 Most digital thermostats have separate programs for heating and cooling, and may feature a digital or manual switch to turn on the furnace blower for air circulation, even when the system isn't heating or cooling.More-sophisticated models may be programmed to run the circulating fan for a brief 5- to 10-minute period in the event a heating or cooling cycle has not taken place during the previous hour.This is particularly useful in buildings subject to stratification where without frequent air circulation, hot air rises and separates from the cooler air that falls.