Controlling minimum ventilation fans with five- or ten-minute interval timers has proven over the years to be a very effective way of controlling air quality and temperature as well as minimizing fuel usage during cold weather. An interval timer provides a grower a high level of control at a minimal cost. Think for a minute how you would ventilate a house during cold weather without exhaust fans controlled by a timer. Negative pressure ventilation would not be practical. If you ran one fan const...antly, a static pressure probably could not be generated, resulting in the fan providing fresh air in only the immediate vicinity of the fan. Even if you could obtain a static pressure with a single fan, it would provide too much fresh air for young birds and not enough for market-age birds. You could run two or three fans and obtain a static pressure, but this would be too much fresh air for all but the oldest birds during mild weather. Of course, you could turn off the fans and rely on a cracked curtain for ventilation and give up virtually all control over air temperature, air quality and fuel usage. Bottom line: exhaust fans on interval timers give the producer control over just how much air enters and help to insure that enough fresh air is brought into a house on a regular basis to control moisture, ammonia, and dust. At the same time, they help to keep fuel usage down with minimal cost and effort.