WHY IS MAKEUP AIR NEEDED? Most exhaust systems are installed for employee comfort and safety or for product process requirements. In the past, when energy was inexpensive and plentiful, exhaust fans were frequently added and often oversized. Little thought was given to bringing in outside air to replace the exhausted amount. If adequate air is not introduced into the building, a negative pressure condition will develop. This causes uncomfortable conditions and reduces the effectiveness of exhaust systems. Traditionally, when exhaust effectiveness is reduced, someone often adds another exhaust fan without compensating air which reduces the exhaust effectiveness further, which causes someone to add another exhaust fan, which.......you get the idea. The end result is an ineffective and uncomfortable system that is extremely wasteful and costly to operate.