im actually working on a building model with both the chiller and evaporative pre cooler in Ahu system , what i want to do is i want the evaporative pre cooler to run when the temperature of the outsider air is between say15 - 20 degree celcius and during that time the chiller should not operate and if the temperature of the outside air becomes more than 20 the chiller should operate and the evaporative pre cooler should stop , i have figured out how to stop the chiller working below 20 degree celcius but im having problem to configure the evaporative pre cooler to work between these temperature , if any one knows how to configure this please let me know