A heating, ventilation and air conditioning (HVAC) device (104) which includes both heating and cooling operating modes provides an easy-to-use interface (110) for selecting the operating parameters of the device (104). The interface (110) allows the input of a setpoint temperature at which the HVAC device (104) conditions the ambient temperature of a space (102). A mode switch-over algorithm (200) uses the setpoint temperature, the sensed temperature from the conditioned space, and prestored threshold values that depend on the device's operating capacities, to determine when to change the device between heating and cooling modes. Also, within each of the respective modes, a heating (300) or cooling (400) algorithm controls the engaging and disengaging of the heating and cooling elements of the device (104) to maintain the temperature of the conditioned space (102) within a desired comfort zone.