is provided to filter incoming air 46 the heat exchanger 18 preferably includes a heating portion 48 and a cooling portion 50 the heat exchanger 18 receives cooling fluid from the cold fluid supply 20 and hot fluid supply 22 in quantities and proportions as dictated by the control unit 34 if the system requires greater heating capabilities the heating portion of the heat exchanger may be adapted to accommodate steam the preferred capacity for the heat exchanger should be capable of shifting the air temperature at a rate 20 u00b0 f min at 4000 cubic feet minute the capacity can be varied however these values were found to be sufficient to provide adequate temperature control for the variety of vehicles tested the control unit 34 communicates a control signal to the cold and hot valves 24 26 the cold and hot valves adjust the flow of fluid into the heat exchanger 18 to condition the air flowing therethrough the cold and hot valves 24 26 may take any form of the many variable flow forms common in the industry one such example would be separate solenoid actuated units model 870010 available from honeywell corporation a pulse width modulated valve may also be used in lieu of the analog voltage operated solenoids described in the preferred embodiment it should also be recognized that a mixing valve could be used to combine the cold and hot fluids and feed a single.