OpenTherm Thermostat Shield
  • 2/25/2021
  • Thermostat, OpenTherm

OpenTherm Thermostat Shield

There's a variety of OpenTherm thermostats out there. Most smart thermostats cost from €100 to €300, which is quite expensive. This article describes how to build your own smart OpenTherm Thermostat with remote control for a cheap price and without any monthly costs.

We provide a simple solution, which gives you ability to integrate thermostat into your favorite home automation system, like Home Assistant, Domoticz, Blynk, Tasmota and so on... In general, it allows you to control not only boilers and domestic water heaters, it can be used with heat pumps and ventilation systems which has OpenTherm protocol support. This is a custom solution, so you can implement the best smart thermostat for your situation.

OpenTherm boiler connection

OpenTherm Boiler Connection In general to connect Master OpenTherm Shield you need 3 pairs of wires. First (GND, 3.3/5V) is for powering shield, depending on your microcontroller you can power up it with 3.3v or 5v. Second (IN,OUT) - for connecting microcontroller input, output pins. Third (B1,B2) - for boiler connection.

Master OpenTherm Shield

Master OpenTherm Shield together your favorite microcontroller such as Arduino, STM32, Espressif ESP8266/ESP32 allows you to build smart OpenTherm thermostat. It can control all OpenTherm compatible boilers, heat pumps and ventilation systems.

You can either write your own firmware to implement some desired control algorithms or use one of our provided below examples for fast start. Using OpenTherm Library you can build and send any OpenTherm commands described in the OpenTherm protocol specification

Master OpenTherm Shield Connection

ESP32/ESP8266 OpenTherm Thermostat

ESP32/ESP8266 OpenTherm Thermostat ESP8266 Thermostat Shield. Same as Master OpenTherm Shield. but already bundled with a temperature sensor.

Arduino OpenTherm Thermostat

Arduino OpenTherm Thermostat Arduino OpenTherm Shield. Can be used for stacking Master OpenTherm Shield together with Arduino UNO or MEGA board for building OpenTherm Thermostat


First of all smart thermostat it is about comfort. ON/Off modulating heating controls adjust the average water temperature within the heating system by cycling your boiler On and Off periodically. That is why set point is not accurately controlled, so overshoots (unnecessary overheating) and undershoots is a common problem of such kind of thermostats. This can feel uncomfortable and normally leads to residents setting the thermostat higher than required.

Smart thermostat uses PI algorithm to calculate boiler water set point and tells to OpenTherm boiler when to adjust their heating water temperature. This is allows smart modulating thermostat to provide efficient climate control and to let your heating system control your home's temperature more smoothly and precisely.

Energy savings

We recommend to use smart thermostats because they can save you money on energy bills. OpenTherm thermostat together with modulating boiler use more or less gas when it's appropriate rather than by cycling it On and Off, to be more efficient while keeping you comfortable. The important difference here is that the boiler will run for longer periods but producing water at lower temperatures resulting in less gas being used and maximizing the time spent in the higher efficiency condensing mode.

Most new boilers are 'modulating', i.e. they can adjust the amount of gas they use, for example between 5kW and 24kW. This makes them very efficient as they can reduce their output during warmer periods. The central heating requirement is calculated, and the boiler will operate at the minimum rate at which this can be achieved which maximizes operating efficiency and reduced fuel costs. So with smart modulating thermostat you can easily control your home temperature and save energy without renouncing your comfort.

Remote control

A smart thermostat uses a Wi-Fi connection so you can control at any time, from anywhere remotely your home's thermostat settings through your smartphone, tablet or personal computer. If like to travel in the winter for the holidays, you need a smart thermostat, because it allows to monitor your HVAC system, having your heating system malfunction while you're gone is the last thing you want, it could cause frozen pipes, which could lead to serious damages of your home.


OpenTherm is a point-to-point communication system and connects gas central heating (condensing) boilers with room thermostats. Traditionally, OpenTherm has been limited to modulation gas boilers, but in theory any type of heating technology could use OpenTherm (oil boilers, ventilation system and heat pumps for example).

OpenTherm is a standard protocol which has sets the rules on how the boiler and the room thermostat communicate with each other. It allows you more precise control resulting in more comfort and energy savings. Heat requirement is calculated by the controller based on the difference between the set (or target) temperature and the actual temperature.

Some of the standardized functions available include setting and reading DHW temperature, reading outdoor temperature and reading fault codes. Also it gives you diagnostics tools to monitor and examine what has happenned in case of any malfunction.

It requires a standard 2-wire low voltage and polarity-free connection and can also be used as a wireless protocol. It guarantees a minimum level of compatibility. Using OpenTherm devices allows many advantages for the end-user and the installer and also minimizes the impact on the environment and increases the life of the boiler.

Currently we have an examples of the thermostat implementation with a following systems:

Basic OpenTherm Sample

OpenTherm Sample is good start with to understand how OpenTherm Library works and ensure right connection of OpenTherm Master shield.

ESP32 Wifi Thermostat

ESP32 Wifi thermostat is a simple thermostat with a web interface, is controlled via wifi from any browser within your network. ESP32 Wi-Fi OpenTherm Thermostat

Home Assistant Thermostat

Home Assistant Thermostat is a complex home automation solution which allows you fine tuning of automation according to your needs. Home Assistant OpenTherm Thermostat

Domoticz Thermostat

Domoticz Thermostat Simple home automation solution. Domoticz OpenTherm Thermostat

Tasmota Thermostat

Tasmota Thermostat Simplifies further integration into home automation systems or can act as an automation tool itself. ESP32 Tasmota OpenTherm Thermostat

Blynk Thermostat

Blynk Thermostat Enables remote control via your smartphone over the internet with a few simple steps. Blynk OpenTherm Thermostat