US thermostats usually use low-voltage wiring for signalling to the furnace or boiler. We usually only see line&#x2F;mains voltage thermostats (that work by limiting current supplied to a electric heater) in cheap apartments.
<a href="https:&#x2F;&#x2F;nest.com&#x2F;support&#x2F;article&#x2F;How-can-I-tell-if-my-current-thermostat-is-low-voltage" rel="nofollow">https:&#x2F;&#x2F;nest.com&#x2F;support&#x2F;article&#x2F;How-can-I-tell-if-my-curren...</a><p>The US version won&#x27;t work with line voltage systems. 
It looks like the UK version <i>only</i> works with line voltage systems.<p>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.<p>Nest US: &quot;It&#x27;s easy to install the Nest Thermostat. Everything you need comes in the box&quot;<p>Nest UK: &quot;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.&quot;