The Difference
Why It Belongs On the System
A smart thermostat on its own is a nicer way to set a schedule. The reason to put climate on the same system as everything else has less to do with convenience than most people expect.
When a monitored smoke or CO alarm goes off, an integrated system shuts the HVAC down. That matters because your ductwork is the fastest path smoke has to reach every room in the house — a furnace fan running through a fire is actively working against you. A thermostat that isn’t on the system has no idea the alarm is sounding, so it keeps running.
The same wiring is what lets the house set itself back when the alarm arms away, back off when a door has been left open, and get ahead of tomorrow’s cold snap instead of reacting to it. None of that is available to a thermostat working alone, however clever it is.
It’s the clearest argument we know for one system rather than several good ones that don’t talk to each other.