apparatus disclosed in u s pat no 3 951 205 to zilbermann and u s pat no 4 203 485 to zilbermann and aronoff heaters 35 and 36 may likewise be similar to the duct heaters described in these u s patents however any suitable mixing box and duct heater may be used in accordance with the following functions mixing boxes 32 and 33 act to enable air from a return plenum to be recirculated back through skin zone ducts 30 and 31 to recover heat which would otherwise be exhausted from a return plenum typically an air volume is located above a floor ceiling structure and serves as the return air plenum heated cooling air tends to naturally rise into the volume for removal or recirculation as hereinafter explained heaters 35 and 36 provide all the heating for the three zones 14 16 and 18 and are energized when the heat available from the return air plenum not shown is less than the heat lost through the building exterior 10 and the temperature within perimeter zone 16 drops to actuate at least one sector thermostat 51 52 53 or 54 as shown in fig1 in a preferred operational sequence the heating and cooling portions are cooperatively staged so that the system does not attempt to heat air which has first been cooled thus as the temperatures begin to decrease in perimeter zone 16 the outputs from thermostats 51 52 53 and 54 act to close air flow dampers 56 57 58 and 59 respectively to decrease the cooling air flow from perimeter zone sector ducts 24 25 26 and 27 the control systems and control boxes 64 and 65 hereinafter explained select the lowest cooling demand from the perimeter zone sector duct systems to control.