Abstract: A thermostat for control of an AC-operated HVAC unit (or a unit providing only heating or only cooling) is provided with load activation detection sensing for increased reliability of latching relay activation. The thermostat includes a sensing transformer in series with the heating or cooling load (or both, if both are in the system), so that, when activation of the load is called for by the thermostat, current flows in a primary winding of the transformer, inducing a current in the secondary winding. A voltage derived from the current in the secondary winding is sensed by the thermostat controller and used to determine whether the heating or cooling load has been properly activated or deactivated. Sensing of the AC power source in the HVAC unit may also be provided, so that the controller can confirm that the absence of the voltage derived from the current in the secondary winding is actually due to the state of the latching relays rather than to a failure of the AC power source.