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OpenTherm library to create thermostat for OpenTherm enabled boilers and HVACs using an OpenTherm controller and an ESP32 or other microcontroller


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OpenTherm Arduino ESP32/ESP8266 Library

Was already working on an ESP32, now ready for an ESP8266 too!

Create your own thermostat with this library and save money on your energy bills while reducing your carbon footprint! Or use a home automation system like Home Assistant and smart thermostatic valves to heat up each room individually.

The library complies with the OpenTherm specification. Control any (condensing) boiler or air conditioner (HVAC) that also meets the OpenTherm specification.

The library can be easily installed in the Arduino IDE. It has been tested on an ESP32-S2, ESP32-C3, ESP32 NodeMCU and an ESP8266 NodeMCU microcontroller and may also work on other MCU's. To connect the boiler, you will need an OpenTherm controller board or an OpenTherm controller Shield.

Fully functioning Home Assistant Boiler Thermostat in examples

Home Assistant logo MQTT logo\

The Advanced_Thermostat.ino example is specially designed for Home Assistant

  • Setup the MQTT integration in Home Assistant, if not already done
  • Set your WiFi network name and password in Advanced_Thermostat.ino
  • Set your MQTT broker hostname or IP address, MQTT user name and MQTT password in Advanced_Thermostat.ino
  • Compile and flash to an ESP32-S2 or an ESP32-C3 with an OpenTherm controller Shield or any other ESP32 or ESP8266 with an OpenTherm controller board
  • Forward the temperature of a room thermometer to the Thermostat using a Home Assistant Automation The thermostat (Climate) integration is automatically added to Home Assistant, together with several sensors. More information in the examples directory.


  • Install the EasyOpenTherm library directly using the Arduino IDE library manager (search for 'EasyOpenTherm')
  • Connect the pins marked 'OT' of the OpenTherm controller with two wires to the boiler or use the screw terminals on the OpenTherm controller shield. You can use the existing wires from your current thermostat. The order of the wires is not important, they are interchangeable
  • If using the board connect the pins marked '3v3' and 'GND' to the ESP32 pins '3v3; and 'GND'. For the shield these pins are already connected
  • If using the board connect the pin marked 'RxD' to a pin supporting OUTPUT of the ESP32 and the pin marked 'TxD' to a pin supporting interrupts. For the shield these pins are already connected. The pins used you use should be defined in the program (see below). In Advanced_Thermostat.ino they default to the right pins for the shield and either an ESP32-S2 mini or an ESP32-C3 mini.


#include <EasyOpenTherm.h>

Select two free GPIO pins, one to send data to the boiler and one to receive data. The pin receiving data must support interrupts. For the pin that sends data, do not use a 'read only' GPIO. Define these pins in the program

#define OT_RX_PIN (35)
#define OT_TX_PIN (33)

In this case GPIO34 is used for receiving and GPIO33 is used for sending data. Note that the Rx pin is connected to the TxD pin of the OpenTherm controller and vice versa!

Create an OpenTherm class instance

OpenTherm thermostat(OT_RX_PIN, OT_TX_PIN);

Make sure that only one instance of this object is alive at a time. So make it global or static like in the examples. Start communicating with the boiler (or HVAC) e.g. request activation of the services of the boiler and request it's status flags

uint8_t primaryFlags = uint8_t(OpenTherm::STATUS_FLAGS::PRIMARY_DHW_ENABLE);  // Enable Domestic Hot Water 
primaryFlags |= uint8_t(OpenTherm::STATUS_FLAGS::PRIMARY_CH_ENABLE);          // Enable Central Heating
primaryFlags |= uint8_t(OpenTherm::STATUS_FLAGS::PRIMARY_COOLING_ENABLE);     // Enable cooling (of your boiler, if available)
primaryFlags |= uint8_t(OpenTherm::STATUS_FLAGS::PRIMARY_OTC_ENABLE);         // Enable Outside Temperature Compensation (ifa available in your boiler)
uint8_t statusFlags;                                                          // Flags returned by the boiler will be stored into this variable
thermostat.status(primaryFlags, statusFlags);

This command will return true on success and false otherwise All other interaction with the boiler is done using the read(), write() or readWrite() functions, e.g.

thermostat.write(OpenTherm::WRITE_DATA_ID::CONTROL_SETPOINT_CH, 40.0); // This will start up the boiler to heat up the boiler water to 40 degrees

All these functions take an OpenTherm DATA-ID as first parameter. The DATA-ID refers to the action requested from the boiler. All known DATA-ID's are defined in EasyOpenTherm.h. The DATA-IDs for reading data from the boiler are defined in enum class READ_DATA_ID, DATA-IDs for writing data to the boiler are defined in enum class WRITE_DATA_ID and DATA-IDs for writing and reading data to and from the boiler are defined in enum class READ_WRITE_DATA_ID. The second parameter and sometimes third parameter defines the value written to the boiler or read from the boiler. The data types are:

  • uint16_t marked as u16 in the comments
  • int16_t marked as s16
  • float marked as f8.8 (because actually it is an int16_t / 256.0)
  • Two times an uint8_t marked as flag8 or u8, or an int8_t as s8. If it is a flag the meaning of bits is defined in an enum class with a name ending in _FLAGS, e.g. enum class STATUS_FLAGS

Error handling

The function error() is used to get information about the last call to one of the functions read(), write() or readWrite(). All these functions return false if an error occurred in the communication between thermostat (primary) and boiler or HVAC (secondary). These functions return true if everything is fine, but also upon an error on application level. You will get this error if e.g. you read out your boiler or HVAC with a DATA-ID that is not supported. Also, if you write data to your boiler or HVAC, the value can be out of range, e.g. a setpoint is too low or too high. In this case error() will return INVALID_DATA.

All error codes:

  • OK: everything is fine!
  • UNKNOWN_DATA_ID: your boiler or HVAC does not support the DATA-ID you requested.
  • INVALID_DATA: the DATA-ID you sent with write() or readWrite() is correct but the value you sent is out of bounds
  • SEND_TIMEOUT: a timeout occurred while sending the request to the boiler or HVAC. Check if the timeout value in the OpenTherm constructor is OK (should be less than 1,000 ms, defaults to 900 ms)
  • RECEIVE_TIMEOUT: a timeout occurred while reading the response from the boiler or HVAC. Most of the time this indicates problems with wiring. If wiring is OK, did you check if your boiler actually does support the OpenTherm protocol?
  • PARITY_ERROR: the parity check failed.


  • primary: the device issuing the requests, in this context also called thermostat
  • secondary: the device handling the requests and sending responses, also called boiler or HVAC
  • CH: Central Heating
  • DHW: Domestic Hot Water
  • OTC: Outside Temperature Compensation
  • HVAC: Heating, Ventilation and Air Conditioning
  • setpoint: the desired value of a parameter, e.g. the desired temperature of the temperature of the water of the boiler is called CH (Central Heating) setpoint
  • on/off: a non digital control mode switching the boiler on and off (by shortening the thermostat wires and opening them)
  • modulation: a technique of lowering the flame when less power is needed
  • flow: water leaving the boiler
  • return: water returning to the boiler


© 2022 Jeroen Döll, licensed under the GNU General Public License. Enjoy using the library, feedback is welcome!