Abstract: A thermostat for heating or cooling, operative to maintain an indoor air temperature to a limited setpoint range, is provided with a comparison circuit to prevent tampering intended to exceed a setpoint limit. A first temperature sensor responds to ambient air temperature and can include a first thermistor. A second thermistor is partly thermally isolated from the air, and senses a reference temperature that lags temperature changes in the ambient air. The two thermistors form a voltage divider coupled to one input of a differential amplifier comparator, the other input of which is coupled to a fixed voltage reference. When the ambient and reference temperatures exceed a predetermined temperature difference, the differential amplifier operates a relay that breaks a circuit including the contacts. This occurs when locally cooling the ambient air to generate a spurious call for heat (or vice versa).