OpenTherm is the protocol used to control HVAC systems. Nowadays it is common to find a central heating boiler that has an OpenTherm interface between the thermostat and the boiler.

There is an open source project for a DIY OpenTherm Gateway. This is an easy start to get acquainted with the OpenTherm protocol.

OpenTherm uses a physical layer based on a 2-wire 20 mA current loop interface using a 1000 bits/s signal with Manchester encoding. Any simple microcontroller can be used to read or write this signal. The interface for the slave (e.g. thermostat unit) has an optical isolated interface.