Interval timers are the most popular method of controlling and limiting the amount of fresh air brought into a house by exhaust fans during cold weather. A 36" fan will move approximately 10,000 cubic feet of air each minute it operates. But there are many times a grower may want considerably less air than this to be brought in. For instance, in the average 400- foot house during the first week of production, we only need about 2,000 cubic feet of fresh air each minute. During the fourth week, w...e may only need 8,000 cubic feet of fresh air each minute. To allow for this, growers place their minimum ventilation exhaust fans on ten-minute interval timers. If a single 36" fan runs for one minute then shuts off for nine, it will move an average of 1,000 ft3/min over the ten-minute period. If we turn it on two minutes and shut it off for eight, it will move an average of 2,000 ft3/min of air over the ten-minute period, and so on.