You’ll need a dry contact relay such as https://en.cozify.fi/products/dry-contact-relay for your boiler.
Then you need in every room a radiator valve like this https://eurotronic.org/produkte/zigbee-heizkoerperthermostat/spirit-zigbee/ and a thermometer for measuring the room temperature.
That combined with a rule, such I’m developing and testing: Smart Virtual Thermostat (beta version) and you’ve got a smart thermostat.
Warning, it’s not a simple plug and play system. It involves a lot of time and effort 
Btw, I’ve giving examples of devices compatible with Zigbee, but with openHAB you can use any type of communication between your devices (even mixed).