An HVAC Controller may include a first sub-assembly and a second sub-assembly capable of releasably engaging the first sub-assembly. The first sub-assembly may include a first housing and a printed circuit board having a controller for providing one or more control signals, where the printed circuit board may be secured relative to the first housing. The second sub-assembly may include a second housing, a power source, one or more output terminals, and a second printed circuit board. The second printed circuit board may be secured relative to the second housing and may have circuitry thereon that is powered by the power source and configured to receive the one or more control signals from the first printed circuit board. The circuitry of the second sub-assembly may set output terminals thereof to a predetermined state when the first sub-assembly is released from the second sub-assembly.