What is a suitable/recommended cable for OpenTherm?
Use standard 1 pair of 0,6 mm^2 'phone' (class3) cable, it'll be enough (70 ohm/km, you'll get 5 ohm with 70m so well over the 50m OT limit) and also suitable for noisy environments as it's twisted. Remember to check that cable insulation is rated at least 250V if you run it in same mains power conduits as required by CEI regulations