote boiler parameters: DHW setpoint read/write

• max_ch_setpoint_rw: Remote boiler parameters: CH maximum setpoint read/write

• service_request: Service required

• lockout_reset: Lockout Reset

• low_water_pressure: Low water pressure fault

• flame_fault: Flame fault

• air_pressure_fault: Air pressure fault

• water_over_temp: Water overtemperature

Sensor

The boiler can also report several numerical values, which are available through sensors. Your boiler may not support all of these values, in which case there won’t be any value published to that sensor. The following sensors are available:

• rel_mod_level: Relative modulation level (%)

• ch_pressure: Water pressure in CH circuit (bar)

• dhw_flow_rate: Water flow rate in DHW circuit (l/min)

• t_boiler: Boiler water temperature (°C)

• t_dhw: DHW temperature (°C)

• t_outside: Outside temperature (°C)

• t_ret: Return water temperature (°C)

• t_storage: Solar storage temperature (°C)

• t_collector: Solar collector temperature (°C)

• t_flow_ch2: Flow water temperature CH2 circuit (°C)

• t_dhw2: Domestic hot water temperature 2 (°C)

• t_exhaust: Boiler exhaust temperature (°C)

• fan_speed: Boiler fan speed (RPM)

• fan_speed_setpoint: Boiler fan speed setpoint (RPM)

• flame_current: Boiler flame current (µA)

• burner_starts: Number of starts burner

• ch_pump_starts: Number of starts CH pump

• dhw_pump_valve_starts: Number of starts DHW pump/valve

• dhw_burner_starts: Number of starts burner during DHW mode

• burner_operation_hours: Number of hours that burner is in operation

• ch_pump_operation_hours: Number of hours that CH pump has been running

• dhw_pump_valve_operation_hours: Number of hours that DHW pump has been running or DHW valve has been opened

• dhw_burner_operation_hours: Number of hours that burner is in operation during DHW mode

• t_dhw_set_ub: Upper bound for adjustment of DHW setpoint (°C)

• t_dhw_set_lb: Lower bound for adjustment of DHW setpoint (°C)

• max_t_set_ub: Upper bound for adjustment of max CH setpoint (°C)

• max_t_set_lb: Lower bound for adjustment of max CH setpoint (°C)

• t_dhw_set: Domestic hot water temperature setpoint (°C)

• max_t_set: Maximum allowable CH water setpoint (°C)

• opentherm_version_device: Version of OpenTherm implemented by device

• device_type: Device product type

• device_version: Device product version

• device_id: Device ID code

• otc_hc_ratio_ub: OTC heat curve ratio upper bound

• otc_hc_ratio_lb: OTC heat curve ratio lower bound

Examples

Minimal example with numeric input

# An extremely minimal configuration which only enables you to set the boiler's
# water temperature setpoint as a number.

opentherm:
  in_pin: GPIOXX
  out_pin: GPIOXX
  ch_enable: true

number:
  - platform: opentherm
    t_set:
      name: "Boiler Control setpoint"

Basic PID thermostat

# A basic thremostat for a boiler with a single central heating circuit and
# domestic hot water. It reports the flame, CH and DHW status, similar to what
# you would expect to see on a thermostat and also reports the internal boiler
# temperatures and the current modulation level. The temperature is regulated
# through a PID Climate controller and the current room temperature is retrieved
# from a sensor in Home Asisstant.

# This configuration should meet most needs and is the recommended starting
# point if you just want a thermostat with an external temperature sensor.

opentherm:
  in_pin: GPIOXX
  out_pin: GPIOXX
  dhw_enable: true    # Note that when we specify an input in hub config with a static value, it can't be
                      # changed without uploading new firmware. If you want to be able to turn things on or off,
                      # use a switch (see the ch_enable switch below).
                      # Also note that when we define an input as a switch (or use other platform), we don't need
                      # to set it at hub level.

output:
  - platform: opentherm
    t_set:
      id: t_set
      min_value: 20
      max_value: 65
      zero_means_zero: true

sensor:
  - platform: opentherm
    rel_mod_level:
      name: "Boiler Relative modulation level"
    t_boiler:
      name: "Boiler water temperature"
    t_ret:
      name: "Boiler Return water temperature"

  - platform: homeassistant
    id: ch_room_temperature
    entity_id: sensor.temperature
    filters:
      # Push room temperature every second to update PID parameters
      - heartbeat: 1s

binary_sensor:
  - platform: opentherm
    ch_active:
      name: "Boiler Central Heating active"
    dhw_active:
      name: "Boiler Domestic Hot Water active"
    flame_on:
      name: "Boiler Flame on"
    fault_indication:
      name: "Boiler Fault indication"
      entity_category: diagnostic
    diagnostic_indication:
      name: "Boiler Diagnostic event"
      entity_category: diagnostic

switch:
  - platform: opentherm
    ch_enable:
      name: "Boiler Central Heating enabled"
      restore_mode: RESTORE_DEFAULT_ON

climate:
  - platform: pid
    name: "Central heating"
    heat_output: t_set
    default_target_temperature: 20
    sensor: ch_room_temperature
    control_parameters:
      kp: 0.4
      ki: 0.004

See Also

• API Reference: OpenthermHub

• API Reference: OpenthermInput

• API Reference: OpenthermNumber

• API Reference: OpenthermOutput

• API Reference: OpenthermSwitch

• OpenTherm thermostat with ESPHome and Home Assistant — real-world use case for this component.

• Development repository — new features will be tested here before proposing them to ESPHome core.

• Edit this page on GitHub