respectively to lines 12 18 with the inverter inputs being connected to 12 volts through resistors r28 r29 r30 and r31 and the inverter outputs being connected to ground through capacitors c5 c6 c7 and c8 in furnishing the one pulse per second signal on line 56 ic lead is connected thereto by inverter i5 the inverter input being connected to 12 volts through resistor r23 in adapting the time slot computer discussed to this juncture to use in controlling the firing of a heat generator in an installation of type having pre existing wiring extending from both day and night thermostats to the heat generator the system of fig1 may be employed a switching unit 126 includes relays cr1 and cr2 connected respectively to lines 90a and 90b for energization the contacts of relay cr1 connect lines 128a the output wire pair of day thermostat 128 to lines 132 extending to the heat generator the contacts of relay cr2 connect lines 130a the output wire pair of night thermostat 130 to lines 132 the heat generator may comprise the customary solenoid in series circuit with the secondary of a transformer whose primary is energized to provide a c voltage 24 v in the secondary in situations wherein the pre existing arrangement is a single thermostat unit with but a single output wire pair the invention contemplates an arrangement shown in fig1 whereby a changeover may be made from day to day night control without the need for opening walls to install a second wire pair a pair of lines 134a and 134b are shown extending from solenoid s controlling the heat generator valve lines 136a and 136b are connected to day thermostat unit 136 including the customary bi metallic element t1 and variable resistor.