Abstract: A microprocessor controlled sensor system for an apartment house periodically samples the temperature of each apartment at preselected sample intervals of time. The on/off state of a furnace calling device such as a thermostatic switch is also periodically sampled at the preselected sample intervals of time. The apartment temperature samples for each apartment are accumulated to provide an indication of the degree days. The number of times that the furnace has been called by each apartment is stored to provide an indication of the frequency of use of the furnace by each apartment. The degree days and the frequency of furnace use provide an indication of the efficiency of the apartment house heating system.