For this project you will need a "server" to perform as MQTT broker, OpenHub2 server and boiler controller.

I am using a Raspberry Pi 2 running DietPi

You can install the mosquitto broker using the software menu in DietPi It is supported

You need to install OpenHub2 (it requires Java) Guide

You need Python but it is probably installed already

Checkout the repo.

The etc folder contains the openhab2 config files ready to be copied.

The controller folder contains the boilerLogic.py script. Copy the script to your prefered location.

Highly recomended to create an init script (or systemd service) to automatically launch the control every boot.

Download all the Adafruit libraries.

Install the Arduino IDE and make it work with the ESP8266: Guide

The code for the remote units is RemoteUnit.c Create a new project and use tthe code from this file.

Remember to change IP, WLAN details, MQTT topic names...

Flash your device and that's it, it should be working.