The invention relates to a thermostat including a user interface module. The user interface module has a display, a plurality of user interface buttons, and a first power and communications interface. The thermostat also includes an I/O module, the I/O module electrically coupled to a HVAC plant and the I/O module having a plurality of switches to control the HVAC plant, and a second power and communications interface. The user interface module and the I/O module are communicatively coupled and exchange power via the power and communications interfaces and the thermostat can be mounted and operated in one of two user selectable installations: a stacked configuration comprising a thermostat having the user interface module directly mounted to the I/O module, or a remote configuration comprising a thermostat having the user interface module remotely mounted from the I/O module.