discussed in terms of cooling the building however the same logic may be applied in heating the building or maintaining one or more other parameter s while not shown specifically in fig4 maximum and minimum fan speeds may be defined according to the above description in reference to fig3 an internal environmental parameter is then sensed as shown at step 174 and an appropriate data signal is communicated to the control unit 72 an external parameter is also sensed as shown at 176 as noted above the external parameter in this example is the ambient temperature outside the building the control unit 72 then determines if the sensed temperature is less than the defined setpoint as indicated at 178 if the result is affirmative then the control unit 72 will decrease the speed of one or more fans 24 and 32 as indicated at 180 following the decrease in fan speed the process returns to step 174 if the inquiry at step 178 is answered negatively then the control unit 72 determines whether the sensed temperature is greater than the defined level as indicated at 182 if the result is negative then the fan speed is maintained as shown at 184 and the process returns to step 176 if however the result is affirmative the control.