Smart thermostats require "low voltage" systems (24V) to operate. At a minimum, you'll need a "C" wire to provide continuous power to the thermostat. The best thing to do is to visit the website of the thermostat you're thinking of buying and look for a wire reference guide. Remove your current thermostat, and compare the wires to the guide. As a general rule, if you already have a digital thermostat, you'll likely have the necessary wires; if you have the old rotary-style thermostat, you will likely not. Most radiator-based systems are millivolt systems and are not compatible with smart thermostats.