In many places it will be. Want to buy a Nest in the UK? Guess what, you can't install it yourself: Nest will send a contractor along to install it for you. Many countries highly regulate heating system installation, for good reasons. Even if you're in a jurisdiction where this isn't a problem, the majority of thermostats get replaced along with other HVAC work (new boiler, new pipework, etc), which is going to be done by a qualified contractor.