Make-Up Air Systems

Makeup or compensating air is outside air which is introduced into the building to replace air that is exhausted from the building through ventilation or combustion processes.


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.


Buildings which have inadequate makeup air (or excess exhaust) and are operating with a negative pressure condition have several common characteristics. These include:

A) The building is uncomfortable and has noticeable temperature fluctuations. The perimeter of the building may be cold and drafty due to high infiltration at the same time that other central areas may be overheated.

B) Exterior doors may be difficult and hazardous to open. A door which slams shut can result in broken bones and serious injuries.

C) Cold drafts can be felt rushing into the building around cracks and openings.

D) Exhaust systems do not perform at the rated design capacity. Inadequate or ineffective exhaust may result in a hazy atmosphere.

E) Gravity vents from heating equipment or process may back-vent into the conditioned space, drawing potentially hazardous products of combustion (Flue Gases) into the building.