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
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.