> i.e. you usually change your thermostat when you change your HVAC This isn't a technical requirement (in the US at least). Thermostat HVAC connections foll

> i.e. you usually change your thermostat when you change your HVAC This isn't a technical requirement (in the US at least). Thermostat HVAC connections follow a simple wiring/control scheme, which is why smart thermostats can be retrofit at all. That said, replacing an inefficient and poorly balanced HVAC system with a newer well designed one, and insulating a house to modern standards will save far more on operating costs than a smart thermostat, but it is orders of magnitude more expensive in upfront costs.