Abstract: The present device incrementally adds or subtracts power units to and from a heating or cooling system and employs a major control board into which are plug-connected a plurality of minor control boards. The minor control boards have varying designs. Some are designed to accept different signals from different types of temperature monitoring means and are designed to convert those different signals into common command signals, while other minor control boards are designed to accept those command signals in order to communicate with and control yet others of said minor control boards to effect a turning on and/or turning off of said heating or cooling power units according to at least two different modes of operation. The major control board provides a power source for the minor control boards and provides the circuit paths to enable the minor control boards to transmit signals among any set of such mounted boards.