At the beginning of the project I need to make some important decisions how to build the thermostat, and what components to use. The first problem is how to control the boiler. I checked the manual of my boiler, and I found a schematic (version in Spanish, but quite obvious):
On the schematic there is a jumper P1 which connects terminals 6 and 9. The boiler was supplied with the jumper in place. Removing the jumper turns off the heating circuit of the combi-boiler, even if heating is enabled on control panel (and domestic water works normally). That's very good news, because the only thing I need to do is to replace the P1 jumper with a relay, and I don't have to care about electrical compatibility of my thermostat and the boiler. Well I just have to choose a relay big enough for the voltage and current present in the circuit.
I measured the voltage between terminals 6 and 9, and my ancient multimeter shows 28V. Quite strange value, but it doesn't matter - 125V or 250V relay will work perfectly.