For projects like that, you don't connect anything to the mains at all. Specifically: - At least in US, thermostats control is low-voltage (24 V AC), and come

For projects like that, you don't connect anything to the mains at all. Specifically: - At least in US, thermostats control is low-voltage (24 V AC), and come from low-power transformer which will just stop working on overload (ask me how I know :) ), so there is no electrical fire danger - You never plug anything into the AC outlet directly -- you always use AC/DC adapters (and not the ones bought on ebay for minimum price, too) - To prevent freeze hazard, you also leave original thermostat hooked up, but set it to really low temperature (10C/50F). - Overheating house is possible, and you definitely need to consider this in your software (for example, require periodic control messages to keep your boiler on). However, I do not think it is an actual fire hazard -- likely, there are hardware safeties in your heater to prevent that. Still, if you are worried about this, you can install a second thermostat in series with your relay and set it to max temperature you ever want.