US thermostats usually use low-voltage wiring for signalling to the furnace or boiler. We usually only see line/mains voltage thermostats (that work by limiting

US thermostats usually use low-voltage wiring for signalling to the furnace or boiler. We usually only see line/mains voltage thermostats (that work by limiting current supplied to a electric heater) in cheap apartments. https://nest.com/support/article/How-can-I-tell-if-my-current-thermostat-is-low-voltage https://nest.com/support/article/How-can-I-tell-if-my-curren... The US version won't work with line voltage systems. It looks like the UK version only works with line voltage systems. This also explains why in the US changing a thermostat is something pretty much anyone can do but the UK version requires a Nest Pro to hook up. Nest US: "It's easy to install the Nest Thermostat. Everything you need comes in the box" Nest UK: "A Nest Pro installer should install your Nest Thermostat. They will identify cables from the boiler, zone valve or junction box to connect to the Heat Link."