A thermostat is configured for automated compatibility with HVAC systems that are either single-HVAC-transformer systems or dual-HVAC-transformer systems. The compatibility is automated in that a manual jumper installation is not required for adaptation to either single-HVAC-transformer systems or dual-HVAC-transformer systems. The thermostat has a plurality of HVAC wire connectors including a first call relay wire connector, a first power return wire connector, a second call relay wire connector, and a second power return wire connector. The thermostat is configured such that if the first and second external wires have been inserted into the first and second power return wire connectors, respectively, then the first and second power return wire connectors are electrically isolated from each other. Otherwise, the first and second power return wire connectors are electrically connected.